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[PATCH v3 0/4] Allow customizable random offset to mmap_base address.

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  [PATCH v3 0/4] Allow customizable random offset to mmap_base address. Daniel Cashman <dcashman@android.com> - 2015-11-19 00:30 +0100
    [PATCH v3 1/4] mm: mmap: Add new /proc tunable for mmap_base ASLR. Daniel Cashman <dcashman@android.com> - 2015-11-19 00:30 +0100
      Re: [PATCH v3 1/4] mm: mmap: Add new /proc tunable for mmap_base  ASLR. Daniel Cashman <dcashman@android.com> - 2015-11-19 01:20 +0100
      Re: [PATCH v3 1/4] mm: mmap: Add new /proc tunable for mmap_base  ASLR. Andrew Morton <akpm@linux-foundation.org> - 2015-11-25 01:50 +0100
        Re: [PATCH v3 1/4] mm: mmap: Add new /proc tunable for mmap_base ASLR. Kees Cook <keescook@chromium.org> - 2015-11-25 01:50 +0100
          Re: [PATCH v3 1/4] mm: mmap: Add new /proc tunable for mmap_base  ASLR. Daniel Cashman <dcashman@android.com> - 2015-11-25 20:20 +0100
      Re: [PATCH v3 1/4] mm: mmap: Add new /proc tunable for mmap_base  ASLR. Michael Ellerman <mpe@ellerman.id.au> - 2015-11-25 05:50 +0100
        Re: [PATCH v3 1/4] mm: mmap: Add new /proc tunable for mmap_base  ASLR. Daniel Cashman <dcashman@android.com> - 2015-11-25 20:40 +0100
    Re: [PATCH v3 0/4] Allow customizable random offset to mmap_base  address. Andrew Morton <akpm@linux-foundation.org> - 2015-11-25 01:40 +0100
      Re: [PATCH v3 0/4] Allow customizable random offset to mmap_base  address. Daniel Cashman <dcashman@android.com> - 2015-11-25 20:10 +0100
        Re: [PATCH v3 0/4] Allow customizable random offset to mmap_base  address. Martin Schwidefsky <schwidefsky@de.ibm.com> - 2015-11-26 16:20 +0100
      Re: [PATCH v3 0/4] Allow customizable random offset to mmap_base  address. Michael Ellerman <mpe@ellerman.id.au> - 2015-11-26 08:10 +0100

#1272677 — [PATCH v3 0/4] Allow customizable random offset to mmap_base address.

From: dcashman <dcashman@google.com>

Address Space Layout Randomization (ASLR) provides a barrier to exploitation of user-space processes in the presence of security vulnerabilities by making it more difficult to find desired code/data which could help an attack. This is done by adding a random offset to the location of regions in the process address space, with a greater range of potential offset values corresponding to better protection/a larger search-space for brute force, but also to greater potential for fragmentation.

The offset added to the mmap_base address, which provides the basis for the majority of the mappings for a process, is set once on process exec in arch_pick_mmap_layout() and is done via hard-coded per-arch values, which reflect, hopefully, the best compromise for all systems. The trade-off between increased entropy in the offset value generation and the corresponding increased variability in address space fragmentation is not absolute, however, and some platforms may tolerate higher amounts of entropy. This patch introduces both new Kconfig values and a sysctl interface which may be used to change the amount of entropy used for offset generation on a system.

The direct motivation for this change was in response to the libstagefright vulnerabilities that affected Android, specifically to information provided by Google's project zero at:

http://googleprojectzero.blogspot.com/2015/09/stagefrightened.html

The attack presented therein, by Google's project zero, specifically targeted the limited randomness used to generate the offset added to the mmap_base address in order to craft a brute-force-based attack. Concretely, the attack was against the mediaserver process, which was limited to respawning every 5 seconds, on an arm device. The hard-coded 8 bits used resulted in an average expected success rate of defeating the mmap ASLR after just over 10 minutes (128 tries at 5 seconds a piece). With this patch, and an accompanying increase in the entropy value to 16 bits, the same attack would take an average expected time of over 45 hours (32768 tries), which makes it both less feasible and more likely to be noticed.

The introduced Kconfig and sysctl options are limited by per-arch minimum and maximum values, the minimum of which was chosen to match the current hard-coded value and the maximum of which was chosen so as to give the greatest flexibility without generating an invalid mmap_base address, generally a 3-4 bits less than the number of bits in the user-space accessible virtual address space.

When decided whether or not to change the default value, a system developer should consider that mmap_base address could be placed anywhere up to 2^(value) bits away from the non-randomized location, which would introduce variable-sized areas above and below the mmap_base address such that the maximum vm_area_struct size may be reduced, preventing very large allocations.

Changes in v3:
* moved sysctl from /proc/sys/kernel to /proc/sys/vm
* added to arch/x86 (both 32 and 64 bit)
* added to arch/arm64
* added ability for arch to specify default value in between max - min

dcashman (4):
  mm: mmap: Add new /proc tunable for mmap_base ASLR.
  arm: mm: support ARCH_MMAP_RND_BITS.
  arm64: mm: support ARCH_MMAP_RND_BITS.
  x86: mm: support ARCH_MMAP_RND_BITS.

 Documentation/sysctl/vm.txt | 29 ++++++++++++++++++++
 arch/Kconfig                | 64 +++++++++++++++++++++++++++++++++++++++++++++
 arch/arm/Kconfig            | 10 +++++++
 arch/arm/mm/mmap.c          |  3 +--
 arch/arm64/Kconfig          | 23 ++++++++++++++++
 arch/arm64/mm/mmap.c        |  6 +++--
 arch/x86/Kconfig            | 16 ++++++++++++
 arch/x86/mm/mmap.c          | 12 ++++-----
 include/linux/mm.h          | 11 ++++++++
 kernel/sysctl.c             | 22 ++++++++++++++++
 mm/mmap.c                   | 12 +++++++++
 11 files changed, 198 insertions(+), 10 deletions(-)

-- 
2.6.0.rc2.230.g3dd15c0

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#1272680 — [PATCH v3 1/4] mm: mmap: Add new /proc tunable for mmap_base ASLR.

From: dcashman <dcashman@google.com>

ASLR currently only uses 8 bits to generate the random offset for the
mmap base address on 32 bit architectures. This value was chosen to
prevent a poorly chosen value from dividing the address space in such
a way as to prevent large allocations. This may not be an issue on all
platforms. Allow the specification of a minimum number of bits so that
platforms desiring greater ASLR protection may determine where to place
the trade-off.

Signed-off-by: Daniel Cashman <dcashman@google.com>
---
 Documentation/sysctl/vm.txt | 29 ++++++++++++++++++++
 arch/Kconfig                | 64 +++++++++++++++++++++++++++++++++++++++++++++
 include/linux/mm.h          | 11 ++++++++
 kernel/sysctl.c             | 22 ++++++++++++++++
 mm/mmap.c                   | 12 +++++++++
 5 files changed, 138 insertions(+)

diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
index f72370b..d77a81a 100644
--- a/Documentation/sysctl/vm.txt
+++ b/Documentation/sysctl/vm.txt
@@ -42,6 +42,8 @@ Currently, these files are in /proc/sys/vm:
 - min_slab_ratio
 - min_unmapped_ratio
 - mmap_min_addr
+- mmap_rnd_bits
+- mmap_rnd_compat_bits
 - nr_hugepages
 - nr_overcommit_hugepages
 - nr_trim_pages         (only if CONFIG_MMU=n)
@@ -485,6 +487,33 @@ against future potential kernel bugs.
 
 ==============================================================
 
+mmap_rnd_bits:
+
+This value can be used to select the number of bits to use to
+determine the random offset to the base address of vma regions
+resulting from mmap allocations on architectures which support
+tuning address space randomization.  This value will be bounded
+by the architecture's minimum and maximum supported values.
+
+This value can be changed after boot using the
+/proc/sys/kernel/mmap_rnd_bits tunable
+
+==============================================================
+
+mmap_rnd_compat_bits:
+
+This value can be used to select the number of bits to use to
+determine the random offset to the base address of vma regions
+resulting from mmap allocations for applications run in
+compatibility mode on architectures which support tuning address
+space randomization.  This value will be bounded by the
+architecture's minimum and maximum supported values.
+
+This value can be changed after boot using the
+/proc/sys/kernel/mmap_rnd_compat_bits tunable
+
+==============================================================
+
 nr_hugepages
 
 Change the minimum size of the hugepage pool.
diff --git a/arch/Kconfig b/arch/Kconfig
index 4e949e5..141823f 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -511,6 +511,70 @@ config ARCH_HAS_ELF_RANDOMIZE
 	  - arch_mmap_rnd()
 	  - arch_randomize_brk()
 
+config HAVE_ARCH_MMAP_RND_BITS
+	bool
+	help
+	  An arch should select this symbol if it supports setting a variable
+	  number of bits for use in establishing the base address for mmap
+	  allocations and provides values for both:
+	  - ARCH_MMAP_RND_BITS_MIN
+	  - ARCH_MMAP_RND_BITS_MAX
+
+config ARCH_MMAP_RND_BITS_MIN
+	int
+
+config ARCH_MMAP_RND_BITS_MAX
+	int
+
+config ARCH_MMAP_RND_BITS_DEFAULT
+	int
+
+config ARCH_MMAP_RND_BITS
+	int "Number of bits to use for ASLR of mmap base address" if EXPERT
+	range ARCH_MMAP_RND_BITS_MIN ARCH_MMAP_RND_BITS_MAX
+	default ARCH_MMAP_RND_BITS_DEFAULT if ARCH_MMAP_RND_BITS_DEFAULT
+	default ARCH_MMAP_RND_BITS_MIN
+	depends on HAVE_ARCH_MMAP_RND_BITS
+	help
+	  This value can be used to select the number of bits to use to
+	  determine the random offset to the base address of vma regions
+	  resulting from mmap allocations. This value will be bounded
+	  by the architecture's minimum and maximum supported values.
+
+	  This value can be changed after boot using the
+	  /proc/sys/kernel/mmap_rnd_bits tunable
+
+config HAVE_ARCH_MMAP_RND_COMPAT_BITS
+	bool
+	help
+	  An arch should select this symbol if it supports running applications
+	  in compatibility mode, supports setting a variable number of bits for
+	  use in establishing the base address for mmap allocations, and
+	  provides values for both:
+	  - ARCH_MMAP_RND_COMPAT_BITS_MIN
+	  - ARCH_MMAP_RND_COMPAT_BITS_MAX
+
+config ARCH_MMAP_RND_COMPAT_BITS_MIN
+	int
+
+config ARCH_MMAP_RND_COMPAT_BITS_MAX
+	int
+
+config ARCH_MMAP_RND_COMPAT_BITS
+	int "Number of bits to use for ASLR of mmap base address for compatible applications" if EXPERT
+	range ARCH_MMAP_RND_COMPAT_BITS_MIN ARCH_MMAP_RND_COMPAT_BITS_MAX
+	default ARCH_MMAP_RND_COMPAT_BITS_MIN
+	depends on HAVE_ARCH_MMAP_RND_COMPAT_BITS
+	help
+	  This value can be used to select the number of bits to use to
+	  determine the random offset to the base address of vma regions
+	  resulting from mmap allocations for compatible applications This
+	  value will be bounded by the architecture's minimum and maximum
+	  supported values.
+
+	  This value can be changed after boot using the
+	  /proc/sys/kernel/mmap_rnd_compat_bits tunable
+
 config HAVE_COPY_THREAD_TLS
 	bool
 	help
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 00bad77..7d39828 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -51,6 +51,17 @@ extern int sysctl_legacy_va_layout;
 #define sysctl_legacy_va_layout 0
 #endif
 
+#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
+extern int mmap_rnd_bits_min;
+extern int mmap_rnd_bits_max;
+extern int mmap_rnd_bits;
+#endif
+#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
+extern int mmap_rnd_compat_bits_min;
+extern int mmap_rnd_compat_bits_max;
+extern int mmap_rnd_compat_bits;
+#endif
+
 #include <asm/page.h>
 #include <asm/pgtable.h>
 #include <asm/processor.h>
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index dc6858d..40e5de6 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1568,6 +1568,28 @@ static struct ctl_table vm_table[] = {
 		.mode		= 0644,
 		.proc_handler	= proc_doulongvec_minmax,
 	},
+#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
+	{
+		.procname	= "mmap_rnd_bits",
+		.data		= &mmap_rnd_bits,
+		.maxlen		= sizeof(mmap_rnd_bits),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= &mmap_rnd_bits_min,
+		.extra2		= &mmap_rnd_bits_max,
+	},
+#endif
+#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
+	{
+		.procname	= "mmap_rnd_compat_bits",
+		.data		= &mmap_rnd_compat_bits,
+		.maxlen		= sizeof(mmap_rnd_compat_bits),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= &mmap_rnd_compat_bits_min,
+		.extra2		= &mmap_rnd_compat_bits_max,
+	},
+#endif
 	{ }
 };
 
diff --git a/mm/mmap.c b/mm/mmap.c
index 2ce04a6..aa49841 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -58,6 +58,18 @@
 #define arch_rebalance_pgtables(addr, len)		(addr)
 #endif
 
+#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
+int mmap_rnd_bits_min = CONFIG_ARCH_MMAP_RND_BITS_MIN;
+int mmap_rnd_bits_max = CONFIG_ARCH_MMAP_RND_BITS_MAX;
+int mmap_rnd_bits = CONFIG_ARCH_MMAP_RND_BITS;
+#endif
+#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
+int mmap_rnd_compat_bits_min = CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MIN;
+int mmap_rnd_compat_bits_max = CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MAX;
+int mmap_rnd_compat_bits = CONFIG_ARCH_MMAP_RND_COMPAT_BITS;
+#endif
+
+
 static void unmap_region(struct mm_struct *mm,
 		struct vm_area_struct *vma, struct vm_area_struct *prev,
 		unsigned long start, unsigned long end);
-- 
2.6.0.rc2.230.g3dd15c0

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#1272701 — Re: [PATCH v3 1/4] mm: mmap: Add new /proc tunable for mmap_base ASLR.

On 11/18/2015 03:20 PM, Daniel Cashman wrote:
>  ==============================================================
>  
> +mmap_rnd_bits:
> +
> +This value can be used to select the number of bits to use to
> +determine the random offset to the base address of vma regions
> +resulting from mmap allocations on architectures which support
> +tuning address space randomization.  This value will be bounded
> +by the architecture's minimum and maximum supported values.
> +
> +This value can be changed after boot using the
> +/proc/sys/kernel/mmap_rnd_bits tunable
> +
> +==============================================================
> +
> +mmap_rnd_compat_bits:
> +
> +This value can be used to select the number of bits to use to
> +determine the random offset to the base address of vma regions
> +resulting from mmap allocations for applications run in
> +compatibility mode on architectures which support tuning address
> +space randomization.  This value will be bounded by the
> +architecture's minimum and maximum supported values.
> +
> +This value can be changed after boot using the
> +/proc/sys/kernel/mmap_rnd_compat_bits tunable
> +
> +==============================================================

As Kees pointed out in my erroneously sent (missing v3 prefix)
patch-set: the /proc/sys/kernel/ entries were not changed to reflect the
move to /proc/sys/vm/.


> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 00bad77..7d39828 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -51,6 +51,17 @@ extern int sysctl_legacy_va_layout;
>  #define sysctl_legacy_va_layout 0
>  #endif
>  
> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
> +extern int mmap_rnd_bits_min;
> +extern int mmap_rnd_bits_max;
> +extern int mmap_rnd_bits;
> +#endif
> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
> +extern int mmap_rnd_compat_bits_min;
> +extern int mmap_rnd_compat_bits_max;
> +extern int mmap_rnd_compat_bits;
> +#endif
> +
>  #include <asm/page.h>
>  #include <asm/pgtable.h>
>  #include <asm/processor.h>
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index dc6858d..40e5de6 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -1568,6 +1568,28 @@ static struct ctl_table vm_table[] = {
>  		.mode		= 0644,
>  		.proc_handler	= proc_doulongvec_minmax,
>  	},
> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
> +	{
> +		.procname	= "mmap_rnd_bits",
> +		.data		= &mmap_rnd_bits,
> +		.maxlen		= sizeof(mmap_rnd_bits),
> +		.mode		= 0644,
> +		.proc_handler	= proc_dointvec_minmax,
> +		.extra1		= &mmap_rnd_bits_min,
> +		.extra2		= &mmap_rnd_bits_max,
> +	},
> +#endif
> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
> +	{
> +		.procname	= "mmap_rnd_compat_bits",
> +		.data		= &mmap_rnd_compat_bits,
> +		.maxlen		= sizeof(mmap_rnd_compat_bits),
> +		.mode		= 0644,
> +		.proc_handler	= proc_dointvec_minmax,
> +		.extra1		= &mmap_rnd_compat_bits_min,
> +		.extra2		= &mmap_rnd_compat_bits_max,
> +	},
> +#endif
>  	{ }
>  };
>  
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 2ce04a6..aa49841 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -58,6 +58,18 @@
>  #define arch_rebalance_pgtables(addr, len)		(addr)
>  #endif
>  
> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
> +int mmap_rnd_bits_min = CONFIG_ARCH_MMAP_RND_BITS_MIN;
> +int mmap_rnd_bits_max = CONFIG_ARCH_MMAP_RND_BITS_MAX;
> +int mmap_rnd_bits = CONFIG_ARCH_MMAP_RND_BITS;
> +#endif
> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
> +int mmap_rnd_compat_bits_min = CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MIN;
> +int mmap_rnd_compat_bits_max = CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MAX;
> +int mmap_rnd_compat_bits = CONFIG_ARCH_MMAP_RND_COMPAT_BITS;
> +#endif
> +

Again from Kees in my erroneously sent (missing v3 prefix) patch-set:
the min/max should be const.
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#1276931 — Re: [PATCH v3 1/4] mm: mmap: Add new /proc tunable for mmap_base ASLR.

On Wed, 18 Nov 2015 15:20:05 -0800 Daniel Cashman <dcashman@android.com> wrote:

> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -1568,6 +1568,28 @@ static struct ctl_table vm_table[] = {
>  		.mode		= 0644,
>  		.proc_handler	= proc_doulongvec_minmax,
>  	},
> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
> +	{
> +		.procname	= "mmap_rnd_bits",
> +		.data		= &mmap_rnd_bits,
> +		.maxlen		= sizeof(mmap_rnd_bits),
> +		.mode		= 0644,

Is there any harm in permitting the attacker to read these values?

And is there any benefit in permitting non-attackers to read them?

> +		.proc_handler	= proc_dointvec_minmax,
> +		.extra1		= &mmap_rnd_bits_min,
> +		.extra2		= &mmap_rnd_bits_max,
> +	},
> +#endif
> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
> +	{
> +		.procname	= "mmap_rnd_compat_bits",
> +		.data		= &mmap_rnd_compat_bits,
> +		.maxlen		= sizeof(mmap_rnd_compat_bits),
> +		.mode		= 0644,
> +		.proc_handler	= proc_dointvec_minmax,
> +		.extra1		= &mmap_rnd_compat_bits_min,
> +		.extra2		= &mmap_rnd_compat_bits_max,
> +	},
> +#endif
>
> ...
>
> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
> +int mmap_rnd_bits_min = CONFIG_ARCH_MMAP_RND_BITS_MIN;
> +int mmap_rnd_bits_max = CONFIG_ARCH_MMAP_RND_BITS_MAX;
> +int mmap_rnd_bits = CONFIG_ARCH_MMAP_RND_BITS;
> +#endif
> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
> +int mmap_rnd_compat_bits_min = CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MIN;
> +int mmap_rnd_compat_bits_max = CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MAX;
> +int mmap_rnd_compat_bits = CONFIG_ARCH_MMAP_RND_COMPAT_BITS;

These could be __read_mostly.

If one believes in such things.  One effect of __read_mostly is to
clump the write-often stuff into the same cachelines and I've never
been convinced that one outweighs the other...

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#1276933 — Re: [PATCH v3 1/4] mm: mmap: Add new /proc tunable for mmap_base ASLR.

On Tue, Nov 24, 2015 at 4:40 PM, Andrew Morton
<akpm@linux-foundation.org> wrote:
> On Wed, 18 Nov 2015 15:20:05 -0800 Daniel Cashman <dcashman@android.com> wrote:
>
>> --- a/kernel/sysctl.c
>> +++ b/kernel/sysctl.c
>> @@ -1568,6 +1568,28 @@ static struct ctl_table vm_table[] = {
>>               .mode           = 0644,
>>               .proc_handler   = proc_doulongvec_minmax,
>>       },
>> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
>> +     {
>> +             .procname       = "mmap_rnd_bits",
>> +             .data           = &mmap_rnd_bits,
>> +             .maxlen         = sizeof(mmap_rnd_bits),
>> +             .mode           = 0644,
>
> Is there any harm in permitting the attacker to read these values?
>
> And is there any benefit in permitting non-attackers to read them?

I'm on the fence. Things like kernel/randomize_va_space is 644. But
since I don't see a benefit in exposing them, let's make them all 600
instead -- it's a new interface, better to keep it narrower now.

>
>> +             .proc_handler   = proc_dointvec_minmax,
>> +             .extra1         = &mmap_rnd_bits_min,
>> +             .extra2         = &mmap_rnd_bits_max,
>> +     },
>> +#endif
>> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
>> +     {
>> +             .procname       = "mmap_rnd_compat_bits",
>> +             .data           = &mmap_rnd_compat_bits,
>> +             .maxlen         = sizeof(mmap_rnd_compat_bits),
>> +             .mode           = 0644,
>> +             .proc_handler   = proc_dointvec_minmax,
>> +             .extra1         = &mmap_rnd_compat_bits_min,
>> +             .extra2         = &mmap_rnd_compat_bits_max,
>> +     },
>> +#endif
>>
>> ...
>>
>> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
>> +int mmap_rnd_bits_min = CONFIG_ARCH_MMAP_RND_BITS_MIN;
>> +int mmap_rnd_bits_max = CONFIG_ARCH_MMAP_RND_BITS_MAX;
>> +int mmap_rnd_bits = CONFIG_ARCH_MMAP_RND_BITS;
>> +#endif
>> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
>> +int mmap_rnd_compat_bits_min = CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MIN;
>> +int mmap_rnd_compat_bits_max = CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MAX;
>> +int mmap_rnd_compat_bits = CONFIG_ARCH_MMAP_RND_COMPAT_BITS;
>
> These could be __read_mostly.
>
> If one believes in such things.  One effect of __read_mostly is to
> clump the write-often stuff into the same cachelines and I've never
> been convinced that one outweighs the other...

The _min and _max values should be const, actually, since they're
build-time selected. The _bits could easily be __read_mostly, yeah.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security
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#1277712 — Re: [PATCH v3 1/4] mm: mmap: Add new /proc tunable for mmap_base ASLR.

On 11/24/2015 04:47 PM, Kees Cook wrote:
> On Tue, Nov 24, 2015 at 4:40 PM, Andrew Morton
> <akpm@linux-foundation.org> wrote:
>> On Wed, 18 Nov 2015 15:20:05 -0800 Daniel Cashman <dcashman@android.com> wrote:
>>
>>> --- a/kernel/sysctl.c
>>> +++ b/kernel/sysctl.c
>>> @@ -1568,6 +1568,28 @@ static struct ctl_table vm_table[] = {
>>>               .mode           = 0644,
>>>               .proc_handler   = proc_doulongvec_minmax,
>>>       },
>>> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
>>> +     {
>>> +             .procname       = "mmap_rnd_bits",
>>> +             .data           = &mmap_rnd_bits,
>>> +             .maxlen         = sizeof(mmap_rnd_bits),
>>> +             .mode           = 0644,
>>
>> Is there any harm in permitting the attacker to read these values?
>>
>> And is there any benefit in permitting non-attackers to read them?
> 
> I'm on the fence. Things like kernel/randomize_va_space is 644. But
> since I don't see a benefit in exposing them, let's make them all 600
> instead -- it's a new interface, better to keep it narrower now.

Is there any harm in allowing the attacker to read these values? Nothing
immediately comes to mind.  It is a form of information leakage, and I
guess a local attacker could use this information to calibrate an attack
or decide whether or not brute-forcing is a worthy approach, but this
easily could be leaked in other ways as well.

Is there a benefit to allowing non-attackers to read them?  Possibly
could be used in tests seeking to verify the system environment, but
again, this could be discovered in other ways.

I like Kees' suggestion of starting narrow and granting if need arises.

>>>
>>> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
>>> +int mmap_rnd_bits_min = CONFIG_ARCH_MMAP_RND_BITS_MIN;
>>> +int mmap_rnd_bits_max = CONFIG_ARCH_MMAP_RND_BITS_MAX;
>>> +int mmap_rnd_bits = CONFIG_ARCH_MMAP_RND_BITS;
>>> +#endif
>>> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
>>> +int mmap_rnd_compat_bits_min = CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MIN;
>>> +int mmap_rnd_compat_bits_max = CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MAX;
>>> +int mmap_rnd_compat_bits = CONFIG_ARCH_MMAP_RND_COMPAT_BITS;
>>
>> These could be __read_mostly.
>>
>> If one believes in such things.  One effect of __read_mostly is to
>> clump the write-often stuff into the same cachelines and I've never
>> been convinced that one outweighs the other...
> 
> The _min and _max values should be const, actually, since they're
> build-time selected. The _bits could easily be __read_mostly, yeah.

Yes, one would generally expect these to never be touched, and even if
they were, the threshold of __read_mostly would certainly be crossed.

-Dan

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#1277014 — Re: [PATCH v3 1/4] mm: mmap: Add new /proc tunable for mmap_base ASLR.

On Wed, 2015-11-18 at 15:20 -0800, Daniel Cashman wrote:

> From: dcashman <dcashman@google.com>
> 
> ASLR currently only uses 8 bits to generate the random offset for the
> mmap base address on 32 bit architectures. This value was chosen to
> prevent a poorly chosen value from dividing the address space in such
> a way as to prevent large allocations. This may not be an issue on all
> platforms. Allow the specification of a minimum number of bits so that
> platforms desiring greater ASLR protection may determine where to place
> the trade-off.

...

> diff --git a/arch/Kconfig b/arch/Kconfig
> index 4e949e5..141823f 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -511,6 +511,70 @@ config ARCH_HAS_ELF_RANDOMIZE
>  	  - arch_mmap_rnd()
>  	  - arch_randomize_brk()
>  
> +config HAVE_ARCH_MMAP_RND_BITS
> +	bool
> +	help
> +	  An arch should select this symbol if it supports setting a variable
> +	  number of bits for use in establishing the base address for mmap
> +	  allocations and provides values for both:
> +	  - ARCH_MMAP_RND_BITS_MIN
> +	  - ARCH_MMAP_RND_BITS_MAX
> +
> +config ARCH_MMAP_RND_BITS_MIN
> +	int
> +
> +config ARCH_MMAP_RND_BITS_MAX
> +	int
> +
> +config ARCH_MMAP_RND_BITS_DEFAULT
> +	int
> +
> +config ARCH_MMAP_RND_BITS
> +	int "Number of bits to use for ASLR of mmap base address" if EXPERT
> +	range ARCH_MMAP_RND_BITS_MIN ARCH_MMAP_RND_BITS_MAX
> +	default ARCH_MMAP_RND_BITS_DEFAULT if ARCH_MMAP_RND_BITS_DEFAULT

Here you support a default which is separate from the minimum.

> +	default ARCH_MMAP_RND_BITS_MIN
> +	depends on HAVE_ARCH_MMAP_RND_BITS

...
> +
> +config ARCH_MMAP_RND_COMPAT_BITS
> +	int "Number of bits to use for ASLR of mmap base address for compatible applications" if EXPERT
> +	range ARCH_MMAP_RND_COMPAT_BITS_MIN ARCH_MMAP_RND_COMPAT_BITS_MAX
> +	default ARCH_MMAP_RND_COMPAT_BITS_MIN

But here you don't.

Just forgot?

I'd like to have a default which is separate from the minimum. That way we can
have a default which is reasonably large, but allow it to be lowered easily if
anything breaks.

cheers

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#1277731 — Re: [PATCH v3 1/4] mm: mmap: Add new /proc tunable for mmap_base ASLR.

On 11/24/2015 08:40 PM, Michael Ellerman wrote:
> On Wed, 2015-11-18 at 15:20 -0800, Daniel Cashman wrote:
> 
>> From: dcashman <dcashman@google.com>
>>
>> ASLR currently only uses 8 bits to generate the random offset for the
>> mmap base address on 32 bit architectures. This value was chosen to
>> prevent a poorly chosen value from dividing the address space in such
>> a way as to prevent large allocations. This may not be an issue on all
>> platforms. Allow the specification of a minimum number of bits so that
>> platforms desiring greater ASLR protection may determine where to place
>> the trade-off.
> 
> ...
> 
>> diff --git a/arch/Kconfig b/arch/Kconfig
>> index 4e949e5..141823f 100644
>> --- a/arch/Kconfig
>> +++ b/arch/Kconfig
>> @@ -511,6 +511,70 @@ config ARCH_HAS_ELF_RANDOMIZE
>>  	  - arch_mmap_rnd()
>>  	  - arch_randomize_brk()
>>  
>> +config HAVE_ARCH_MMAP_RND_BITS
>> +	bool
>> +	help
>> +	  An arch should select this symbol if it supports setting a variable
>> +	  number of bits for use in establishing the base address for mmap
>> +	  allocations and provides values for both:
>> +	  - ARCH_MMAP_RND_BITS_MIN
>> +	  - ARCH_MMAP_RND_BITS_MAX
>> +
>> +config ARCH_MMAP_RND_BITS_MIN
>> +	int
>> +
>> +config ARCH_MMAP_RND_BITS_MAX
>> +	int
>> +
>> +config ARCH_MMAP_RND_BITS_DEFAULT
>> +	int
>> +
>> +config ARCH_MMAP_RND_BITS
>> +	int "Number of bits to use for ASLR of mmap base address" if EXPERT
>> +	range ARCH_MMAP_RND_BITS_MIN ARCH_MMAP_RND_BITS_MAX
>> +	default ARCH_MMAP_RND_BITS_DEFAULT if ARCH_MMAP_RND_BITS_DEFAULT
> 
> Here you support a default which is separate from the minimum.
> 
>> +	default ARCH_MMAP_RND_BITS_MIN
>> +	depends on HAVE_ARCH_MMAP_RND_BITS
> 
> ...
>> +
>> +config ARCH_MMAP_RND_COMPAT_BITS
>> +	int "Number of bits to use for ASLR of mmap base address for compatible applications" if EXPERT
>> +	range ARCH_MMAP_RND_COMPAT_BITS_MIN ARCH_MMAP_RND_COMPAT_BITS_MAX
>> +	default ARCH_MMAP_RND_COMPAT_BITS_MIN
> 
> But here you don't.
> 
> Just forgot?

Yes.  Good catch.

> I'd like to have a default which is separate from the minimum. That way we can
> have a default which is reasonably large, but allow it to be lowered easily if
> anything breaks.

Will add it, along w/the documentation cleanup and other changes.

Thank You,
Dan
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#1276929 — Re: [PATCH v3 0/4] Allow customizable random offset to mmap_base address.

On Wed, 18 Nov 2015 15:20:04 -0800 Daniel Cashman <dcashman@android.com> wrote:

> Address Space Layout Randomization (ASLR) provides a barrier to
> exploitation of user-space processes in the presence of security
> vulnerabilities by making it more difficult to find desired code/data
> which could help an attack.  This is done by adding a random offset to the
> location of regions in the process address space, with a greater range of
> potential offset values corresponding to better protection/a larger
> search-space for brute force, but also to greater potential for
> fragmentation.
> 
> The offset added to the mmap_base address, which provides the basis for
> the majority of the mappings for a process, is set once on process exec in
> arch_pick_mmap_layout() and is done via hard-coded per-arch values, which
> reflect, hopefully, the best compromise for all systems.  The trade-off
> between increased entropy in the offset value generation and the
> corresponding increased variability in address space fragmentation is not
> absolute, however, and some platforms may tolerate higher amounts of
> entropy.  This patch introduces both new Kconfig values and a sysctl
> interface which may be used to change the amount of entropy used for
> offset generation on a system.
> 
> The direct motivation for this change was in response to the
> libstagefright vulnerabilities that affected Android, specifically to
> information provided by Google's project zero at:
> 
> http://googleprojectzero.blogspot.com/2015/09/stagefrightened.html
> 
> The attack presented therein, by Google's project zero, specifically
> targeted the limited randomness used to generate the offset added to the
> mmap_base address in order to craft a brute-force-based attack. 
> Concretely, the attack was against the mediaserver process, which was
> limited to respawning every 5 seconds, on an arm device.  The hard-coded 8
> bits used resulted in an average expected success rate of defeating the
> mmap ASLR after just over 10 minutes (128 tries at 5 seconds a piece). 
> With this patch, and an accompanying increase in the entropy value to 16
> bits, the same attack would take an average expected time of over 45 hours
> (32768 tries), which makes it both less feasible and more likely to be
> noticed.
> 
> The introduced Kconfig and sysctl options are limited by per-arch minimum
> and maximum values, the minimum of which was chosen to match the current
> hard-coded value and the maximum of which was chosen so as to give the
> greatest flexibility without generating an invalid mmap_base address,
> generally a 3-4 bits less than the number of bits in the user-space
> accessible virtual address space.
> 
> When decided whether or not to change the default value, a system
> developer should consider that mmap_base address could be placed anywhere
> up to 2^(value) bits away from the non-randomized location, which would
> introduce variable-sized areas above and below the mmap_base address such
> that the maximum vm_area_struct size may be reduced, preventing very large
> allocations. 

Nice, thanks.

mips, powerpc and s390 also implement arch_mmap_rnd().  Are there any
special considerations here, or it just a matter of maintainers wiring
it up and testing it?

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#1277705 — Re: [PATCH v3 0/4] Allow customizable random offset to mmap_base address.

On 11/24/2015 04:39 PM, Andrew Morton wrote:

> mips, powerpc and s390 also implement arch_mmap_rnd().  Are there any
> special considerations here, or it just a matter of maintainers wiring
> it up and testing it?

I had not yet looked at those at all, as I had no way to do even a
rudimentary "does it boot" test and opted to post v3 first.  Upon first
glance, it should just be a matter of wiring it up:

Mips is divided into 12/16 bits for 32/64 bit (assume baseline 4k page)
w/COMPAT kconfig,  powerpc is 11/18 w/COMPAT, s390 is 11/11 w/COMPAT.
s390 is a bit strange as COMPAT is for a 31-bit address space, although
is_32bit_task() is used to determine which mask to use, and the mask
itself for 64-bit only introduces 11 bits of entropy, but while still
affecting larger chunks of the address space (mask is 0x3ff80, resulting
in an effective 0x7ff shift of PAGE_SIZE + 7 bits).

I could go ahead and add these to patchset v4 and as with the previous
architectures, rely on feedback from arch-specific maintainers to help
tune and test the values.

-Dan
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#1278281 — Re: [PATCH v3 0/4] Allow customizable random offset to mmap_base address.

On Wed, 25 Nov 2015 11:07:09 -0800
Daniel Cashman <dcashman@android.com> wrote:

> On 11/24/2015 04:39 PM, Andrew Morton wrote:
> 
> > mips, powerpc and s390 also implement arch_mmap_rnd().  Are there any
> > special considerations here, or it just a matter of maintainers wiring
> > it up and testing it?
> 
> I had not yet looked at those at all, as I had no way to do even a
> rudimentary "does it boot" test and opted to post v3 first.  Upon first
> glance, it should just be a matter of wiring it up:
> 
> Mips is divided into 12/16 bits for 32/64 bit (assume baseline 4k page)
> w/COMPAT kconfig,  powerpc is 11/18 w/COMPAT, s390 is 11/11 w/COMPAT.
> s390 is a bit strange as COMPAT is for a 31-bit address space, although
> is_32bit_task() is used to determine which mask to use, and the mask
> itself for 64-bit only introduces 11 bits of entropy, but while still
> affecting larger chunks of the address space (mask is 0x3ff80, resulting
> in an effective 0x7ff shift of PAGE_SIZE + 7 bits).

s390 uses a mmap randomization of 11 bits but applies it to different
bits dependent if the task is a compat task or not. From the machine
perspective we would like to always use the randomization bits for
normal, non-compat tasks. But as the 2GB address space for compat tasks
is really limited the randomization is applied in bits 2^12..2^22 for
compat tasks vs 2^19..2^29 for normal tasks at the cost of performance.
This has to do with the cache aliasing on z13.

By the way we will replace is_32bit_task with() is_compat_task(), I have
a patch from Heiko pending for that.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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#1278010 — Re: [PATCH v3 0/4] Allow customizable random offset to mmap_base address.

On Tue, 2015-11-24 at 16:39 -0800, Andrew Morton wrote:
> On Wed, 18 Nov 2015 15:20:04 -0800 Daniel Cashman <dcashman@android.com> wrote:
> > Address Space Layout Randomization (ASLR) provides a barrier to
> > exploitation of user-space processes in the presence of security
> > vulnerabilities by making it more difficult to find desired code/data
> > which could help an attack.  This is done by adding a random offset to the
> > location of regions in the process address space, with a greater range of
> > potential offset values corresponding to better protection/a larger
> > search-space for brute force, but also to greater potential for
> > fragmentation.
> 
> mips, powerpc and s390 also implement arch_mmap_rnd().  Are there any
> special considerations here, or it just a matter of maintainers wiring
> it up and testing it?

I had a quick stab at powerpc. It seems to work OK, though I've only tested on
64-bit 64K pages.

I'll update this when Daniel does a version which supports a DEFAULT for both
MIN values.

cheers

From 7c42636d5df21203977900d283c722116f06310c Mon Sep 17 00:00:00 2001
From: Michael Ellerman <mpe@ellerman.id.au>
Date: Thu, 26 Nov 2015 17:40:00 +1100
Subject: [PATCH] powerpc/mm: Use ARCH_MMCAP_RND_BITS

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
 arch/powerpc/Kconfig   | 32 ++++++++++++++++++++++++++++++++
 arch/powerpc/mm/mmap.c | 12 +++++++-----
 2 files changed, 39 insertions(+), 5 deletions(-)

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index db49e0d796b1..e796d6c4055c 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -27,6 +27,36 @@ config MMU
 	bool
 	default y
 
+config ARCH_MMAP_RND_BITS_MIN
+	# On 64-bit up to 1G of address space (2^30)
+	default 12 if 64BIT && PPC_256K_PAGES 	# 256K (2^18), = 30 - 18 = 12
+	default 14 if 64BIT && PPC_64K_PAGES  	# 64K  (2^16), = 30 - 16 = 14
+	default 16 if 64BIT && PPC_16K_PAGES  	# 16K  (2^14), = 30 - 14 = 16
+	default 18 if 64BIT			# 4K   (2^12), = 30 - 12 = 18
+	default ARCH_MMAP_RND_COMPAT_BITS_MIN
+
+config ARCH_MMAP_RND_BITS_MAX
+	# On 64-bit up to 32T of address space (2^45)
+	default 27 if 64BIT && PPC_256K_PAGES   # 256K (2^18), = 45 - 18 = 27
+	default 29 if 64BIT && PPC_64K_PAGES    # 64K  (2^16), = 45 - 16 = 29
+	default 31 if 64BIT && PPC_16K_PAGES    # 16K  (2^14), = 45 - 14 = 31
+	default 33 if 64BIT			# 4K   (2^12), = 45 - 12 = 33
+	default ARCH_MMAP_RND_COMPAT_BITS_MAX
+
+config ARCH_MMAP_RND_COMPAT_BITS_MIN
+	# Up to 8MB of address space (2^23)
+	default 5 if PPC_256K_PAGES             # 256K (2^18), = 23 - 18 = 5
+	default 7 if PPC_64K_PAGES              # 64K  (2^16), = 23 - 16 = 7
+	default 9 if PPC_16K_PAGES              # 16K  (2^14), = 23 - 14 = 9
+	default 11				# 4K   (2^12), = 23 - 12 = 11
+
+config ARCH_MMAP_RND_COMPAT_BITS_MAX
+	# Up to 2G of address space (2^31)
+	default 13 if PPC_256K_PAGES            # 256K (2^18), = 31 - 18 = 13
+	default 15 if PPC_64K_PAGES             # 64K  (2^16), = 31 - 16 = 15
+	default 17 if PPC_16K_PAGES             # 16K  (2^14), = 31 - 14 = 17
+	default 19				# 4K   (2^12), = 31 - 12 = 19
+
 config HAVE_SETUP_PER_CPU_AREA
 	def_bool PPC64
 
@@ -160,6 +190,8 @@ config PPC
 	select EDAC_ATOMIC_SCRUB
 	select ARCH_HAS_DMA_SET_COHERENT_MASK
 	select HAVE_ARCH_SECCOMP_FILTER
+	select HAVE_ARCH_MMAP_RND_BITS
+	select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
 
 config GENERIC_CSUM
 	def_bool CPU_LITTLE_ENDIAN
diff --git a/arch/powerpc/mm/mmap.c b/arch/powerpc/mm/mmap.c
index 0f0502e12f6c..269f7bcd2702 100644
--- a/arch/powerpc/mm/mmap.c
+++ b/arch/powerpc/mm/mmap.c
@@ -55,13 +55,15 @@ static inline int mmap_is_legacy(void)
 
 unsigned long arch_mmap_rnd(void)
 {
-	unsigned long rnd;
+	unsigned long shift, rnd;
 
-	/* 8MB for 32bit, 1GB for 64bit */
+	shift = mmap_rnd_bits;
+#ifdef CONFIG_COMPAT
 	if (is_32bit_task())
-		rnd = (unsigned long)get_random_int() % (1<<(23-PAGE_SHIFT));
-	else
-		rnd = (unsigned long)get_random_int() % (1<<(30-PAGE_SHIFT));
+		shift = mmap_rnd_compat_bits;
+#endif
+
+	rnd = (unsigned long)get_random_int() % (1 << shift);
 
 	return rnd << PAGE_SHIFT;
 }
-- 
2.5.0


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