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[PATCH v6 1/4] mm: mmap: Add new /proc tunable for mmap_base ASLR.

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  [PATCH v6 1/4] mm: mmap: Add new /proc tunable for mmap_base ASLR. Daniel Cashman <dcashman@android.com> - 2015-12-11 19:00 +0100
    [PATCH v6 4/4] x86: mm: support ARCH_MMAP_RND_BITS. Daniel Cashman <dcashman@android.com> - 2015-12-11 19:00 +0100
      Re: [PATCH v6 4/4] x86: mm: support ARCH_MMAP_RND_BITS. "H. Peter Anvin" <hpa@zytor.com> - 2015-12-14 20:10 +0100
        Re: [PATCH v6 4/4] x86: mm: support ARCH_MMAP_RND_BITS. Daniel Cashman <dcashman@android.com> - 2015-12-14 22:00 +0100
    [PATCH v6 2/4] arm: mm: support ARCH_MMAP_RND_BITS. Daniel Cashman <dcashman@android.com> - 2015-12-11 19:10 +0100
      [PATCH v6 3/4] arm64: mm: support ARCH_MMAP_RND_BITS. Daniel Cashman <dcashman@android.com> - 2015-12-11 19:10 +0100
        Re: [PATCH v6 3/4] arm64: mm: support ARCH_MMAP_RND_BITS. Will Deacon <will.deacon@arm.com> - 2015-12-14 12:20 +0100
          Re: [PATCH v6 3/4] arm64: mm: support ARCH_MMAP_RND_BITS. Daniel Cashman <dcashman@android.com> - 2015-12-14 21:50 +0100

#1289775 — [PATCH v6 1/4] mm: mmap: Add new /proc tunable for mmap_base ASLR.

From: dcashman <dcashman@google.com>

ASLR  only uses as few as 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@android.com>
---
 Documentation/sysctl/vm.txt | 29 +++++++++++++++++++
 arch/Kconfig                | 68 +++++++++++++++++++++++++++++++++++++++++++++
 include/linux/mm.h          | 11 ++++++++
 kernel/sysctl.c             | 22 +++++++++++++++
 mm/mmap.c                   | 12 ++++++++
 5 files changed, 142 insertions(+)

diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
index f72370b..ee763f3 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/vm/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/vm/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..ba1b626 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -511,6 +511,74 @@ 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, has MMU enabled 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/vm/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, has MMU
+	  enabled 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_DEFAULT
+	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_DEFAULT if ARCH_MMAP_RND_COMPAT_BITS_DEFAULT
+	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/vm/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..6f6dd6e 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 const int mmap_rnd_bits_min;
+extern const int mmap_rnd_bits_max;
+extern int mmap_rnd_bits __read_mostly;
+#endif
+#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
+extern const int mmap_rnd_compat_bits_min;
+extern const int mmap_rnd_compat_bits_max;
+extern int mmap_rnd_compat_bits __read_mostly;
+#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..d5e7e24 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		= 0600,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= (void *)&mmap_rnd_bits_min,
+		.extra2		= (void *)&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		= 0600,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= (void *)&mmap_rnd_compat_bits_min,
+		.extra2		= (void *)&mmap_rnd_compat_bits_max,
+	},
+#endif
 	{ }
 };
 
diff --git a/mm/mmap.c b/mm/mmap.c
index 2ce04a6..fe3816c 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
+const int mmap_rnd_bits_min = CONFIG_ARCH_MMAP_RND_BITS_MIN;
+const int mmap_rnd_bits_max = CONFIG_ARCH_MMAP_RND_BITS_MAX;
+int mmap_rnd_bits __read_mostly = CONFIG_ARCH_MMAP_RND_BITS;
+#endif
+#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
+const int mmap_rnd_compat_bits_min = CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MIN;
+const int mmap_rnd_compat_bits_max = CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MAX;
+int mmap_rnd_compat_bits __read_mostly = 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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#1289778 — [PATCH v6 4/4] x86: mm: support ARCH_MMAP_RND_BITS.

From: dcashman <dcashman@google.com>

x86: arch_mmap_rnd() uses hard-coded values, 8 for 32-bit and 28 for
64-bit, to generate the random offset for the mmap base address.
This value represents a compromise between increased ASLR
effectiveness and avoiding address-space fragmentation. Replace it
with a Kconfig option, which is sensibly bounded, so that platform
developers may choose where to place this compromise. Keep default
values as new minimums.

Signed-off-by: Daniel Cashman <dcashman@android.com>
---
 arch/x86/Kconfig   | 16 ++++++++++++++++
 arch/x86/mm/mmap.c | 12 ++++++------
 2 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index db3622f..bdc67d2 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -82,6 +82,8 @@ config X86
 	select HAVE_ARCH_KASAN			if X86_64 && SPARSEMEM_VMEMMAP
 	select HAVE_ARCH_KGDB
 	select HAVE_ARCH_KMEMCHECK
+	select HAVE_ARCH_MMAP_RND_BITS		if MMU
+	select HAVE_ARCH_MMAP_RND_COMPAT_BITS	if MMU && COMPAT
 	select HAVE_ARCH_SECCOMP_FILTER
 	select HAVE_ARCH_SOFT_DIRTY		if X86_64
 	select HAVE_ARCH_TRACEHOOK
@@ -183,6 +185,20 @@ config HAVE_LATENCYTOP_SUPPORT
 config MMU
 	def_bool y
 
+config ARCH_MMAP_RND_BITS_MIN
+       default 28 if 64BIT
+       default 8
+
+config ARCH_MMAP_RND_BITS_MAX
+	default 32 if 64BIT
+	default 16
+
+config ARCH_MMAP_RND_COMPAT_BITS_MIN
+	default 8
+
+config ARCH_MMAP_RND_COMPAT_BITS_MAX
+	default 16
+
 config SBUS
 	bool
 
diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
index 844b06d..647fecf 100644
--- a/arch/x86/mm/mmap.c
+++ b/arch/x86/mm/mmap.c
@@ -69,14 +69,14 @@ unsigned long arch_mmap_rnd(void)
 {
 	unsigned long rnd;
 
-	/*
-	 *  8 bits of randomness in 32bit mmaps, 20 address space bits
-	 * 28 bits of randomness in 64bit mmaps, 40 address space bits
-	 */
 	if (mmap_is_ia32())
-		rnd = (unsigned long)get_random_int() % (1<<8);
+#ifdef CONFIG_COMPAT
+		rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_compat_bits);
+#else
+		rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
+#endif
 	else
-		rnd = (unsigned long)get_random_int() % (1<<28);
+		rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
 
 	return rnd << PAGE_SHIFT;
 }
-- 
2.6.0.rc2.230.g3dd15c0

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#1291470 — Re: [PATCH v6 4/4] x86: mm: support ARCH_MMAP_RND_BITS.

On 12/11/15 09:52, Daniel Cashman wrote:
> From: dcashman <dcashman@google.com>
> 
> x86: arch_mmap_rnd() uses hard-coded values, 8 for 32-bit and 28 for
> 64-bit, to generate the random offset for the mmap base address.
> This value represents a compromise between increased ASLR
> effectiveness and avoiding address-space fragmentation. Replace it
> with a Kconfig option, which is sensibly bounded, so that platform
> developers may choose where to place this compromise. Keep default
> values as new minimums.
> 
> Signed-off-by: Daniel Cashman <dcashman@android.com>

OK, this is around the time when I make a lecture about the danger of
expecting the compiler to make certain transformations:

> diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
> index 844b06d..647fecf 100644
> --- a/arch/x86/mm/mmap.c
> +++ b/arch/x86/mm/mmap.c
> @@ -69,14 +69,14 @@ unsigned long arch_mmap_rnd(void)
>  {
>  	unsigned long rnd;
>  
> -	/*
> -	 *  8 bits of randomness in 32bit mmaps, 20 address space bits
> -	 * 28 bits of randomness in 64bit mmaps, 40 address space bits
> -	 */
>  	if (mmap_is_ia32())
> -		rnd = (unsigned long)get_random_int() % (1<<8);
> +#ifdef CONFIG_COMPAT
> +		rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_compat_bits);
> +#else
> +		rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
> +#endif
>  	else
> -		rnd = (unsigned long)get_random_int() % (1<<28);
> +		rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
>  
>  	return rnd << PAGE_SHIFT;
>  }
> 

Now, you and I know that both variants can be implemented with a simple
AND, but I have a strong suspicion that once this is turned into a
variable, this will in fact be changed from an AND to a divide.

So I'd prefer to use the
"get_random_int() & ((1UL << mmap_rnd_bits) - 1)" construct instead.

	-hpa

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#1291550 — Re: [PATCH v6 4/4] x86: mm: support ARCH_MMAP_RND_BITS.

On 12/14/2015 10:58 AM, H. Peter Anvin wrote:
> On 12/11/15 09:52, Daniel Cashman wrote:
>> diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
>> index 844b06d..647fecf 100644
>> --- a/arch/x86/mm/mmap.c
>> +++ b/arch/x86/mm/mmap.c
>> @@ -69,14 +69,14 @@ unsigned long arch_mmap_rnd(void)
>>  {
>>  	unsigned long rnd;
>>  
>> -	/*
>> -	 *  8 bits of randomness in 32bit mmaps, 20 address space bits
>> -	 * 28 bits of randomness in 64bit mmaps, 40 address space bits
>> -	 */
>>  	if (mmap_is_ia32())
>> -		rnd = (unsigned long)get_random_int() % (1<<8);
>> +#ifdef CONFIG_COMPAT
>> +		rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_compat_bits);
>> +#else
>> +		rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
>> +#endif
>>  	else
>> -		rnd = (unsigned long)get_random_int() % (1<<28);
>> +		rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
>>  
>>  	return rnd << PAGE_SHIFT;
>>  }
>>
> 
> Now, you and I know that both variants can be implemented with a simple
> AND, but I have a strong suspicion that once this is turned into a
> variable, this will in fact be changed from an AND to a divide.
> 
> So I'd prefer to use the
> "get_random_int() & ((1UL << mmap_rnd_bits) - 1)" construct instead.

Good point.  Will change in v7 across patch-set.

Thank You,
Dan

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#1289791 — [PATCH v6 2/4] arm: mm: support ARCH_MMAP_RND_BITS.

From: dcashman <dcashman@google.com>

arm: arch_mmap_rnd() uses a hard-code value of 8 to generate the
random offset for the mmap base address.  This value represents a
compromise between increased ASLR effectiveness and avoiding
address-space fragmentation. Replace it with a Kconfig option, which
is sensibly bounded, so that platform developers may choose where to
place this compromise. Keep 8 as the minimum acceptable value.

Signed-off-by: Daniel Cashman <dcashman@android.com>
---
 arch/arm/Kconfig   | 9 +++++++++
 arch/arm/mm/mmap.c | 3 +--
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 34e1569..a1b8ca1 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -35,6 +35,7 @@ config ARM
 	select HAVE_ARCH_BITREVERSE if (CPU_32v7M || CPU_32v7) && !CPU_32v6
 	select HAVE_ARCH_JUMP_LABEL if !XIP_KERNEL && !CPU_ENDIAN_BE32
 	select HAVE_ARCH_KGDB if !CPU_ENDIAN_BE32
+	select HAVE_ARCH_MMAP_RND_BITS if MMU
 	select HAVE_ARCH_SECCOMP_FILTER if (AEABI && !OABI_COMPAT)
 	select HAVE_ARCH_TRACEHOOK
 	select HAVE_BPF_JIT
@@ -308,6 +309,14 @@ config MMU
 	  Select if you want MMU-based virtualised addressing space
 	  support by paged memory management. If unsure, say 'Y'.
 
+config ARCH_MMAP_RND_BITS_MIN
+	default 8
+
+config ARCH_MMAP_RND_BITS_MAX
+	default 14 if PAGE_OFFSET=0x40000000
+	default 15 if PAGE_OFFSET=0x80000000
+	default 16
+
 #
 # The "ARM system type" choice list is ordered alphabetically by option
 # text.  Please add new entries in the option alphabetic order.
diff --git a/arch/arm/mm/mmap.c b/arch/arm/mm/mmap.c
index 407dc78..c938693 100644
--- a/arch/arm/mm/mmap.c
+++ b/arch/arm/mm/mmap.c
@@ -173,8 +173,7 @@ unsigned long arch_mmap_rnd(void)
 {
 	unsigned long rnd;
 
-	/* 8 bits of randomness in 20 address space bits */
-	rnd = (unsigned long)get_random_int() % (1 << 8);
+	rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
 
 	return rnd << PAGE_SHIFT;
 }
-- 
2.6.0.rc2.230.g3dd15c0

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#1289794 — [PATCH v6 3/4] arm64: mm: support ARCH_MMAP_RND_BITS.

From: dcashman <dcashman@google.com>

arm64: arch_mmap_rnd() uses STACK_RND_MASK to generate the
random offset for the mmap base address.  This value represents a
compromise between increased ASLR effectiveness and avoiding
address-space fragmentation. Replace it with a Kconfig option, which
is sensibly bounded, so that platform developers may choose where to
place this compromise. Keep default values as new minimums.

Signed-off-by: Daniel Cashman <dcashman@android.com>
---
 arch/arm64/Kconfig   | 33 +++++++++++++++++++++++++++++++++
 arch/arm64/mm/mmap.c |  8 ++++++--
 2 files changed, 39 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 871f217..0cc9c24 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -51,6 +51,8 @@ config ARM64
 	select HAVE_ARCH_JUMP_LABEL
 	select HAVE_ARCH_KASAN if SPARSEMEM_VMEMMAP && !(ARM64_16K_PAGES && ARM64_VA_BITS_48)
 	select HAVE_ARCH_KGDB
+	select HAVE_ARCH_MMAP_RND_BITS
+	select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
 	select HAVE_ARCH_SECCOMP_FILTER
 	select HAVE_ARCH_TRACEHOOK
 	select HAVE_BPF_JIT
@@ -104,6 +106,37 @@ config ARCH_PHYS_ADDR_T_64BIT
 config MMU
 	def_bool y
 
+config ARCH_MMAP_RND_BITS_MIN
+       default 14 if ARM64_64K_PAGES
+       default 16 if ARM64_16K_PAGES
+       default 18
+
+# max bits determined by the following formula:
+#  VA_BITS - PAGE_SHIFT - 3
+config ARCH_MMAP_RND_BITS_MAX
+       default 19 if ARM64_VA_BITS=36
+       default 20 if ARM64_64K_PAGES && ARM64_VA_BITS=39
+       default 22 if ARM64_16K_PAGES && ARM64_VA_BITS=39
+       default 24 if ARM64_VA_BITS=39
+       default 23 if ARM64_64K_PAGES && ARM64_VA_BITS=42
+       default 25 if ARM64_16K_PAGES && ARM64_VA_BITS=42
+       default 27 if ARM64_VA_BITS=42
+       default 30 if ARM64_VA_BITS=47
+       default 29 if ARM64_64K_PAGES && ARM64_VA_BITS=48
+       default 31 if ARM64_16K_PAGES && ARM64_VA_BITS=48
+       default 33 if ARM64_VA_BITS=48
+       default 15 if ARM64_64K_PAGES
+       default 17 if ARM64_16K_PAGES
+       default 18
+
+config ARCH_MMAP_RND_COMPAT_BITS_MIN
+       default 7 if ARM64_64K_PAGES
+       default 9 if ARM64_16K_PAGES
+       default 11
+
+config ARCH_MMAP_RND_COMPAT_BITS_MAX
+       default 16
+
 config NO_IOPORT_MAP
 	def_bool y if !PCI
 
diff --git a/arch/arm64/mm/mmap.c b/arch/arm64/mm/mmap.c
index ed17747..e59a75a 100644
--- a/arch/arm64/mm/mmap.c
+++ b/arch/arm64/mm/mmap.c
@@ -51,8 +51,12 @@ unsigned long arch_mmap_rnd(void)
 {
 	unsigned long rnd;
 
-	rnd = (unsigned long)get_random_int() & STACK_RND_MASK;
-
+#ifdef CONFIG_COMPAT
+	if (test_thread_flag(TIF_32BIT))
+		rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_compat_bits);
+	else
+#endif
+		rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
 	return rnd << PAGE_SHIFT;
 }
 
-- 
2.6.0.rc2.230.g3dd15c0

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#1291117 — Re: [PATCH v6 3/4] arm64: mm: support ARCH_MMAP_RND_BITS.

Hi Daniel,

On Fri, Dec 11, 2015 at 09:52:17AM -0800, Daniel Cashman wrote:
> From: dcashman <dcashman@google.com>
> 
> arm64: arch_mmap_rnd() uses STACK_RND_MASK to generate the
> random offset for the mmap base address.  This value represents a
> compromise between increased ASLR effectiveness and avoiding
> address-space fragmentation. Replace it with a Kconfig option, which
> is sensibly bounded, so that platform developers may choose where to
> place this compromise. Keep default values as new minimums.
> 
> Signed-off-by: Daniel Cashman <dcashman@android.com>
> ---
>  arch/arm64/Kconfig   | 33 +++++++++++++++++++++++++++++++++
>  arch/arm64/mm/mmap.c |  8 ++++++--
>  2 files changed, 39 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 871f217..0cc9c24 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -51,6 +51,8 @@ config ARM64
>  	select HAVE_ARCH_JUMP_LABEL
>  	select HAVE_ARCH_KASAN if SPARSEMEM_VMEMMAP && !(ARM64_16K_PAGES && ARM64_VA_BITS_48)
>  	select HAVE_ARCH_KGDB
> +	select HAVE_ARCH_MMAP_RND_BITS
> +	select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
>  	select HAVE_ARCH_SECCOMP_FILTER
>  	select HAVE_ARCH_TRACEHOOK
>  	select HAVE_BPF_JIT
> @@ -104,6 +106,37 @@ config ARCH_PHYS_ADDR_T_64BIT
>  config MMU
>  	def_bool y
>  
> +config ARCH_MMAP_RND_BITS_MIN
> +       default 14 if ARM64_64K_PAGES
> +       default 16 if ARM64_16K_PAGES
> +       default 18
> +
> +# max bits determined by the following formula:
> +#  VA_BITS - PAGE_SHIFT - 3

Now that we have this comment, I think we can drop the unsupported
combinations from the list below. That means we just end up with:

> +config ARCH_MMAP_RND_BITS_MAX
> +       default 19 if ARM64_VA_BITS=36
> +       default 24 if ARM64_VA_BITS=39
> +       default 27 if ARM64_VA_BITS=42
> +       default 30 if ARM64_VA_BITS=47
> +       default 29 if ARM64_VA_BITS=48 && ARM64_64K_PAGES
> +       default 31 if ARM64_VA_BITS=48 && ARM64_16K_PAGES
> +       default 33 if ARM64_VA_BITS=48

With that:

  Acked-by: Will Deacon <will.deacon@arm.com>

Will
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#1291545 — Re: [PATCH v6 3/4] arm64: mm: support ARCH_MMAP_RND_BITS.

On 12/14/2015 03:19 AM, Will Deacon wrote:
>> +# max bits determined by the following formula:
>> +#  VA_BITS - PAGE_SHIFT - 3
> 
> Now that we have this comment, I think we can drop the unsupported
> combinations from the list below. That means we just end up with:
> 
>> +config ARCH_MMAP_RND_BITS_MAX
>> +       default 19 if ARM64_VA_BITS=36
>> +       default 24 if ARM64_VA_BITS=39
>> +       default 27 if ARM64_VA_BITS=42
>> +       default 30 if ARM64_VA_BITS=47
>> +       default 29 if ARM64_VA_BITS=48 && ARM64_64K_PAGES
>> +       default 31 if ARM64_VA_BITS=48 && ARM64_16K_PAGES
>> +       default 33 if ARM64_VA_BITS=48

Unless you object, I'd like to keep the last 3 as well, to mirror the
min bits, should any new configurations be added but not reflected here:
+       default 15 if ARM64_64K_PAGES
+       default 17 if ARM64_16K_PAGES
+       default 18

The first two of these three should be changed as well to 14 and 16.

Thanks,
Dan
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