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| Started by | Kees Cook <keescook@chromium.org> |
|---|---|
| First post | 2015-12-01 01:10 +0100 |
| Last post | 2015-12-01 19:20 +0100 |
| Articles | 2 — 2 participants |
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Re: [PATCH v4 4/4] x86: mm: support ARCH_MMAP_RND_BITS. Kees Cook <keescook@chromium.org> - 2015-12-01 01:10 +0100
Re: [PATCH v4 4/4] x86: mm: support ARCH_MMAP_RND_BITS. Daniel Cashman <dcashman@android.com> - 2015-12-01 19:20 +0100
| From | Kees Cook <keescook@chromium.org> |
|---|---|
| Date | 2015-12-01 01:10 +0100 |
| Subject | Re: [PATCH v4 4/4] x86: mm: support ARCH_MMAP_RND_BITS. |
| Message-ID | <qAGG5-2ec-15@gated-at.bofh.it> |
On Thu, Nov 26, 2015 at 2:59 PM, Daniel Cashman <dcashman@android.com> wrote:
> 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..12768c4 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
> + select HAVE_ARCH_MMAP_RND_COMPAT_BITS if 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
>
Can you rework this logic to look more like the arm64 one? I think
it's more readable as:
#ifdef CONFIG_COMPAT
if (mmap_is_ia32())
rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_compat_bits);
else
#endif
rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
-Kees
--
Kees Cook
Chrome OS & Brillo Security
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| From | Daniel Cashman <dcashman@android.com> |
|---|---|
| Date | 2015-12-01 19:20 +0100 |
| Message-ID | <qAXGW-4Gh-15@gated-at.bofh.it> |
| In reply to | #1280415 |
On 11/30/2015 04:03 PM, Kees Cook wrote:
> On Thu, Nov 26, 2015 at 2:59 PM, Daniel Cashman <dcashman@android.com> 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;
>> }
>> --
>> 2.6.0.rc2.230.g3dd15c0
>>
>
> Can you rework this logic to look more like the arm64 one? I think
> it's more readable as:
>
> #ifdef CONFIG_COMPAT
> if (mmap_is_ia32())
> rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_compat_bits);
> else
> #endif
> rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
>
> -Kees
>
There is a subtle difference between the two that requires this
difference. the x86 code was written to be used by both 32-bit and
64-bit kernels, whereas the arm64 code runs only for 64-bit. The
assumption I've made with arm64 is that TIF_32BIT should never be set if
CONFIG_COMPAT is not set, but with x86 we could encounter a 32-bit
application without CONFIG_COMPAT, in which case it should use the
default mmap_rnd_bits, not compat, since there is no compat.
-Dan
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