[PATCH] arm: fix integer overflow in ELF_ET_DYN_BASE

[Kees Cook]

On Fri, Mar 20, 2015 at 4:12 AM, Andrey Ryabinin <a.ryabinin at samsung.com> wrote:
> Usually ELF_ET_DYN_BASE is 2/3 of TASK_SIZE. With 3G/1G user/kernel
> split this is not so, because 2*TASK_SIZE overflows 32 bits,
> so the actual value of ELF_ET_DYN_BASE is:
>         (2 * TASK_SIZE / 3) = 0x2a000000
>
> When ASLR is disabled PIE binaries will load at ELF_ET_DYN_BASE address.
> On 32bit platforms AddressSanitzer uses addresses [0x20000000 - 0x40000000]
> for shadow memory [1]. So ASan doesn't work for PIE binaries when ASLR disabled
> as it fails to map shadow memory.
> Also after Kees's 'split ET_DYN ASLR from mmap ASLR' patchset PIE binaries
> has a high chance of loading somewhere in between [0x2a000000 - 0x40000000]
> even if ASLR enabled. This makes ASan with PIE absolutely incompatible.
>
> Fix overflow by dividing TASK_SIZE prior to multiplying.
> After this patch ELF_ET_DYN_BASE equals to (for CONFIG_VMSPLIT_3G=y):
>         (TASK_SIZE / 3 * 2) = 0x7f555554
>
> [1] https://code.google.com/p/address-sanitizer/wiki/AddressSanitizerAlgorithm#Mapping
>
> Signed-off-by: Andrey Ryabinin <a.ryabinin at samsung.com>
> Reported-by: Maria Guseva <m.guseva at samsung.com>
> Cc: stable at vger.kernel.org
> ---
>  arch/arm/include/asm/elf.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/arm/include/asm/elf.h b/arch/arm/include/asm/elf.h
> index c1ff8ab..1984a92 100644
> --- a/arch/arm/include/asm/elf.h
> +++ b/arch/arm/include/asm/elf.h
> @@ -115,7 +115,7 @@ int dump_task_regs(struct task_struct *t, elf_gregset_t *elfregs);
>     the loader.  We need to make sure that it is out of the way of the program
>     that it will "exec", and that there is sufficient room for the brk.  */
>
> -#define ELF_ET_DYN_BASE        (2 * TASK_SIZE / 3)
> +#define ELF_ET_DYN_BASE        (TASK_SIZE / 3 * 2)
>
>  /* When the program starts, a1 contains a pointer to a function to be
>     registered with atexit, as per the SVR4 ABI.  A value of 0 means we

Acked-by: Kees Cook <keescook at chromium.org>

Though, yes, as mentioned in the other replies, please send fixes for
the other architectures too.

Thanks for catching this!

-Kees

-- 
Kees Cook
Chrome OS Security