Re: [PATCH] ASLR: fix stack randomization on 64-bit systems

[Borislav Petkov]

On Sat, Feb 14, 2015 at 09:33:50AM -0800, Kees Cook wrote:
> From: Hector Marco-Gisbert <hecmargi@xxxxxx>
> 
> The issue is that the stack for processes is not properly randomized on 64 bit
> architectures due to an integer overflow.
> 
> The affected function is randomize_stack_top() in file "fs/binfmt_elf.c":
> 
> static unsigned long randomize_stack_top(unsigned long stack_top)
> {
>          unsigned int random_variable = 0;
> 
>          if ((current->flags & PF_RANDOMIZE) &&
>                  !(current->personality & ADDR_NO_RANDOMIZE)) {
>                  random_variable = get_random_int() & STACK_RND_MASK;
>                  random_variable <<= PAGE_SHIFT;
>          }
>          return PAGE_ALIGN(stack_top) + random_variable;
>          return PAGE_ALIGN(stack_top) - random_variable;
> }
> 
> Note that, it declares the "random_variable" variable as "unsigned int". Since
> the result of the shifting operation between STACK_RND_MASK (which is
> 0x3fffff on x86_64, 22 bits) and PAGE_SHIFT (which is 12 on x86_64):
> 
> random_variable <<= PAGE_SHIFT;
> 
> then the two leftmost bits are dropped when storing the result in the
> "random_variable". This variable shall be at least 34 bits long to hold the
> (22+12) result.
> 
> These two dropped bits have an impact on the entropy of process stack.
> Concretely, the total stack entropy is reduced by four: from 2^28 to 2^30 (One
> fourth of expected entropy).
> 
> This patch restores back the entropy by correcting the types involved in the
> operations in the functions randomize_stack_top() and stack_maxrandom_size().
> 
> The successful fix can be tested with:
> $ for i in `seq 1 10`; do cat /proc/self/maps | grep stack; done
> 7ffeda566000-7ffeda587000 rw-p 00000000 00:00 0                          [stack]
> 7fff5a332000-7fff5a353000 rw-p 00000000 00:00 0                          [stack]
> 7ffcdb7a1000-7ffcdb7c2000 rw-p 00000000 00:00 0                          [stack]
> 7ffd5e2c4000-7ffd5e2e5000 rw-p 00000000 00:00 0                          [stack]
> ...
> 
> Once corrected, the leading bytes should be between 7ffc and 7fff, rather
> than always being 7fff.
> 
> CVE-2015-1593
> 
> Signed-off-by: Hector Marco-Gisbert <hecmargi@xxxxxx>
> Signed-off-by: Ismael Ripoll <iripoll@xxxxxx>
> [kees: rebase, fix 80 char, clean up commit message, add test example, cve]
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx

Ok, I'm picking this up. Do scream if someone else wants to do that,
otherwise it is going to tip next week, after the merge window is over.

Thanks.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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