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

[Andy Lutomirski]

On 02/14/2015 09:33 AM, 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

Awesome.  So the vdso randomization *and* the stack randomization 
implementations were buggy.  Anyone want to check the mmap and brk 
randomization implementations?

--Andy
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