Re: [PATCH 4/5] mm: split ET_DYN ASLR from mmap ASLR

[Kees Cook]

On Tue, Mar 3, 2015 at 8:16 PM, Michael Ellerman <mpe@xxxxxxxxxxxxxx> wrote:
> On Mon, 2015-03-02 at 16:19 -0800, Kees Cook wrote:
>> This fixes the "offset2lib" weakness in ASLR for arm, arm64, mips,
>> powerpc, and x86. The problem is that if there is a leak of ASLR from
>> the executable (ET_DYN), it means a leak of shared library offset as
>> well (mmap), and vice versa. Further details and a PoC of this attack
>> are available here:
>> http://cybersecurity.upv.es/attacks/offset2lib/offset2lib.html
>>
>> With this patch, a PIE linked executable (ET_DYN) has its own ASLR region:
>>
>> $ ./show_mmaps_pie
>> 54859ccd6000-54859ccd7000 r-xp  ...  /tmp/show_mmaps_pie
>> 54859ced6000-54859ced7000 r--p  ...  /tmp/show_mmaps_pie
>> 54859ced7000-54859ced8000 rw-p  ...  /tmp/show_mmaps_pie
>
> Just to be clear, it's the fact that the above vmas are in a different
> address range to those below that shows the patch is working, right?

That's correct, yes. I've called this out explicitly now in the 9/10
patch in v4.

>
>> 7f75be764000-7f75be91f000 r-xp  ...  /lib/x86_64-linux-gnu/libc.so.6
>> 7f75be91f000-7f75beb1f000 ---p  ...  /lib/x86_64-linux-gnu/libc.so.6
>
>
> On powerpc I'm seeing:
>
> # /bin/dash
> # cat /proc/$$/maps
> 524e0000-52510000 r-xp 00000000 08:03 129814                             /bin/dash
> 52510000-52520000 rw-p 00020000 08:03 129814                             /bin/dash
> 10034f20000-10034f50000 rw-p 00000000 00:00 0                            [heap]
> 3fffaeaf0000-3fffaeca0000 r-xp 00000000 08:03 13529                      /lib/powerpc64le-linux-gnu/libc-2.19.so
> 3fffaeca0000-3fffaecb0000 rw-p 001a0000 08:03 13529                      /lib/powerpc64le-linux-gnu/libc-2.19.so
> 3fffaecc0000-3fffaecd0000 rw-p 00000000 00:00 0
> 3fffaecd0000-3fffaecf0000 r-xp 00000000 00:00 0                          [vdso]
> 3fffaecf0000-3fffaed20000 r-xp 00000000 08:03 13539                      /lib/powerpc64le-linux-gnu/ld-2.19.so
> 3fffaed20000-3fffaed30000 rw-p 00020000 08:03 13539                      /lib/powerpc64le-linux-gnu/ld-2.19.so
> 3fffc7070000-3fffc70a0000 rw-p 00000000 00:00 0                          [stack]
>
>
> Whereas previously the /bin/dash vmas were up at 3fff..

Fantastic! Thanks very much for testing!

>
> So looks good to me for powerpc.
>
> Acked-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>

I had a question in the powerpc-specific change that may have gone unnoticed:

Can mmap ASLR be safely enabled in the legacy mmap case here? Other archs
use "mm->mmap_base = TASK_UNMAPPED_BASE + random_factor".

Separate from this series, do you happen to know if this improvement
can be made, or if the legacy mmap on powerpc can't handle this?

Thanks!

-Kees

-- 
Kees Cook
Chrome OS Security
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