Re: [PATCH v2] binfmt_elf: Move brk out of mmap when doing direct loader exec

[Guenter Roeck]

On 4/22/19 3:57 PM, Kees Cook wrote:
> Commit eab09532d400 ("binfmt_elf: use ELF_ET_DYN_BASE only for PIE"),
> made changes in the rare case when the ELF loader was directly invoked
> (e.g to set a non-inheritable LD_LIBRARY_PATH, testing new versions of
> the loader), by moving into the mmap region to avoid both ET_EXEC and PIE
> binaries. This had the effect of also moving the brk region into mmap,
> which could lead to the stack and brk being arbitrarily close to each
> other. An unlucky process wouldn't get its requested stack size and stack
> allocations could end up scribbling on the heap.
>
> This is illustrated here. In the case of using the loader directly, brk
> (so helpfully identified as "[heap]") is allocated with the _loader_
> not the binary. For example, with ASLR entirely disabled, you can see
> this more clearly:
>
> $ /bin/cat /proc/self/maps
> 555555554000-55555555c000 r-xp 00000000 ... /bin/cat
> 55555575b000-55555575c000 r--p 00007000 ... /bin/cat
> 55555575c000-55555575d000 rw-p 00008000 ... /bin/cat
> 55555575d000-55555577e000 rw-p 00000000 ... [heap]
> ...
> 7ffff7ff7000-7ffff7ffa000 r--p 00000000 ... [vvar]
> 7ffff7ffa000-7ffff7ffc000 r-xp 00000000 ... [vdso]
> 7ffff7ffc000-7ffff7ffd000 r--p 00027000 ... /lib/x86_64-linux-gnu/ld-2.27.so
> 7ffff7ffd000-7ffff7ffe000 rw-p 00028000 ... /lib/x86_64-linux-gnu/ld-2.27.so
> 7ffff7ffe000-7ffff7fff000 rw-p 00000000 ...
> 7ffffffde000-7ffffffff000 rw-p 00000000 ... [stack]
>
> $ /lib/x86_64-linux-gnu/ld-2.27.so /bin/cat /proc/self/maps
> ...
> 7ffff7bcc000-7ffff7bd4000 r-xp 00000000 ... /bin/cat
> 7ffff7bd4000-7ffff7dd3000 ---p 00008000 ... /bin/cat
> 7ffff7dd3000-7ffff7dd4000 r--p 00007000 ... /bin/cat
> 7ffff7dd4000-7ffff7dd5000 rw-p 00008000 ... /bin/cat
> 7ffff7dd5000-7ffff7dfc000 r-xp 00000000 ... /lib/x86_64-linux-gnu/ld-2.27.so
> 7ffff7fb2000-7ffff7fd6000 rw-p 00000000 ...
> 7ffff7ff7000-7ffff7ffa000 r--p 00000000 ... [vvar]
> 7ffff7ffa000-7ffff7ffc000 r-xp 00000000 ... [vdso]
> 7ffff7ffc000-7ffff7ffd000 r--p 00027000 ... /lib/x86_64-linux-gnu/ld-2.27.so
> 7ffff7ffd000-7ffff7ffe000 rw-p 00028000 ... /lib/x86_64-linux-gnu/ld-2.27.so
> 7ffff7ffe000-7ffff8020000 rw-p 00000000 ... [heap]
> 7ffffffde000-7ffffffff000 rw-p 00000000 ... [stack]
>
> The solution is to move brk out of mmap and into ELF_ET_DYN_BASE since
> nothing is there in the direct loader case (and ET_EXEC is still far
> away at 0x400000). Anything that ran before should still work (i.e. the
> ultimately-launched binary already had the brk very far from its text, so
> this should be no different from a COMPAT_BRK standpoint). The only risk
> I see here is that if someone started to suddenly depend on the entire
> memory space lower than the mmap region being available when launching
> binaries via a direct loader execs which seems highly unlikely, I'd hope:
> this would mean a binary would _not_ work when exec()ed normally.
>
> (Note that this is only done under CONFIG_ARCH_HAS_ELF_RANDOMIZATION when
> randomization is turned on.)
>
> Reported-by: Ali Saidi <alisaidi@xxxxxxxxxx>
> Link: https://lkml.kernel.org/r/CAGXu5jJ5sj3emOT2QPxQkNQk0qbU6zEfu9=Omfhx_p0nCKPSjA@xxxxxxxxxxxxxx
> Fixes: eab09532d400 ("binfmt_elf: use ELF_ET_DYN_BASE only for PIE")
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
> v2: limit effect to only architectures that are expecting it! (Gunter)

This patch applies cleanly on top of next-20190418, and the crashes
observed with my xtensa boot tests no longer occur. I didn't test
any other architectures.

I don't know the base of Andrew's modification - the code snipped he says
isn't there anymore is still present in next-20190418.

Guenter