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

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Date:   Tue, 16 Apr 2019 16:04:07 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Ali Saidi <alisaidi@...zon.com>, Michal Hocko <mhocko@...e.com>,
        Matthew Wilcox <willy@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jann Horn <jannh@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] binfmt_elf: Move brk out of mmap when doing direct
 loader exec

On Mon, 15 Apr 2019 21:23:20 -0700 Kees Cook <keescook@...omium.org> 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 this direct loader case (and ET_EXEC 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 above 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 execed normally.
> 
> Reported-by: Ali Saidi <alisaidi@...zon.com>
> Link: https://lkml.kernel.org/r/CAGXu5jJ5sj3emOT2QPxQkNQk0qbU6zEfu9=Omfhx_p0nCKPSjA@mail.gmail.com
> Fixes: eab09532d400 ("binfmt_elf: use ELF_ET_DYN_BASE only for PIE")
> Signed-off-by: Kees Cook <keescook@...omium.org>

No cc:stable?