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 18:14:00 -0500
From:   Kees Cook <keescook@...omium.org>
To:     Andrew Morton <akpm@...ux-foundation.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>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] binfmt_elf: Move brk out of mmap when doing direct loader exec

On Tue, Apr 16, 2019 at 6:04 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> 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?

Probably it should be, yes. I think I'm just shy about that when
poking ELF mappings. :)

-- 
Kees Cook