From: Guenter Roeck <firstname.lastname@example.org> To: Kees Cook <email@example.com>, Andrew Morton <firstname.lastname@example.org> Cc: Ali Saidi <email@example.com>, Michal Hocko <firstname.lastname@example.org>, Matthew Wilcox <email@example.com>, Thomas Gleixner <firstname.lastname@example.org>, Jann Horn <email@example.com>, firstname.lastname@example.org Subject: Re: [PATCH v2] binfmt_elf: Move brk out of mmap when doing direct loader exec Date: Mon, 22 Apr 2019 18:23:46 -0700 [thread overview] Message-ID: <email@example.com> (raw) In-Reply-To: <20190422225727.GA21011@beast> 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 <firstname.lastname@example.org> > 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 <email@example.com> > --- > 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
prev parent reply other threads:[~2019-04-23 1:23 UTC|newest] Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-04-22 22:57 Kees Cook 2019-04-22 23:24 ` Andrew Morton 2019-04-23 1:23 ` Guenter Roeck [this message]
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