The patch titled
Subject: fs/binfmt_elf.c: move brk out of mmap when doing direct loader exec
has been added to the -mm tree. Its filename is
binfmt_elf-move-brk-out-of-mmap-when-doing-direct-loader-exec.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/binfmt_elf-move-brk-out-of-mmap-when-doing-direct-loader-exec.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/binfmt_elf-move-brk-out-of-mmap-when-doing-direct-loader-exec.patch
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------------------------------------------------------
From: Kees Cook <keescook@xxxxxxxxxxxx>
Subject: fs/binfmt_elf.c: move brk out of mmap when doing direct loader exec
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.
Link: http://lkml.kernel.org/r/20190416042320.GA36924@beast
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>
Reported-by: Ali Saidi <alisaidi@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
fs/binfmt_elf.c | 9 +++++++++
1 file changed, 9 insertions(+)
--- a/fs/binfmt_elf.c~binfmt_elf-move-brk-out-of-mmap-when-doing-direct-loader-exec
+++ a/fs/binfmt_elf.c
@@ -1131,6 +1131,15 @@ out_free_interp:
current->mm->end_data = end_data;
current->mm->start_stack = bprm->p;
+ /*
+ * When executing a loader directly (ET_DYN without Interp), move
+ * the brk area out of the mmap region (since it grows up, and may
+ * collide early with the stack growing down), and into the unused
+ * ELF_ET_DYN_BASE region.
+ */
+ if (!elf_interpreter)
+ current->mm->brk = current->mm->start_brk = ELF_ET_DYN_BASE;
+
if ((current->flags & PF_RANDOMIZE) && (randomize_va_space > 1)) {
current->mm->brk = current->mm->start_brk =
arch_randomize_brk(current->mm);
_
Patches currently in -mm which might be from keescook@xxxxxxxxxxxx are
binfmt_elf-move-brk-out-of-mmap-when-doing-direct-loader-exec.patch
