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Hugh Dickins cfc0eb4038 mm: larger stack guard gap, between vmas 
commit 1be7107fbe upstream.

Stack guard page is a useful feature to reduce a risk of stack smashing
into a different mapping. We have been using a single page gap which
is sufficient to prevent having stack adjacent to a different mapping.
But this seems to be insufficient in the light of the stack usage in
userspace. E.g. glibc uses as large as 64kB alloca() in many commonly
used functions. Others use constructs liks gid_t buffer[NGROUPS_MAX]
which is 256kB or stack strings with MAX_ARG_STRLEN.

This will become especially dangerous for suid binaries and the default
no limit for the stack size limit because those applications can be
tricked to consume a large portion of the stack and a single glibc call
could jump over the guard page. These attacks are not theoretical,
unfortunatelly.

Make those attacks less probable by increasing the stack guard gap
to 1MB (on systems with 4k pages; but make it depend on the page size
because systems with larger base pages might cap stack allocations in
the PAGE_SIZE units) which should cover larger alloca() and VLA stack
allocations. It is obviously not a full fix because the problem is
somehow inherent, but it should reduce attack space a lot.

One could argue that the gap size should be configurable from userspace,
but that can be done later when somebody finds that the new 1MB is wrong
for some special case applications.  For now, add a kernel command line
option (stack_guard_gap) to specify the stack gap size (in page units).

Implementation wise, first delete all the old code for stack guard page:
because although we could get away with accounting one extra page in a
stack vma, accounting a larger gap can break userspace - case in point,
a program run with "ulimit -S -v 20000" failed when the 1MB gap was
counted for RLIMIT_AS; similar problems could come with RLIMIT_MLOCK
and strict non-overcommit mode.

Instead of keeping gap inside the stack vma, maintain the stack guard
gap as a gap between vmas: using vm_start_gap() in place of vm_start
(or vm_end_gap() in place of vm_end if VM_GROWSUP) in just those few
places which need to respect the gap - mainly arch_get_unmapped_area(),
and and the vma tree's subtree_gap support for that.

Original-patch-by: Oleg Nesterov <oleg@redhat.com>
Original-patch-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Tested-by: Helge Deller <deller@gmx.de> # parisc
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[wt: backport to 4.11: adjust context]
[wt: backport to 4.9: adjust context ; kernel doc was not in admin-guide]
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
History 2017-06-24 07:11:18 +02:00
..
boards sh: add Kconfig option for J-Core SoC core drivers 2016-10-18 18:53:37 -04:00
boot sh: add device tree source for J2 FPGA on Mimas v2 board 2016-08-05 03:38:26 +00:00
cchips genirq: Remove irq argument from irq flow handlers 2015-09-16 15:47:51 +02:00
configs sh: add earlycon support to j2_defconfig 2016-10-18 18:54:56 -04:00
drivers These changes improve device tree support (including builtin DTB), add 2016-08-06 09:00:05 -04:00
include Merge branch 'work.uaccess2' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2016-10-11 23:38:39 -07:00
kernel nmi_backtrace: generate one-line reports for idle cpus 2016-10-07 18:46:30 -07:00
lib sh: New gcc support 2016-03-17 19:45:44 +00:00
lib64 sh64: Kill off old exception debugging helpers. 2012-06-14 14:12:18 +09:00
math-emu sh: remove unused do_fpu_error 2014-04-03 16:20:52 -07:00
mm mm: larger stack guard gap, between vmas 2017-06-24 07:11:18 +02:00
oprofile oprofile, arm/sh: Fix oprofile_arch_exit() linkage issue 2011-12-23 11:58:34 +01:00
tools sh: Add RSK2+SH7269 board 2012-05-10 17:20:01 +09:00
Kconfig sh: add SMP support for J2 2016-08-05 03:29:39 +00:00
Kconfig.cpu arch/sh: remove depends on CONFIG_EXPERIMENTAL 2013-01-21 14:43:13 -08:00
Kconfig.debug sh: dwarf unwinder depends on SHcompact. 2012-03-30 19:31:22 +09:00
Makefile sh: support CPU_J2 when compiler lacks -mj2 2016-10-13 16:15:41 -04:00