linux/arch
Hugh Dickins 1be7107fbe mm: larger stack guard gap, between vmas
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>
2017-06-19 21:50:20 +08:00
..
alpha osf_wait4(): fix infoleak 2017-05-21 13:10:07 -04:00
arc mm: larger stack guard gap, between vmas 2017-06-19 21:50:20 +08:00
arm mm: larger stack guard gap, between vmas 2017-06-19 21:50:20 +08:00
arm64 ARM: SoC fixes 2017-06-19 16:50:09 +08:00
blackfin uapi: export all headers under uapi directories 2017-05-11 00:21:54 +09:00
c6x uapi: export all headers under uapi directories 2017-05-11 00:21:54 +09:00
cris devicetree: Move include prefixes from arch to separate directory 2017-05-18 23:55:48 -07:00
frv mm: larger stack guard gap, between vmas 2017-06-19 21:50:20 +08:00
h8300 uapi: export all headers under uapi directories 2017-05-11 00:21:54 +09:00
hexagon hexagon: Use raw_copy_to_user 2017-06-10 19:10:31 -07:00
ia64 Kbuild UAPI header export updates for v4.12 2017-05-10 20:45:36 -07:00
m32r uapi: export all headers under uapi directories 2017-05-11 00:21:54 +09:00
m68k uapi: export all headers under uapi directories 2017-05-11 00:21:54 +09:00
metag devicetree: Move include prefixes from arch to separate directory 2017-05-18 23:55:48 -07:00
microblaze uapi: export all headers under uapi directories 2017-05-11 00:21:54 +09:00
mips mm: larger stack guard gap, between vmas 2017-06-19 21:50:20 +08:00
mn10300 uapi: export all headers under uapi directories 2017-05-11 00:21:54 +09:00
nios2 nios2 update for v4.12-rc1 2017-05-12 09:53:16 -07:00
openrisc kthread: fix boot hang (regression) on MIPS/OpenRISC 2017-05-29 09:40:54 -07:00
parisc mm: larger stack guard gap, between vmas 2017-06-19 21:50:20 +08:00
powerpc mm: larger stack guard gap, between vmas 2017-06-19 21:50:20 +08:00
s390 mm: larger stack guard gap, between vmas 2017-06-19 21:50:20 +08:00
score uapi: export all headers under uapi directories 2017-05-11 00:21:54 +09:00
sh mm: larger stack guard gap, between vmas 2017-06-19 21:50:20 +08:00
sparc mm: larger stack guard gap, between vmas 2017-06-19 21:50:20 +08:00
tile mm: larger stack guard gap, between vmas 2017-06-19 21:50:20 +08:00
um Merge branch 'for-linus-4.12-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/rw/uml 2017-05-13 10:20:02 -07:00
unicore32 Kbuild UAPI header export updates for v4.12 2017-05-10 20:45:36 -07:00
x86 mm: larger stack guard gap, between vmas 2017-06-19 21:50:20 +08:00
xtensa mm: larger stack guard gap, between vmas 2017-06-19 21:50:20 +08:00
.gitignore
Kconfig Merge branch 'core-rcu-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2017-05-10 10:30:46 -07:00