mm, mempolicy: fix uninit memory access

Syzbot with KMSAN reports (excerpt):

==================================================================
BUG: KMSAN: uninit-value in mpol_rebind_policy mm/mempolicy.c:353 [inline]
BUG: KMSAN: uninit-value in mpol_rebind_mm+0x249/0x370 mm/mempolicy.c:384
CPU: 1 PID: 17420 Comm: syz-executor4 Not tainted 4.20.0-rc7+ #15
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Call Trace:
  __dump_stack lib/dump_stack.c:77 [inline]
  dump_stack+0x173/0x1d0 lib/dump_stack.c:113
  kmsan_report+0x12e/0x2a0 mm/kmsan/kmsan.c:613
  __msan_warning+0x82/0xf0 mm/kmsan/kmsan_instr.c:295
  mpol_rebind_policy mm/mempolicy.c:353 [inline]
  mpol_rebind_mm+0x249/0x370 mm/mempolicy.c:384
  update_tasks_nodemask+0x608/0xca0 kernel/cgroup/cpuset.c:1120
  update_nodemasks_hier kernel/cgroup/cpuset.c:1185 [inline]
  update_nodemask kernel/cgroup/cpuset.c:1253 [inline]
  cpuset_write_resmask+0x2a98/0x34b0 kernel/cgroup/cpuset.c:1728

...

Uninit was created at:
  kmsan_save_stack_with_flags mm/kmsan/kmsan.c:204 [inline]
  kmsan_internal_poison_shadow+0x92/0x150 mm/kmsan/kmsan.c:158
  kmsan_kmalloc+0xa6/0x130 mm/kmsan/kmsan_hooks.c:176
  kmem_cache_alloc+0x572/0xb90 mm/slub.c:2777
  mpol_new mm/mempolicy.c:276 [inline]
  do_mbind mm/mempolicy.c:1180 [inline]
  kernel_mbind+0x8a7/0x31a0 mm/mempolicy.c:1347
  __do_sys_mbind mm/mempolicy.c:1354 [inline]

As it's difficult to report where exactly the uninit value resides in
the mempolicy object, we have to guess a bit.  mm/mempolicy.c:353
contains this part of mpol_rebind_policy():

        if (!mpol_store_user_nodemask(pol) &&
            nodes_equal(pol->w.cpuset_mems_allowed, *newmask))

"mpol_store_user_nodemask(pol)" is testing pol->flags, which I couldn't
ever see being uninitialized after leaving mpol_new().  So I'll guess
it's actually about accessing pol->w.cpuset_mems_allowed on line 354,
but still part of statement starting on line 353.

For w.cpuset_mems_allowed to be not initialized, and the nodes_equal()
reachable for a mempolicy where mpol_set_nodemask() is called in
do_mbind(), it seems the only possibility is a MPOL_PREFERRED policy
with empty set of nodes, i.e.  MPOL_LOCAL equivalent, with MPOL_F_LOCAL
flag.  Let's exclude such policies from the nodes_equal() check.  Note
the uninit access should be benign anyway, as rebinding this kind of
policy is always a no-op.  Therefore no actual need for stable
inclusion.

Link: http://lkml.kernel.org/r/[email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Vlastimil Babka <[email protected]>
Reported-by: [email protected]
Cc: Alexander Potapenko <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Yisheng Xie <[email protected]>
Cc: zhong jiang <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

1 files changed, 1 insertions, 1 deletions

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 76e7e4bc3335..af171ccb56a2 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -350,7 +350,7 @@ static void mpol_rebind_policy(struct mempolicy *pol, const nodemask_t *newmask)
 {
 	if (!pol)
 		return;
-	if (!mpol_store_user_nodemask(pol) &&
+	if (!mpol_store_user_nodemask(pol) && !(pol->flags & MPOL_F_LOCAL) &&
 	    nodes_equal(pol->w.cpuset_mems_allowed, *newmask))
 		return;