On Fri, Oct 18, 2019 at 09:52:31AM -0700, Shakeel Butt wrote:

__mem_cgroup_free() can be called on the failure path in mem_cgroup_alloc(). However memcg_flush_percpu_vmstats() and memcg_flush_percpu_vmevents() which are called from __mem_cgroup_free() access the fields of memcg which can potentially be null if called from failure path from mem_cgroup_alloc(). Indeed syzbot has reported the following crash:

R13: 00000000004bf27d R14: 00000000004db028 R15: 0000000000000003 kasan: CONFIG_KASAN_INLINE enabled kasan: GPF could be caused by NULL-ptr deref or user memory access general protection fault: 0000 [#1] PREEMPT SMP KASAN CPU: 0 PID: 30393 Comm: syz-executor.1 Not tainted 5.4.0-rc2+ #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 RIP: 0010:memcg_flush_percpu_vmstats+0x4ae/0x930 mm/memcontrol.c:3436 Code: 05 41 89 c0 41 0f b6 04 24 41 38 c7 7c 08 84 c0 0f 85 5d 03 00 00 44 3b 05 33 d5 12 08 0f 83 e2 00 00 00 4c 89 f0 48 c1 e8 03 <42> 80 3c 28 00 0f 85 91 03 00 00 48 8b 85 10 fe ff ff 48 8b b0 90 RSP: 0018:ffff888095c27980 EFLAGS: 00010206 RAX: 0000000000000012 RBX: ffff888095c27b28 RCX: ffffc90008192000 RDX: 0000000000040000 RSI: ffffffff8340fae7 RDI: 0000000000000007 RBP: ffff888095c27be0 R08: 0000000000000000 R09: ffffed1013f0da33 R10: ffffed1013f0da32 R11: ffff88809f86d197 R12: fffffbfff138b760 R13: dffffc0000000000 R14: 0000000000000090 R15: 0000000000000007 FS: 00007f5027170700(0000) GS:ffff8880ae800000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000000000710158 CR3: 00000000a7b18000 CR4: 00000000001406f0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 Call Trace: __mem_cgroup_free+0x1a/0x190 mm/memcontrol.c:5021 mem_cgroup_free mm/memcontrol.c:5033 [inline] mem_cgroup_css_alloc+0x3a1/0x1ae0 mm/memcontrol.c:5160 css_create kernel/cgroup/cgroup.c:5156 [inline] cgroup_apply_control_enable+0x44d/0xc40 kernel/cgroup/cgroup.c:3119 cgroup_mkdir+0x899/0x11b0 kernel/cgroup/cgroup.c:5401 kernfs_iop_mkdir+0x14d/0x1d0 fs/kernfs/dir.c:1124 vfs_mkdir+0x42e/0x670 fs/namei.c:3807 do_mkdirat+0x234/0x2a0 fs/namei.c:3830 __do_sys_mkdir fs/namei.c:3846 [inline] __se_sys_mkdir fs/namei.c:3844 [inline] __x64_sys_mkdir+0x5c/0x80 fs/namei.c:3844 do_syscall_64+0xfa/0x760 arch/x86/entry/common.c:290 entry_SYSCALL_64_after_hwframe+0x49/0xbe RIP: 0033:0x459a59 Code: fd b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 cb b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00 RSP: 002b:00007f502716fc78 EFLAGS: 00000246 ORIG_RAX: 0000000000000053 RAX: ffffffffffffffda RBX: 00007f502716fc90 RCX: 0000000000459a59 RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000020000180 RBP: 000000000075bf20 R08: 0000000000000000 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000246 R12: 00007f50271706d4 R13: 00000000004bf27d R14: 00000000004db028 R15: 0000000000000003

Fixing this by moving the flush to mem_cgroup_free as there is no need to flush anything if we see failure in mem_cgroup_alloc().

Reported-by: syzbot+515d5bcfe179cdf049b2@syzkaller.appspotmail.com Fixes: bb65f89b7d3d ("mm: memcontrol: flush percpu vmevents before releasing memcg") Fixes: c350a99ea2b1 ("mm: memcontrol: flush percpu vmstats before releasing memcg") Signed-off-by: Shakeel Butt shakeelb@google.com Cc: Roman Gushchin guro@fb.com Cc: Michal Hocko mhocko@suse.com Cc: Johannes Weiner hannes@cmpxchg.org Cc: Vladimir Davydov vdavydov.dev@gmail.com Cc: stable@vger.kernel.org Cc: Andrew Morton akpm@linux-foundation.org

---

mm/memcontrol.c | 12 ++++++------ 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c index bdac56009a38..13cb4c1e9f2a 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -5014,12 +5014,6 @@ static void __mem_cgroup_free(struct mem_cgroup *memcg) { int node;

• /*

• * Flush percpu vmstats and vmevents to guarantee the value correctness
  

• * on parent's and all ancestor levels.
  

• */
  

• memcg_flush_percpu_vmstats(memcg, false);
• memcg_flush_percpu_vmevents(memcg); for_each_node(node) free_mem_cgroup_per_node_info(memcg, node); free_percpu(memcg->vmstats_percpu);

@@ -5030,6 +5024,12 @@ static void __mem_cgroup_free(struct mem_cgroup *memcg) static void mem_cgroup_free(struct mem_cgroup *memcg) { memcg_wb_domain_exit(memcg);

• /*

• * Flush percpu vmstats and vmevents to guarantee the value correctness
  

• * on parent's and all ancestor levels.
  

• */
  

• memcg_flush_percpu_vmstats(memcg, false);
• memcg_flush_percpu_vmevents(memcg); __mem_cgroup_free(memcg);

}

Good catch!

Reviewed-by: Roman Gushchin guro@fb.com

Thank you, Shakeel!