On Fri, Jun 8, 2018 at 10:06 AM Roman Gushchin <guro@xxxxxx> wrote: > > Shakeel reported a crash in mem_cgroup_protected(), which > can be triggered by memcg reclaim if the legacy cgroup v1 > use_hierarchy=0 mode is used: > > [ 226.060572] BUG: unable to handle kernel NULL pointer dereference > at 0000000000000120 > [ 226.068310] PGD 8000001ff55da067 P4D 8000001ff55da067 PUD 1fdc7df067 PMD 0 > [ 226.075191] Oops: 0000 [#4] SMP PTI > [ 226.078637] CPU: 0 PID: 15581 Comm: bash Tainted: G D > 4.17.0-smp-clean #5 > [ 226.086635] Hardware name: ... > [ 226.094546] RIP: 0010:mem_cgroup_protected+0x54/0x130 > [ 226.099533] Code: 4c 8b 8e 00 01 00 00 4c 8b 86 08 01 00 00 48 8d > 8a 08 ff ff ff 48 85 d2 ba 00 00 00 00 48 0f 44 ca 48 39 c8 0f 84 cf > 00 00 00 <48> 8b 81 20 01 00 00 4d 89 ca 4c 39 c8 4c 0f 46 d0 4d 85 d2 > 74 05 > [ 226.118194] RSP: 0000:ffffabe64dfafa58 EFLAGS: 00010286 > [ 226.123358] RAX: ffff9fb6ff03d000 RBX: ffff9fb6f5b1b000 RCX: 0000000000000000 > [ 226.130406] RDX: 0000000000000000 RSI: ffff9fb6f5b1b000 RDI: ffff9fb6f5b1b000 > [ 226.137454] RBP: ffffabe64dfafb08 R08: 0000000000000000 R09: 0000000000000000 > [ 226.144503] R10: 0000000000000000 R11: 000000000000c800 R12: ffffabe64dfafb88 > [ 226.151551] R13: ffff9fb6f5b1b000 R14: ffffabe64dfafb88 R15: ffff9fb77fffe000 > [ 226.158602] FS: 00007fed1f8ac700(0000) GS:ffff9fb6ff400000(0000) > knlGS:0000000000000000 > [ 226.166594] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > [ 226.172270] CR2: 0000000000000120 CR3: 0000001fdcf86003 CR4: 00000000001606f0 > [ 226.179317] Call Trace: > [ 226.181732] ? shrink_node+0x194/0x510 > [ 226.185435] do_try_to_free_pages+0xfd/0x390 > [ 226.189653] try_to_free_mem_cgroup_pages+0x123/0x210 > [ 226.194643] try_charge+0x19e/0x700 > [ 226.198088] mem_cgroup_try_charge+0x10b/0x1a0 > [ 226.202478] wp_page_copy+0x134/0x5b0 > [ 226.206094] do_wp_page+0x90/0x460 > [ 226.209453] __handle_mm_fault+0x8e3/0xf30 > [ 226.213498] handle_mm_fault+0xfe/0x220 > [ 226.217285] __do_page_fault+0x262/0x500 > [ 226.221158] do_page_fault+0x28/0xd0 > [ 226.224689] ? page_fault+0x8/0x30 > [ 226.228048] page_fault+0x1e/0x30 > [ 226.231323] RIP: 0033:0x485b72 > > The problem happens because parent_mem_cgroup() returns a NULL > pointer, which is dereferenced later without a check. > > As cgroup v1 has no memory guarantee support, let's make > mem_cgroup_protected() immediately return MEMCG_PROT_NONE, > if the given cgroup has no parent (non-hierarchical mode is used). > > Reported-by: Shakeel Butt <shakeelb@xxxxxxxxxx> > Signed-off-by: Roman Gushchin <guro@xxxxxx> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx> > Cc: Michal Hocko <mhocko@xxxxxxxxxx> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Tested-by: Shakeel Butt <shakeelb@xxxxxxxxxx> > --- > mm/memcontrol.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 6c9fb4e47be3..6205ba512928 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -5750,6 +5750,9 @@ enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root, > elow = memcg->memory.low; > > parent = parent_mem_cgroup(memcg); > + if (!parent) > + return MEMCG_PROT_NONE; > + > if (parent == root_mem_cgroup) > goto exit; > > -- > 2.14.3 >
