[PATCH] mm/memcontrol: fix NULL pointer dereference when use_hierarchy is 0

[PATCH] mm/memcontrol: fix NULL pointer dereference when use_hierarchy is 0

[Joonsoo Kim]

It can be possible to return NULL in parent_mem_cgroup()
if use_hierarchy is 0. So, we need to check NULL in the loop on
mem_cgroup_low(). Without it, following NULL pointer dereference happens.

[   33.607531] BUG: unable to handle kernel NULL pointer dereference at 00000000000000b0
[   33.608008] IP: [<ffffffff811dcf60>] mem_cgroup_low+0x40/0x90
[   33.608008] PGD 1d893067 PUD 1cf41067 PMD 0
[   33.608008] Oops: 0000 [#12] SMP
[   33.608008] Modules linked in:
[   33.608008] CPU: 1 PID: 3936 Comm: as Tainted: G      D         3.19.0-next-20150216 #156
[   33.608008] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
[   33.608008] task: ffff88001d9c8000 ti: ffff88000cb14000 task.ti: ffff88000cb14000
[   33.608008] RIP: 0010:[<ffffffff811dcf60>]  [<ffffffff811dcf60>] mem_cgroup_low+0x40/0x90
[   33.608008] RSP: 0000:ffff88000cb17a88  EFLAGS: 00010286
[   33.608008] RAX: 0000000000000000 RBX: ffff88000cb17bc0 RCX: 0000000000000000
[   33.608008] RDX: ffff88001f491400 RSI: 0000000000000000 RDI: 0000000000000000
[   33.608008] RBP: ffff88000cb17a88 R08: 0000000000000160 R09: 0000000000000000
[   33.608008] R10: 0000000000000000 R11: 0000000002b8c101 R12: 0000000000000000
[   33.608008] R13: 0000000000000000 R14: ffff88001fff9e08 R15: ffff88001da95800
[   33.608008] FS:  00002b7a12715380(0000) GS:ffff88001fa40000(0000) knlGS:0000000000000000
[   33.608008] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   33.608008] CR2: 00000000000000b0 CR3: 000000000762f000 CR4: 00000000000007e0
[   33.608008] Stack:
[   33.608008]  ffff88000cb17b18 ffffffff811838ec ffff88000cb17cd8 0000000000000000
[   33.608008]  0000000000000000 0001000000000000 000280da00000000 ffff88001fff8780
[   33.608008]  ffff88000cb17af8 ffffffff810e1d7e ffff88001fff8780 000000030000000c
[   33.608008] Call Trace:
[   33.608008]  [<ffffffff811838ec>] shrink_zone+0xac/0x2d0
[   33.608008]  [<ffffffff810e1d7e>] ? ktime_get+0x3e/0xa0
[   33.608008]  [<ffffffff81183e94>] do_try_to_free_pages+0x174/0x440
[   33.608008]  [<ffffffff8117f1a8>] ? throttle_direct_reclaim+0x98/0x250
[   33.608008]  [<ffffffff8118421a>] try_to_free_pages+0xba/0x150
[   33.608008]  [<ffffffff81176d10>] __alloc_pages_nodemask+0x5a0/0x950
[   33.608008]  [<ffffffff811c09ff>] alloc_pages_vma+0xaf/0x200
[   33.608008]  [<ffffffff811a0717>] handle_mm_fault+0x1287/0x17e0
[   33.608008]  [<ffffffff81059e9e>] ? kvm_clock_read+0x1e/0x20
[   33.608008]  [<ffffffff81059e9e>] ? kvm_clock_read+0x1e/0x20
[   33.608008]  [<ffffffff8101e6a9>] ? sched_clock+0x9/0x10
[   33.608008]  [<ffffffff810605f1>] __do_page_fault+0x191/0x440
[   33.608008]  [<ffffffff81060955>] trace_do_page_fault+0x45/0x100
[   33.608008]  [<ffffffff8105968e>] do_async_page_fault+0x1e/0xd0
[   33.608008]  [<ffffffff8176f628>] async_page_fault+0x28/0x30
[   33.608008] Code: 48 8b 15 cc 21 b4 00 48 39 d6 74 53 48 8b 8e b0 00 00 00 48 39 8e 28 01 00 00 72 43 31 c9 48 39 fe 75 1d eb 35 66 0f 1f 44 00 00 <48> 8b 86 b0 00 00 00 48 39 86 28 01 00 00 72 30 48 39 f7 74 1a
[   33.608008] RIP  [<ffffffff811dcf60>] mem_cgroup_low+0x40/0x90
[   33.608008]  RSP <ffff88000cb17a88>
[   33.608008] CR2: 00000000000000b0
[   33.608008] BUG: unable to handle kernel [   33.653499] ---[ end trace e264a32717ffda51 ]---

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim-Hm3cg6mZ9cc@public.gmane.org>
---
 mm/memcontrol.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index d18d3a6..507cfea 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5431,6 +5431,8 @@ bool mem_cgroup_low(struct mem_cgroup *root, struct mem_cgroup *memcg)
 
 	while (memcg != root) {
 		memcg = parent_mem_cgroup(memcg);
+		if (!memcg)
+			break;
 
 		if (memcg == root_mem_cgroup)
 			break;
-- 
1.9.1

Re: [PATCH] mm/memcontrol: fix NULL pointer dereference when use_hierarchy is 0

[Michal Hocko]

On Tue 17-02-15 14:24:59, Joonsoo Kim wrote:
> It can be possible to return NULL in parent_mem_cgroup()
> if use_hierarchy is 0.

This alone is not sufficient because the low limit is present only in
the unified hierarchy API and there is no use_hierarchy there. The
primary issue here is that the memcg has 0 usage so the previous
check for usage will not stop us. And that is bug IMO.

I think that the following patch would be more correct from semantic
POV:
---
From f5d74671d30e44c50b45b4464c92f536f1dbdff6 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko-AlSwsSmVLrQ@public.gmane.org>
Date: Tue, 17 Feb 2015 08:02:12 +0100
Subject: [PATCH] memcg: fix low limit calculation

A memcg is considered low limited even when the current usage is equal
to the low limit. This leads to interesting side effects e.g.
groups/hierarchies with no memory accounted are considered protected and
so the reclaim will emit MEMCG_LOW event when encountering them.

Another and much bigger issue was reported by Joonsoo Kim. He has hit a
NULL ptr dereference with the legacy cgroup API which even doesn't have
low limit exposed. The limit is 0 by default but the initial check fails
for memcg with 0 consumption and parent_mem_cgroup() would return NULL
if use_hierarchy is 0 and so page_counter_read would try to dereference
NULL.

I suppose that the current implementation is just an overlook because
the documentation in Documentation/cgroups/unified-hierarchy.txt says:
"
The memory.low boundary on the other hand is a top-down allocated
reserve.  A cgroup enjoys reclaim protection when it and all its
ancestors are below their low boundaries
"

Fix the usage and the low limit comparision in mem_cgroup_low accordingly.

Fixes: 241994ed8649 (mm: memcontrol: default hierarchy interface for memory)
Reported-by: Joonsoo Kim <js1304-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Signed-off-by: Michal Hocko <mhocko-AlSwsSmVLrQ@public.gmane.org>
---
 mm/memcontrol.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 0b436bc02ba4..079b5c02e245 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5426,7 +5426,7 @@ bool mem_cgroup_low(struct mem_cgroup *root, struct mem_cgroup *memcg)
 	if (memcg == root_mem_cgroup)
 		return false;
 
-	if (page_counter_read(&memcg->memory) > memcg->low)
+	if (page_counter_read(&memcg->memory) >= memcg->low)
 		return false;
 
 	while (memcg != root) {
@@ -5435,7 +5435,7 @@ bool mem_cgroup_low(struct mem_cgroup *root, struct mem_cgroup *memcg)
 		if (memcg == root_mem_cgroup)
 			break;
 
-		if (page_counter_read(&memcg->memory) > memcg->low)
+		if (page_counter_read(&memcg->memory) >= memcg->low)
 			return false;
 	}
 	return true;
-- 
2.1.4

-- 
Michal Hocko
SUSE Labs

Re: [PATCH] mm/memcontrol: fix NULL pointer dereference when use_hierarchy is 0

[Johannes Weiner]

On Tue, Feb 17, 2015 at 09:33:27AM +0100, Michal Hocko wrote:
> On Tue 17-02-15 14:24:59, Joonsoo Kim wrote:
> > It can be possible to return NULL in parent_mem_cgroup()
> > if use_hierarchy is 0.
> 
> This alone is not sufficient because the low limit is present only in
> the unified hierarchy API and there is no use_hierarchy there. The
> primary issue here is that the memcg has 0 usage so the previous
> check for usage will not stop us. And that is bug IMO.

Yes, empty groups shouldn't be considered low.

> From f5d74671d30e44c50b45b4464c92f536f1dbdff6 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko-AlSwsSmVLrQ@public.gmane.org>
> Date: Tue, 17 Feb 2015 08:02:12 +0100
> Subject: [PATCH] memcg: fix low limit calculation
> 
> A memcg is considered low limited even when the current usage is equal
> to the low limit. This leads to interesting side effects e.g.
> groups/hierarchies with no memory accounted are considered protected and
> so the reclaim will emit MEMCG_LOW event when encountering them.
> 
> Another and much bigger issue was reported by Joonsoo Kim. He has hit a
> NULL ptr dereference with the legacy cgroup API which even doesn't have
> low limit exposed. The limit is 0 by default but the initial check fails
> for memcg with 0 consumption and parent_mem_cgroup() would return NULL
> if use_hierarchy is 0 and so page_counter_read would try to dereference
> NULL.
> 
> I suppose that the current implementation is just an overlook because
> the documentation in Documentation/cgroups/unified-hierarchy.txt says:
> "
> The memory.low boundary on the other hand is a top-down allocated
> reserve.  A cgroup enjoys reclaim protection when it and all its
> ancestors are below their low boundaries
> "
> 
> Fix the usage and the low limit comparision in mem_cgroup_low accordingly.
> 
> Fixes: 241994ed8649 (mm: memcontrol: default hierarchy interface for memory)
> Reported-by: Joonsoo Kim <js1304-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> Signed-off-by: Michal Hocko <mhocko-AlSwsSmVLrQ@public.gmane.org>

Acked-by: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>

Re: [PATCH] mm/memcontrol: fix NULL pointer dereference when use_hierarchy is 0

[Joonsoo Kim]

On Tue, Feb 17, 2015 at 09:33:27AM +0100, Michal Hocko wrote:
> On Tue 17-02-15 14:24:59, Joonsoo Kim wrote:
> > It can be possible to return NULL in parent_mem_cgroup()
> > if use_hierarchy is 0.
> 
> This alone is not sufficient because the low limit is present only in
> the unified hierarchy API and there is no use_hierarchy there. The
> primary issue here is that the memcg has 0 usage so the previous
> check for usage will not stop us. And that is bug IMO.
> 
> I think that the following patch would be more correct from semantic
> POV:
> ---
> >From f5d74671d30e44c50b45b4464c92f536f1dbdff6 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko-AlSwsSmVLrQ@public.gmane.org>
> Date: Tue, 17 Feb 2015 08:02:12 +0100
> Subject: [PATCH] memcg: fix low limit calculation
> 
> A memcg is considered low limited even when the current usage is equal
> to the low limit. This leads to interesting side effects e.g.
> groups/hierarchies with no memory accounted are considered protected and
> so the reclaim will emit MEMCG_LOW event when encountering them.
> 
> Another and much bigger issue was reported by Joonsoo Kim. He has hit a
> NULL ptr dereference with the legacy cgroup API which even doesn't have
> low limit exposed. The limit is 0 by default but the initial check fails
> for memcg with 0 consumption and parent_mem_cgroup() would return NULL
> if use_hierarchy is 0 and so page_counter_read would try to dereference
> NULL.
> 
> I suppose that the current implementation is just an overlook because
> the documentation in Documentation/cgroups/unified-hierarchy.txt says:
> "
> The memory.low boundary on the other hand is a top-down allocated
> reserve.  A cgroup enjoys reclaim protection when it and all its
> ancestors are below their low boundaries
> "
> 
> Fix the usage and the low limit comparision in mem_cgroup_low accordingly.
> 
> Fixes: 241994ed8649 (mm: memcontrol: default hierarchy interface for memory)
> Reported-by: Joonsoo Kim <js1304-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> Signed-off-by: Michal Hocko <mhocko-AlSwsSmVLrQ@public.gmane.org>

Good! This fixes my issue.

Thanks.