Re: [PATCH] memcg: do not expose uninitialized mem_cgroup_per_nodeto world

From: Igor Mammedov
Date: Thu Jun 09 2011 - 04:12:41 EST

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On 06/08/2011 05:35 AM, KAMEZAWA Hiroyuki wrote:

On Tue, 07 Jun 2011 15:25:59 +0200
Igor Mammedov<imammedo@xxxxxxxxxx> wrote:

Sorry for late reply,

On 06/03/2011 03:00 PM, Hiroyuki Kamezawa wrote:
2011/6/3 Igor Mammedov<imammedo@xxxxxxxxxx>:
On 06/02/2011 01:10 AM, Hiroyuki Kamezawa wrote:
pc = list_entry(list->prev, struct page_cgroup, lru);
Hmm, I disagree your patch is a fix for mainline. At least, a cgroup
before completion of
create() is not populated to userland and you never be able to rmdir()
it because you can't
find it.

>26: e8 7d 12 30 00 call 0x3012a8
>2b:* 8b 73 08 mov 0x8(%ebx),%esi<-- trapping
instruction
>2e: 8b 7c 24 24 mov 0x24(%esp),%edi
>32: 8b 07 mov (%edi),%eax

Hm, what is the call 0x3012a8 ?

pc = list_entry(list->prev, struct page_cgroup, lru);
if (busy == pc) {
list_move(&pc->lru, list);
busy = 0;
spin_unlock_irqrestore(&zone->lru_lock, flags);
continue;
}
spin_unlock_irqrestore(&zone->lru_lock, flags);<---- is
call 0x3012a8
ret = mem_cgroup_move_parent(pc, mem, GFP_KERNEL);

and mov 0x8(%ebx),%esi
is dereferencing of 'pc' in inlined mem_cgroup_move_parent

Ah, thank you for input..then panicd at accessing pc->page and "pc"
was 0xfffffff4.
it means list->prev was NULL.

yes, that's the case.
I've looked at vmcore once more and indeed there isn't any parallel task
that touches cgroups code path.
Will investigate if it is xen to blame for incorrect data in place.

Thanks very much for your opinion.
What curious to me is that the fact "list->prev" is NULL.
I can see why you doubt the initialization code ....the list pointer never
contains NULL once it's used....
it smells like memory corruption or some to me. If you have vmcore,
what the problematic mem_cgroup_per_zone(node) contains ?

it has all zeros except for last field:

crash> rd f3446a00 62
f3446a00: 00000000 00000000 00000000 00000000 ................
f3446a10: 00000000 00000000 00000000 00000000 ................
f3446a20: 00000000 00000000 00000000 00000000 ................
f3446a30: 00000000 00000000 00000000 00000000 ................
f3446a40: 00000000 00000000 00000000 00000000 ................
f3446a50: 00000000 00000000 00000000 00000000 ................
f3446a60: 00000000 00000000 00000000 00000000 ................
f3446a70: 00000000 00000000 f36ef800 f3446a7c ..........n.|jD.
f3446a80: f3446a7c f3446a84 f3446a84 f3446a8c |jD..jD..jD..jD.
f3446a90: f3446a8c f3446a94 f3446a94 f3446a9c .jD..jD..jD..jD.
f3446aa0: f3446a9c 00000000 00000000 00000000 .jD.............
f3446ab0: 00000000 00000000 00000000 00000000 ................
f3446ac0: 00000000 00000000 00000000 00000000 ................
f3446ad0: 00000000 00000000 00000000 00000000 ................
f3446ae0: 00000000 00000000 00000000 00000000 ................
f3446af0: 00000000 f36ef800

crash> struct mem_cgroup f36ef800
struct mem_cgroup {
...
info = {
nodeinfo = {0xf3446a00}
},
...

It looks like a very targeted corruption of the first zone except of
the last field, while the second zone and the rest are perfectly
normal (i.e. have empty initialized lists).

Hmm, ok, thank you. Then, mem_cgroup_pre_zone[] was initialized once.
In this kind of case, I tend to check slab header of memory object f3446a00,
or check whether f3446a00 is an alive slab object or not.

It looks like f3446a00 alive/allocated object

crash> kmem f3446a00
CACHE NAME OBJSIZE ALLOCATED TOTAL SLABS SSIZE
f7000c80 size-512 512 2251 2616 327 4k
SLAB MEMORY TOTAL ALLOCATED FREE
f3da6540 f3446000 8 1 7
FREE / [ALLOCATED]
[f3446a00]

PAGE PHYSICAL MAPPING INDEX CNT FLAGS
c1fa58c0 33446000 0 70 1 2800080

However I have a related crash that can lead to not initialized lists of the first entry
(i.e. to what we see at f3446a00), debug kernel sometimes will crash at
alloc_mem_cgroup_per_zone_info:

XXX: pn: f208dc00, phy: 3208dc00
XXX: pn: f2e85a00, phy: 32e85a00
BUG: unable to handle kernel paging request at 9b74e240
IP: [<c080b95f>] mem_cgroup_create0x+0xef/0x350
*pdpt = 0000000033542001 *pde = 0000000000000000
Oops: 0002 [#1] SMP
...

Pid: 1823, comm: libvirtd Tainted: G ---------------- T
(2.6.32.700565 #21) HVM domU
EIP: 0060:[<c080b95f>] EFLAGS: 00210297 CPU: 3
EIP is at mem_cgroup_create+0xef/0x350
EAX: 9b74e240 EBX: f2e85a00 ECX: 00000001 EDX: 00000001
ESI: a88c8840 EDI: a88c8840 EBP: f201deb4 ESP: f201de8c
DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068
Process libvirtd (pid: 1823, ti=f201c000 task=f3642ab0 task.ti=f201c000)
Stack:
c09579b2 f2e85a00 32e85a00 f3455800 00000000 f2e85a00 f2c14ac0 c0a5a820
<0> fffffff4 f2c14ac0 f201def8 c049d3a7 00000000 00000000 00000000 000001ed
<0> f2c14ac8 f5fa4400 f24fe954 f3502000 f2c14e40 f24f5608 f3502010 f2c14ac0
Call Trace:
[<c049d3a7>] cgroup_mkdir+0xf7/0x450
[<c05318e3>] vfs_mkdir+0x93/0xf0
[<c0533787>] ? lookup_hash+0x27/0x30
[<c053390e>] sys_mkdirat+0xde/0x100
[<c04b5d4d>] ? call_rcu_sched+0xd/0x10
[<c04b5d58>] ? call_rcu+0x8/0x10
[<c047ab9f>] ? __put_cred+0x2f/0x50
[<c0524ded>] ? sys_faccessat+0x14d/0x180
[<c0523fb7>] ? filp_close+0x47/0x70
[<c0533950>] sys_mkdir+0x20/0x30
[<c0409b5f>] sysenter_do_call+0x12/0x28

static int alloc_mem_cgroup_per_zone_info(struct mem_cgroup *mem, int node)
{
...
memset(pn, 0, sizeof(*pn));

for (zone = 0; zone< MAX_NR_ZONES; zone++) {
mz =&pn->zoneinfo[zone];
for_each_lru(l)
INIT_LIST_HEAD(&mz->lists[l]);<- crash here
mz->usage_in_excess = 0;
mz->on_tree = false;
mz->mem = mem;
}
...

crash> dis 0xc080b93e 15
0xc080b93e<mem_cgroup_create+206>: movl $0x0,-0x18(%ebp)
0xc080b945<mem_cgroup_create+213>: mov %esi,-0x1c(%ebp)
0xc080b948<mem_cgroup_create+216>: imul $0x7c,-0x18(%ebp),%edi
0xc080b94c<mem_cgroup_create+220>: xor %ecx,%ecx
0xc080b94e<mem_cgroup_create+222>: xor %edx,%edx
0xc080b950<mem_cgroup_create+224>: lea (%edi,%edx,8),%esi
0xc080b953<mem_cgroup_create+227>: add $0x1,%ecx
0xc080b956<mem_cgroup_create+230>: lea (%ebx,%esi,1),%eax
0xc080b959<mem_cgroup_create+233>: add $0x1,%edx
0xc080b95c<mem_cgroup_create+236>: cmp $0x5,%ecx
0xc080b95f<mem_cgroup_create+239>: mov %eax,(%ebx,%esi,1)
0xc080b962<mem_cgroup_create+242>: mov %eax,0x4(%eax)
0xc080b965<mem_cgroup_create+245>: jne 0xc080b950
0xc080b967<mem_cgroup_create+247>: mov -0x14(%ebp),%eax
0xc080b96a<mem_cgroup_create+250>: movl $0x0,0x6c(%eax)

EDI on the first iteration should be 0 however it is a88c8840 according to Oops
dump and looking at -0x18(%ebp) in core shows 0 as it should be:

crash> x/xw 0xf201deb4-0x18
0xf201de9c: 0x00000000

so it looks like EDI is incorrectly restored by Xen or at the moment when 0xc080b948
was executed -0x18(%ebp) had that weird value.

It is possible that invalid EDI value and following

0xc080b950<mem_cgroup_create+224>: lea (%edi,%edx,8),%esi

<https://bugzilla.redhat.com/show_bug.cgi?id=700565#c36>lead to some accessible page and writes

0xc080b95f<mem_cgroup_create+239>: mov %eax,(%ebx,%esi,1)
0xc080b962<mem_cgroup_create+242>: mov %eax,0x4(%eax)

silently go to that page. Than after init lists loop it uses correct pn offset from
-0x14(%ebp) and initialises the rest fields of structure on the correct page.

mz->usage_in_excess = 0;
mz->on_tree = false;
mz->mem = mem;

0xc080b967<mem_cgroup_create+247>: mov -0x14(%ebp),%eax<-
0xc080b96a<mem_cgroup_create+250>: movl $0x0,0x6c(%eax)
0xc080b971<mem_cgroup_create+257>: movl $0x0,0x70(%eax)
0xc080b978<mem_cgroup_create+264>: movb $0x0,0x74(%eax)
0xc080b97c<mem_cgroup_create+268>: mov -0x1c(%ebp),%edx
0xc080b97f<mem_cgroup_create+271>: mov %edx,0x78(%eax)
0xc080b982<mem_cgroup_create+274>: add $0x7c,%eax
0xc080b985<mem_cgroup_create+277>: addl $0x1,-0x18(%ebp)
0xc080b989<mem_cgroup_create+281>: cmpl $0x4,-0x18(%ebp)
0xc080b98d<mem_cgroup_create+285>: mov %eax,-0x14(%ebp)
0xc080b990<mem_cgroup_create+288>: jne 0xc080b948

which could lead to the 0-ed list entries of the first zone
and the originally reported Oops in mem_cgroup_force_empty.
Afterwards it looks like:

0xc080b985<mem_cgroup_create+277>: addl $0x1,-0x18(%ebp)

-0x18(%ebp) is read correctly and the rest of 3 mz entries are initialized as
expected.

So question is why and how
0xc080b948<mem_cgroup_create+216>: imul $0x7c,-0x18(%ebp),%edi
may be screwed up

PS:
However, memory search for the went astray writes of the first entry
i.e. sequesnce f3446a00 f3446a00 in a couple of vmcores didn't give
any positive results.

Thanks,
-Kame

PS:
It most easily reproduced only on xen hvm 32bit guest under heavy
vcpus contention for real cpus resources (i.e. I had to overcommit
cpus and run several cpu hog tasks on host to make guest crash on
reboot cycle).
And from last experiments, crash happens only on on hosts that
doesn't have hap feature or if hap is disabled in hypervisor.

Thanks,
-Kame

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Next message: david: "Re: [PATCH v1 00/30] Ext4 snapshots"
Previous message: Stephen Rothwell: "linux-next: Tree for June 9"
In reply to: Igor Mammedov: "Re: [PATCH] memcg: do not expose uninitialized mem_cgroup_per_nodeto world"
Next in thread: Stefano Stabellini: "Possible shadow bug (was: Re: [PATCH] memcg: do not expose uninitializedmem_cgroup_per_node to world)"
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