Possible shadow bug (was: Re: [PATCH] memcg: do not expose uninitializedmem_cgroup_per_node to world)

From: Stefano Stabellini
Date: Thu Jun 09 2011 - 08:37:06 EST

CC'ing xen-devel and Tim.

This is a comment from a previous email in the thread:

> 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.

it makes me think that it is a shadow pagetables bug; see details below.
You can find more details on it following this thread on the lkml.




On Thu, 9 Jun 2011, Igor Mammedov wrote:
> 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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