在 2020/2/19 9:58, Sean Christopherson 写道:
> On Wed, Feb 19, 2020 at 09:39:59AM +0800, Longpeng (Mike) wrote:
>> 在 2020/2/19 4:37, Sean Christopherson 写道:
>>> On Tue, Feb 18, 2020 at 08:10:25PM +0800, Longpeng(Mike) wrote:
>>>> Our machine encountered a panic after run for a long time and
>>>> the calltrace is:
>>>
>>> What's the actual panic? Is it a BUG() in hugetlb_fault(), a bad pointer
>>> dereference, etc...?
>>>
>> A bad pointer dereference.
>>
>> pgd -> pud -> user 1G hugepage
>> huge_pte_offset() wants to return NULL or pud (point to the entry), but it maybe
>> return the a bad pointer of the user 1G hugepage.
>>
>>>> RIP: 0010:[<ffffffff9dff0587>] [<ffffffff9dff0587>] hugetlb_fault+0x307/0xbe0
>>>> RSP: 0018:ffff9567fc27f808 EFLAGS: 00010286
>>>> RAX: e800c03ff1258d48 RBX: ffffd3bb003b69c0 RCX: e800c03ff1258d48
>>>> RDX: 17ff3fc00eda72b7 RSI: 00003ffffffff000 RDI: e800c03ff1258d48
>>>> RBP: ffff9567fc27f8c8 R08: e800c03ff1258d48 R09: 0000000000000080
>>>> R10: ffffaba0704c22a8 R11: 0000000000000001 R12: ffff95c87b4b60d8
>>>> R13: 00005fff00000000 R14: 0000000000000000 R15: ffff9567face8074
>>>> FS: 00007fe2d9ffb700(0000) GS:ffff956900e40000(0000) knlGS:0000000000000000
>>>> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>> CR2: ffffd3bb003b69c0 CR3: 000000be67374000 CR4: 00000000003627e0
>>>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>>>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>>>> Call Trace:
>>>> [<ffffffff9df9b71b>] ? unlock_page+0x2b/0x30
>>>> [<ffffffff9dff04a2>] ? hugetlb_fault+0x222/0xbe0
>>>> [<ffffffff9dff1405>] follow_hugetlb_page+0x175/0x540
>>>> [<ffffffff9e15b825>] ? cpumask_next_and+0x35/0x50
>>>> [<ffffffff9dfc7230>] __get_user_pages+0x2a0/0x7e0
>>>> [<ffffffff9dfc648d>] __get_user_pages_unlocked+0x15d/0x210
>>>> [<ffffffffc068cfc5>] __gfn_to_pfn_memslot+0x3c5/0x460 [kvm]
>>>> [<ffffffffc06b28be>] try_async_pf+0x6e/0x2a0 [kvm]
>>>> [<ffffffffc06b4b41>] tdp_page_fault+0x151/0x2d0 [kvm]
>>>> [<ffffffffc075731c>] ? vmx_vcpu_run+0x2ec/0xc80 [kvm_intel]
>>>> [<ffffffffc0757328>] ? vmx_vcpu_run+0x2f8/0xc80 [kvm_intel]
>>>> [<ffffffffc06abc11>] kvm_mmu_page_fault+0x31/0x140 [kvm]
>>>> [<ffffffffc074d1ae>] handle_ept_violation+0x9e/0x170 [kvm_intel]
>>>> [<ffffffffc075579c>] vmx_handle_exit+0x2bc/0xc70 [kvm_intel]
>>>> [<ffffffffc074f1a0>] ? __vmx_complete_interrupts.part.73+0x80/0xd0 [kvm_intel]
>>>> [<ffffffffc07574c0>] ? vmx_vcpu_run+0x490/0xc80 [kvm_intel]
>>>> [<ffffffffc069f3be>] vcpu_enter_guest+0x7be/0x13a0 [kvm]
>>>> [<ffffffffc06cf53e>] ? kvm_check_async_pf_completion+0x8e/0xb0 [kvm]
>>>> [<ffffffffc06a6f90>] kvm_arch_vcpu_ioctl_run+0x330/0x490 [kvm]
>>>> [<ffffffffc068d919>] kvm_vcpu_ioctl+0x309/0x6d0 [kvm]
>>>> [<ffffffff9deaa8c2>] ? dequeue_signal+0x32/0x180
>>>> [<ffffffff9deae34d>] ? do_sigtimedwait+0xcd/0x230
>>>> [<ffffffff9e03aed0>] do_vfs_ioctl+0x3f0/0x540
>>>> [<ffffffff9e03b0c1>] SyS_ioctl+0xa1/0xc0
>>>> [<ffffffff9e53879b>] system_call_fastpath+0x22/0x27
>>>>
>>>> ( The kernel we used is older, but we think the latest kernel also has this
>>>> bug after dig into this problem. )
>>>>
>>>> For 1G hugepages, huge_pte_offset() wants to return NULL or pudp, but it
>>>> may return a wrong 'pmdp' if there is a race. Please look at the following
>>>> code snippet:
>>>> ...
>>>> pud = pud_offset(p4d, addr);
>>>> if (sz != PUD_SIZE && pud_none(*pud))
>>>> return NULL;
>>>> /* hugepage or swap? */
>>>> if (pud_huge(*pud) || !pud_present(*pud))
>>>> return (pte_t *)pud;
>>>>
>>>> pmd = pmd_offset(pud, addr);
>>>> if (sz != PMD_SIZE && pmd_none(*pmd))
>>>> return NULL;
>>>> /* hugepage or swap? */
>>>> if (pmd_huge(*pmd) || !pmd_present(*pmd))
>>>> return (pte_t *)pmd;
>>>> ...
>>>>
>>>> The following sequence would trigger this bug:
>>>> 1. CPU0: sz = PUD_SIZE and *pud = 0 , continue
>>>> 1. CPU0: "pud_huge(*pud)" is false
>>>> 2. CPU1: calling hugetlb_no_page and set *pud to xxxx8e7(PRESENT)
>>>> 3. CPU0: "!pud_present(*pud)" is false, continue
>>>> 4. CPU0: pmd = pmd_offset(pud, addr) and maybe return a wrong pmdp
>>>> However, we want CPU0 to return NULL or pudp.
>>>>
>>>> We can avoid this race by read the pud only once.
>>>
>>> Are there any other options for avoiding the panic you hit? I ask because
>>> there are a variety of flows that use a very similar code pattern, e.g.
>>> lookup_address_in_pgd(), and using READ_ONCE() in huge_pte_offset() but not
>>> other flows could be confusing (or in my case, anxiety inducing[*]). At
>>> the least, adding a comment in huge_pte_offset() to explain the need for
>>> READ_ONCE() would be helpful.
>>>
>> I hope the hugetlb and mm maintainers could give some other options if they
>> approve this bug.
>
> The race and the fix make sense. I assumed dereferencing garbage from the
> huge page was the issue, but I wasn't 100% that was the case, which is why
> I asked about alternative fixes.
>
>> We change the code from
>> if (pud_huge(*pud) || !pud_present(*pud))
>> to
>> if (pud_huge(*pud)
>> return (pte_t *)pud;
>> busy loop for 500ms
>> if (!pud_present(*pud))
>> return (pte_t *)pud;
>> and the panic will be hit quickly.
>>
>> ARM64 has already use READ/WRITE_ONCE to access the pagetable, look at this
>> commit 20a004e7 (arm64: mm: Use READ_ONCE/WRITE_ONCE when accessing page tables).
>>
>> The root cause is: 'if (pud_huge(*pud) || !pud_present(*pud))' read entry from
>> pud twice and the *pud maybe change in a race, so if we only read the pud once.
>> I use READ_ONCE here is just for safe, to prevents the complier mischief if
>> possible.
>
> FWIW, I'd be in favor of going the READ/WRITE_ONCE() route for x86, e.g.
> convert everything as a follow-up patch (or patches). I'm fairly confident
> that KVM's usage of lookup_address_in_mm() is safe, but I wouldn't exactly
> bet my life on it. I'd much rather the failing scenario be that KVM uses
> a sub-optimal page size as opposed to exploding on a bad pointer.
>
Um...our testcase starts 50 VMs with 2U4G(use 1G hugepage) and then do
live-upgrade(private feature that just modify the qemu and libvirt) and
live-migrate in turns for each one. However our live upgraded new QEMU won't do
touch_all_pages.
Suppose we start a VM without touch_all_pages in QEMU, the VM's guest memory is
not mapped in the CR3 pagetable at the moment. When the 2 vcpus running, they
could access some pages belong to the same 1G-hugepage, both of them will vmexit
due to ept_violation and then call gup-->follow_hugetlb_page-->hugetlb_fault, so
the race may encounter, right?
>> I'll add comments in v2.
>>
>>> [*] In kernel 5.6, KVM is moving to using lookup_address_in_pgd() (via
>>> lookup_address_in_mm()) to identify large page mappings. The function
>>> itself is susceptible to such a race, but KVM only does the lookup
>>> after it has done gup() and also ensures any zapping of ptes will cause
>>> KVM to restart the faulting (guest) instruction or that the zap will be
>>> blocked until after KVM does the lookup, i.e. racing with a transition
>>> from !PRESENT -> PRESENT should be impossible (in theory).
>>>
>> This bug is from hugetlb core, we could trigger it in other usages even if the
>> latest KVM won't.
>
> I was actually worried about the opposite, introducing a bug by moving to
> lookup_address_in_mm().
>
>>>> Signed-off-by: Longpeng(Mike) <longpeng2@xxxxxxxxxx>
>>>> ---
>>>> mm/hugetlb.c | 34 ++++++++++++++++++----------------
>>>> 1 file changed, 18 insertions(+), 16 deletions(-)
>>>>
>>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>>> index dd8737a..3bde229 100644
>>>> --- a/mm/hugetlb.c
>>>> +++ b/mm/hugetlb.c
>>>> @@ -4908,31 +4908,33 @@ pte_t *huge_pte_alloc(struct mm_struct *mm,
>>>> pte_t *huge_pte_offset(struct mm_struct *mm,
>>>> unsigned long addr, unsigned long sz)
>>>> {
>>>> - pgd_t *pgd;
>>>> - p4d_t *p4d;
>>>> - pud_t *pud;
>>>> - pmd_t *pmd;
>>>> + pgd_t *pgdp;
>>>> + p4d_t *p4dp;
>>>> + pud_t *pudp, pud;
>>>> + pmd_t *pmdp, pmd;
>>>>
>>>> - pgd = pgd_offset(mm, addr);
>>>> - if (!pgd_present(*pgd))
>>>> + pgdp = pgd_offset(mm, addr);
>>>> + if (!pgd_present(*pgdp))
>>>> return NULL;
>>>> - p4d = p4d_offset(pgd, addr);
>>>> - if (!p4d_present(*p4d))
>>>> + p4dp = p4d_offset(pgdp, addr);
>>>> + if (!p4d_present(*p4dp))
>>>> return NULL;
>>>>
>>>> - pud = pud_offset(p4d, addr);
>>>> - if (sz != PUD_SIZE && pud_none(*pud))
>>>> + pudp = pud_offset(p4dp, addr);
>>>> + pud = READ_ONCE(*pudp);
>>>> + if (sz != PUD_SIZE && pud_none(pud))
>>>> return NULL;
>>>> /* hugepage or swap? */
>>>> - if (pud_huge(*pud) || !pud_present(*pud))
>>>> - return (pte_t *)pud;
>>>> + if (pud_huge(pud) || !pud_present(pud))
>>>> + return (pte_t *)pudp;
>>>>
>>>> - pmd = pmd_offset(pud, addr);
>>>> - if (sz != PMD_SIZE && pmd_none(*pmd))
>>>> + pmdp = pmd_offset(pudp, addr);
>>>> + pmd = READ_ONCE(*pmdp);
>>>> + if (sz != PMD_SIZE && pmd_none(pmd))
>>>> return NULL;
>>>> /* hugepage or swap? */
>>>> - if (pmd_huge(*pmd) || !pmd_present(*pmd))
>>>> - return (pte_t *)pmd;
>>>> + if (pmd_huge(pmd) || !pmd_present(pmd))
>>>> + return (pte_t *)pmdp;
>>>>
>>>> return NULL;
>>>> }
>>>> --
>>>> 1.8.3.1
>>>>
>>>>
>>>
>>> .
>>>
>>
>>
>> --
>> Regards,
>> Longpeng(Mike)
>>
>
--
Regards,
Longpeng(Mike)
