+ Kirill On 2/18/20 5:58 PM, Sean Christopherson wrote: > 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. Longpeng(Mike) asked in another e-mail specifically about making similar changes to lookup_address_in_mm(). Replying here as there is more context. I 'think' lookup_address_in_mm is safe from this issue. Why? IIUC, the problem with the huge_pte_offset routine is that the pud changes from pud_none() to pud_huge() in the middle of 'if (pud_huge(*pud) || !pud_present(*pud))'. In the case of lookup_address_in_mm, we know pud was not pud_none() as it was previously checked. I am not aware of any other state transitions which could cause us trouble. However, I am no expert in this area. -- Mike Kravetz
