在 2020/2/20 3:33, Mike Kravetz 写道: > + 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. > So... I need just fix huge_pte_offset in mm/hugetlb.c, right? Is it possible the pud changes from pud_huge() to pud_none() while another CPU is walking the pagetable ? -- Regards, Longpeng(Mike)
