On 2020/3/22 7:38, Mike Kravetz wrote: > On 2/21/20 7:33 PM, Longpeng(Mike) wrote: >> From: Longpeng <longpeng2@xxxxxxxxxx> >> >> Our machine encountered a panic(addressing exception) after run >> for a long time and the calltrace is: >> 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. What's more, we also use >> READ_ONCE to access the entries for safe(e.g. avoid the compilier mischief) >> >> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> >> Cc: Sean Christopherson <sean.j.christopherson@xxxxxxxxx> >> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx> >> Cc: stable@xxxxxxxxxxxxxxx >> Signed-off-by: Longpeng <longpeng2@xxxxxxxxxx> > > Andrew dropped this patch from his tree which caused me to go back and > look at the status of this patch/issue. > > It is pretty obvious that code in the current huge_pte_offset routine > is racy. I checked out the assembly code produced by my compiler and > verified that the line, > > if (pud_huge(*pud) || !pud_present(*pud)) > > does actually dereference *pud twice. So, the value could change between > those two dereferences. Longpeng (Mike) could easlily recreate the issue > if he put a delay between the two dereferences. I believe the only > reservations/concerns about the patch below was the use of READ_ONCE(). > Is that correct? > Hi Mike, It seems I've missed your another mail in my client, I found it here (https://lkml.org/lkml/2020/2/27/1927) just now. I think we have reached an agreement that the pud/pmd need READ_ONCE in huge_pte_offset() and disagreement is whether the pgd/p4d also need READ_ONCE, right ? > Are there any objections to the patch if the READ_ONCE() calls are removed? > Because the pgd/p4g are only accessed and dereferenced once here, so some guys want to remove it. But we must make sure they are *really* accessed once, in other words, this makes we need to care about both the implementation of pgd_present/p4d_present and the behavior of any compiler, for example: ''' static inline int func(int val) { return subfunc1(val) & subfunc2(val); } func(*p); // int *p ''' We must make sure there's no strange compiler to generate an assemble code that access and dereference 'p' more than once. I've not found any backwards with READ_ONCE here. However, if you also agree to remove READ_ONCE around pgd/p4d, I'll do. > Longpeng (Mike), can you recreate the issue by adding the delay and removing > the READ_ONCE() calls? > I think remove the READ_ONCE around pgd/p4d won't cause any fucntional change. --- Regards, Longpeng(Mike)
