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. > 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) >