The patch titled Subject: mm/hugetlb.c: fix a addressing exception caused by huge_pte_offset() has been added to the -mm tree. Its filename is mm-hugetlb-fix-a-addressing-exception-caused-by-huge_pte_offset.patch

This patch should soon appear at http://ozlabs.org/~akpm/mmots/broken-out/mm-hugetlb-fix-a-addressing-excepti... and later at http://ozlabs.org/~akpm/mmotm/broken-out/mm-hugetlb-fix-a-addressing-excepti...

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------------------------------------------------------ From: Longpeng longpeng2@huawei.com Subject: mm/hugetlb.c: fix a addressing exception caused by huge_pte_offset()

Our machine encountered a panic(addressing exception) after running for a long time. 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 digging 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 reading the pud only once. What's more, we also use READ_ONCE to access the entries for safety (i.e. avoid the compilier mischief)

Link: http://lkml.kernel.org/r/1582342427-230392-1-git-send-email-longpeng2@huawei... Signed-off-by: Longpeng longpeng2@huawei.com Cc: Matthew Wilcox willy@infradead.org Cc: Sean Christopherson sean.j.christopherson@intel.com Cc: Mike Kravetz mike.kravetz@oracle.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/hugetlb.c | 18 ++++++++++-------- 1 file changed, 10 insertions(+), 8 deletions(-)

--- a/mm/hugetlb.c~mm-hugetlb-fix-a-addressing-exception-caused-by-huge_pte_offset +++ a/mm/hugetlb.c @@ -4910,28 +4910,30 @@ pte_t *huge_pte_offset(struct mm_struct { pgd_t *pgd; p4d_t *p4d; - pud_t *pud; - pmd_t *pmd; + pud_t *pud, pud_entry; + pmd_t *pmd, pmd_entry;

pgd = pgd_offset(mm, addr); - if (!pgd_present(*pgd)) + if (!pgd_present(READ_ONCE(*pgd))) return NULL; p4d = p4d_offset(pgd, addr); - if (!p4d_present(*p4d)) + if (!p4d_present(READ_ONCE(*p4d))) return NULL;

pud = pud_offset(p4d, addr); - if (sz != PUD_SIZE && pud_none(*pud)) + pud_entry = READ_ONCE(*pud); + if (sz != PUD_SIZE && pud_none(pud_entry)) return NULL; /* hugepage or swap? */ - if (pud_huge(*pud) || !pud_present(*pud)) + if (pud_huge(pud_entry) || !pud_present(pud_entry)) return (pte_t *)pud;

pmd = pmd_offset(pud, addr); - if (sz != PMD_SIZE && pmd_none(*pmd)) + pmd_entry = READ_ONCE(*pmd); + if (sz != PMD_SIZE && pmd_none(pmd_entry)) return NULL; /* hugepage or swap? */ - if (pmd_huge(*pmd) || !pmd_present(*pmd)) + if (pmd_huge(pmd_entry) || !pmd_present(pmd_entry)) return (pte_t *)pmd;

return NULL; _

Patches currently in -mm which might be from longpeng2@huawei.com are

mm-hugetlb-fix-a-addressing-exception-caused-by-huge_pte_offset.patch