6810c247a1
The mailing list thread [0] (found by Friedrich, many thanks!) leading up to this patch sounds very familiar to issues users reported in the community forum [1] and enterprise support channel, where a VM would be stuck for no discernable reason with all vCPU threads spinning. [0]: https://lore.kernel.org/all/f023d927-52aa-7e08-2ee5-59a2fbc65953@gameservers.com/T/#u [1]: https://forum.proxmox.com/threads/127459/ Suggested-by: Friedrich Weber <f.weber@proxmox.com> Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
76 lines
3.6 KiB
Diff
76 lines
3.6 KiB
Diff
From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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From: Sean Christopherson <seanjc@google.com>
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Date: Wed, 23 Aug 2023 18:01:04 -0700
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Subject: [PATCH] KVM: x86/mmu: Fix an sign-extension bug with mmu_seq that
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hangs vCPUs
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MIME-Version: 1.0
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Content-Type: text/plain; charset=UTF-8
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Content-Transfer-Encoding: 8bit
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Upstream commit ba6e3fe25543 ("KVM: x86/mmu: Grab mmu_invalidate_seq in
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kvm_faultin_pfn()") unknowingly fixed the bug in v6.3 when refactoring
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how KVM tracks the sequence counter snapshot.
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Take the vCPU's mmu_seq snapshot as an "unsigned long" instead of an "int"
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when checking to see if a page fault is stale, as the sequence count is
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stored as an "unsigned long" everywhere else in KVM. This fixes a bug
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where KVM will effectively hang vCPUs due to always thinking page faults
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are stale, which results in KVM refusing to "fix" faults.
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mmu_invalidate_seq (née mmu_notifier_seq) is a sequence counter used when
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KVM is handling page faults to detect if userspace mappings relevant to
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the guest were invalidated between snapshotting the counter and acquiring
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mmu_lock, i.e. to ensure that the userspace mapping KVM is using to
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resolve the page fault is fresh. If KVM sees that the counter has
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changed, KVM simply resumes the guest without fixing the fault.
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What _should_ happen is that the source of the mmu_notifier invalidations
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eventually goes away, mmu_invalidate_seq becomes stable, and KVM can once
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again fix guest page fault(s).
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But for a long-lived VM and/or a VM that the host just doesn't particularly
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like, it's possible for a VM to be on the receiving end of 2 billion (with
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a B) mmu_notifier invalidations. When that happens, bit 31 will be set in
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mmu_invalidate_seq. This causes the value to be turned into a 32-bit
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negative value when implicitly cast to an "int" by is_page_fault_stale(),
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and then sign-extended into a 64-bit unsigned when the signed "int" is
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implicitly cast back to an "unsigned long" on the call to
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mmu_invalidate_retry_hva().
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As a result of the casting and sign-extension, given a sequence counter of
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e.g. 0x8002dc25, mmu_invalidate_retry_hva() ends up doing
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if (0x8002dc25 != 0xffffffff8002dc25)
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and signals that the page fault is stale and needs to be retried even
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though the sequence counter is stable, and KVM effectively hangs any vCPU
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that takes a page fault (EPT violation or #NPF when TDP is enabled).
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Reported-by: Brian Rak <brak@vultr.com>
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Reported-by: Amaan Cheval <amaan.cheval@gmail.com>
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Reported-by: Eric Wheeler <kvm@lists.ewheeler.net>
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Closes: https://lore.kernel.org/all/f023d927-52aa-7e08-2ee5-59a2fbc65953@gameservers.com
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Fixes: a955cad84cda ("KVM: x86/mmu: Retry page fault if root is invalidated by memslot update")
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Signed-off-by: Sean Christopherson <seanjc@google.com>
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Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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(cherry-picked from commit 82d811ff566594de3676f35808e8a9e19c5c864c in stable v6.1.51)
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Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
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---
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arch/x86/kvm/mmu/mmu.c | 3 ++-
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1 file changed, 2 insertions(+), 1 deletion(-)
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diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
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index 3220c1285984..c42ba5cde7a4 100644
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--- a/arch/x86/kvm/mmu/mmu.c
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+++ b/arch/x86/kvm/mmu/mmu.c
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@@ -4261,7 +4261,8 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
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* root was invalidated by a memslot update or a relevant mmu_notifier fired.
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*/
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static bool is_page_fault_stale(struct kvm_vcpu *vcpu,
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- struct kvm_page_fault *fault, int mmu_seq)
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+ struct kvm_page_fault *fault,
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+ unsigned long mmu_seq)
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{
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struct kvm_mmu_page *sp = to_shadow_page(vcpu->arch.mmu->root.hpa);
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