From 17fb2cfe978ed1525d1b8d56cde6bb5ca7130c10 Mon Sep 17 00:00:00 2001 From: Andrew Lukoshko Date: Thu, 30 Nov 2023 14:46:50 +0000 Subject: [PATCH] KVM: x86/mmu: Fix an sign-extension bug with mmu_seq that hangs vCPUs --- ...an-sign-extension-bug-with-mmu_seq-t.patch | 76 +++++++++++++++++++ SPECS/kernel.spec | 3 + 2 files changed, 79 insertions(+) create mode 100644 SOURCES/1100-KVM-x86-mmu-Fix-an-sign-extension-bug-with-mmu_seq-t.patch diff --git a/SOURCES/1100-KVM-x86-mmu-Fix-an-sign-extension-bug-with-mmu_seq-t.patch b/SOURCES/1100-KVM-x86-mmu-Fix-an-sign-extension-bug-with-mmu_seq-t.patch new file mode 100644 index 0000000..b0fd224 --- /dev/null +++ b/SOURCES/1100-KVM-x86-mmu-Fix-an-sign-extension-bug-with-mmu_seq-t.patch @@ -0,0 +1,76 @@ +From 82d811ff566594de3676f35808e8a9e19c5c864c Mon Sep 17 00:00:00 2001 +From: Sean Christopherson +Date: Wed, 23 Aug 2023 18:01:04 -0700 +Subject: [PATCH] KVM: x86/mmu: Fix an sign-extension bug with mmu_seq that + hangs vCPUs +MIME-Version: 1.0 +Content-Type: text/plain; charset=UTF-8 +Content-Transfer-Encoding: 8bit + +Upstream commit ba6e3fe25543 ("KVM: x86/mmu: Grab mmu_invalidate_seq in +kvm_faultin_pfn()") unknowingly fixed the bug in v6.3 when refactoring +how KVM tracks the sequence counter snapshot. + +Take the vCPU's mmu_seq snapshot as an "unsigned long" instead of an "int" +when checking to see if a page fault is stale, as the sequence count is +stored as an "unsigned long" everywhere else in KVM. This fixes a bug +where KVM will effectively hang vCPUs due to always thinking page faults +are stale, which results in KVM refusing to "fix" faults. + +mmu_invalidate_seq (née mmu_notifier_seq) is a sequence counter used when +KVM is handling page faults to detect if userspace mappings relevant to +the guest were invalidated between snapshotting the counter and acquiring +mmu_lock, i.e. to ensure that the userspace mapping KVM is using to +resolve the page fault is fresh. If KVM sees that the counter has +changed, KVM simply resumes the guest without fixing the fault. + +What _should_ happen is that the source of the mmu_notifier invalidations +eventually goes away, mmu_invalidate_seq becomes stable, and KVM can once +again fix guest page fault(s). + +But for a long-lived VM and/or a VM that the host just doesn't particularly +like, it's possible for a VM to be on the receiving end of 2 billion (with +a B) mmu_notifier invalidations. When that happens, bit 31 will be set in +mmu_invalidate_seq. This causes the value to be turned into a 32-bit +negative value when implicitly cast to an "int" by is_page_fault_stale(), +and then sign-extended into a 64-bit unsigned when the signed "int" is +implicitly cast back to an "unsigned long" on the call to +mmu_invalidate_retry_hva(). + +As a result of the casting and sign-extension, given a sequence counter of +e.g. 0x8002dc25, mmu_invalidate_retry_hva() ends up doing + + if (0x8002dc25 != 0xffffffff8002dc25) + +and signals that the page fault is stale and needs to be retried even +though the sequence counter is stable, and KVM effectively hangs any vCPU +that takes a page fault (EPT violation or #NPF when TDP is enabled). + +Reported-by: Brian Rak +Reported-by: Amaan Cheval +Reported-by: Eric Wheeler +Closes: https://lore.kernel.org/all/f023d927-52aa-7e08-2ee5-59a2fbc65953@gameservers.com +Fixes: a955cad84cda ("KVM: x86/mmu: Retry page fault if root is invalidated by memslot update") +Signed-off-by: Sean Christopherson +Signed-off-by: Greg Kroah-Hartman +--- + arch/x86/kvm/mmu/mmu.c | 3 ++- + 1 file changed, 2 insertions(+), 1 deletion(-) + +diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c +index 230108a90cf3..beca03556379 100644 +--- a/arch/x86/kvm/mmu/mmu.c ++++ b/arch/x86/kvm/mmu/mmu.c +@@ -4212,7 +4212,8 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault) + * root was invalidated by a memslot update or a relevant mmu_notifier fired. + */ + static bool is_page_fault_stale(struct kvm_vcpu *vcpu, +- struct kvm_page_fault *fault, int mmu_seq) ++ struct kvm_page_fault *fault, ++ unsigned long mmu_seq) + { + struct kvm_mmu_page *sp = to_shadow_page(vcpu->arch.mmu->root.hpa); + +-- +2.27.0 + diff --git a/SPECS/kernel.spec b/SPECS/kernel.spec index 0c19a7f..5701fd1 100644 --- a/SPECS/kernel.spec +++ b/SPECS/kernel.spec @@ -527,6 +527,8 @@ Patch999999: linux-kernel-test.patch Patch1000: debrand-single-cpu.patch Patch1002: debrand-rh-i686-cpu.patch +Patch1100: 1100-KVM-x86-mmu-Fix-an-sign-extension-bug-with-mmu_seq-t.patch + # END OF PATCH DEFINITIONS BuildRoot: %{_tmppath}/%{name}-%{KVERREL}-root @@ -1092,6 +1094,7 @@ cd linux-%{KVERREL} ApplyOptionalPatch debrand-single-cpu.patch ApplyOptionalPatch debrand-rh-i686-cpu.patch +ApplyOptionalPatch 1100-KVM-x86-mmu-Fix-an-sign-extension-bug-with-mmu_seq-t.patch ApplyOptionalPatch linux-kernel-test.patch # END OF PATCH APPLICATIONS