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Thu, 06 Aug 2026 14:46:22 -0700 (PDT) Date: Thu, 6 Aug 2026 14:46:18 -0700 Mime-Version: 1.0 X-Mailer: git-send-email 2.55.0.679.g6767b8d81c-goog Message-ID: <20260806214618.82180-1-seanjc@google.com> Subject: [PATCH] KVM: Never clear KVM_REQ_VM_DEAD from a vCPU's requests From: Sean Christopherson To: Marc Zyngier , Oliver Upton , Sean Christopherson , Paolo Bonzini Cc: Joey Gouly , Steffen Eiden , Suzuki K Poulose , Zenghui Yu , linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, kvm@vger.kernel.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260806_144624_817834_5C002B6C X-CRM114-Status: GOOD ( 15.89 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Sean Christopherson Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Use kvm_test_request() instead of kvm_check_request() when querying KVM_REQ_VM_DEAD, i.e. don't clear KVM_REQ_VM_DEAD, as the entire purpose of KVM_REQ_VM_DEAD is to prevent the vCPU from enterring the guest ever again, even if userspace insists on redoing KVM_RUN. Ensuring KVM_REQ_VM_DEAD is never cleared will allow relaxing KVM's rule that ioctls can't be invoked on dead VMs, to only disallow ioctls if the VM is bugged, i.e. if KVM hit a KVM_BUG_ON(). Opportunistically add compile-time assertions to guard against clearing KVM_REQ_VM_DEAD through the standard APIs. Signed-off-by: Sean Christopherson --- This was previously posted as part of a series to add an ioctl to allow userspace to terminate a TDX VM. I'm posting it separately because it fixes a very real flaw, although the flaw is probably benign in practice? I don't want to find out though, i.e. I don't want to wait to get this merged. This came back on my radar when fixing a bug in x86's pre-fault code, and looking at that code made it pretty clear that clearing KVM_REQ_VM_DEAD could make a bad situation worse. arch/arm64/kvm/arm.c | 2 +- arch/x86/kvm/mmu/mmu.c | 2 +- arch/x86/kvm/vmx/tdx.c | 2 +- arch/x86/kvm/x86.c | 2 +- include/linux/kvm_host.h | 9 +++++++-- 5 files changed, 11 insertions(+), 6 deletions(-) diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c index 50adfff75be8..a56ead423897 100644 --- a/arch/arm64/kvm/arm.c +++ b/arch/arm64/kvm/arm.c @@ -1108,7 +1108,7 @@ static int kvm_vcpu_suspend(struct kvm_vcpu *vcpu) static int check_vcpu_requests(struct kvm_vcpu *vcpu) { if (kvm_request_pending(vcpu)) { - if (kvm_check_request(KVM_REQ_VM_DEAD, vcpu)) + if (kvm_test_request(KVM_REQ_VM_DEAD, vcpu)) return -EIO; if (kvm_check_request(KVM_REQ_SLEEP, vcpu)) diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index c519e8e8d646..ec34caa06b6a 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -5033,7 +5033,7 @@ static int kvm_tdp_page_prefault(struct kvm_vcpu *vcpu, gpa_t gpa, if (signal_pending(current)) return -EINTR; - if (kvm_check_request(KVM_REQ_VM_DEAD, vcpu)) + if (kvm_test_request(KVM_REQ_VM_DEAD, vcpu)) return -EIO; cond_resched(); diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c index b272c20586a7..6c842e9191a5 100644 --- a/arch/x86/kvm/vmx/tdx.c +++ b/arch/x86/kvm/vmx/tdx.c @@ -1995,7 +1995,7 @@ static int tdx_handle_ept_violation(struct kvm_vcpu *vcpu) if (kvm_vcpu_has_events(vcpu) || signal_pending(current)) break; - if (kvm_check_request(KVM_REQ_VM_DEAD, vcpu)) { + if (kvm_test_request(KVM_REQ_VM_DEAD, vcpu)) { ret = -EIO; break; } diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index d94b59140c45..0af4b4a95c2d 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -8060,7 +8060,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu) bool req_immediate_exit = false; if (kvm_request_pending(vcpu)) { - if (kvm_check_request(KVM_REQ_VM_DEAD, vcpu)) { + if (kvm_test_request(KVM_REQ_VM_DEAD, vcpu)) { r = -EIO; goto out; } diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index 03bfc92864b6..cf7fe835c4ad 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -2324,13 +2324,18 @@ static inline bool kvm_test_request(int req, struct kvm_vcpu *vcpu) return test_bit(req & KVM_REQUEST_MASK, (void *)&vcpu->requests); } -static inline void kvm_clear_request(int req, struct kvm_vcpu *vcpu) +static __always_inline void kvm_clear_request(int req, struct kvm_vcpu *vcpu) { + BUILD_BUG_ON(req == KVM_REQ_VM_DEAD); + clear_bit(req & KVM_REQUEST_MASK, (void *)&vcpu->requests); } -static inline bool kvm_check_request(int req, struct kvm_vcpu *vcpu) +static __always_inline bool kvm_check_request(int req, struct kvm_vcpu *vcpu) { + /* Once a VM is dead, it needs to stay dead. */ + BUILD_BUG_ON(req == KVM_REQ_VM_DEAD); + if (kvm_test_request(req, vcpu)) { kvm_clear_request(req, vcpu); base-commit: a806d364ef288a6443a1337820ea8410a7ccc6b3 -- 2.55.0.679.g6767b8d81c-goog