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Thu, 02 Apr 2026 20:11:11 -0700 (PDT) From: Ethan Yang To: kvm@vger.kernel.org Cc: seanjc@google.com, pbonzini@redhat.com, syzbot+bc0e18379a290e5edfe4@syzkaller.appspotmail.com, linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com, x86@kernel.org, tglx@kernel.org, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, glider@google.com Subject: [PATCH] KVM: x86: Don't leave APF half-enabled on bad APF data GPA Date: Fri, 3 Apr 2026 03:11:11 +0000 Message-ID: <20260403031111.3171-1-ethan.yang.kernel@gmail.com> X-Mailer: git-send-email @GIT_VERSION@ In-Reply-To: <6877331d.a00a0220.3af5df.000c.GAE@google.com> References: <6877331d.a00a0220.3af5df.000c.GAE@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit kvm_pv_enable_async_pf() updates vcpu->arch.apf.msr_en_val before initializing the APF data gfn_to_hva cache. If userspace provides an invalid GPA, kvm_gfn_to_hva_cache_init() fails, but msr_en_val stays enabled and leaves APF state half-initialized. Later APF paths can then try to use the empty cache and trigger WARN_ON() in kvm_read_guest_offset_cached(). Determine the new APF enabled state from the incoming MSR value, do cache initialization first on the enable path, and commit msr_en_val only after successful initialization. Keep the disable path behavior unchanged. Reported-by: syzbot+bc0e18379a290e5edfe4@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=bc0e18379a290e5edfe4 Link: https://lore.kernel.org/r/aHfD3MczrDpzDX9O@google.com Suggested-by: Sean Christopherson Signed-off-by: Ethan Yang --- arch/x86/kvm/x86.c | 19 +++++++++++++------ 1 file changed, 13 insertions(+), 6 deletions(-) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index fd1c4a36b59..c355bbd36c3 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -1041,11 +1041,15 @@ bool kvm_require_dr(struct kvm_vcpu *vcpu, int dr) } EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_require_dr); -static bool kvm_pv_async_pf_enabled(struct kvm_vcpu *vcpu) +static bool kvm_pv_async_pf_enabled_data(u64 data) { u64 mask = KVM_ASYNC_PF_ENABLED | KVM_ASYNC_PF_DELIVERY_AS_INT; + return (data & mask) == mask; +} - return (vcpu->arch.apf.msr_en_val & mask) == mask; +static bool kvm_pv_async_pf_enabled(struct kvm_vcpu *vcpu) +{ + return kvm_pv_async_pf_enabled_data(vcpu->arch.apf.msr_en_val); } static inline u64 pdptr_rsvd_bits(struct kvm_vcpu *vcpu) @@ -3645,17 +3649,20 @@ static int kvm_pv_enable_async_pf(struct kvm_vcpu *vcpu, u64 data) if (!lapic_in_kernel(vcpu)) return data ? 1 : 0; + bool new_enabled = kvm_pv_async_pf_enabled_data(data); + + if (new_enabled && + kvm_gfn_to_hva_cache_init(vcpu->kvm, &vcpu->arch.apf.data, gpa, + sizeof(u64))) + return 1; vcpu->arch.apf.msr_en_val = data; - if (!kvm_pv_async_pf_enabled(vcpu)) { + if (!new_enabled) { kvm_clear_async_pf_completion_queue(vcpu); kvm_async_pf_hash_reset(vcpu); return 0; } - if (kvm_gfn_to_hva_cache_init(vcpu->kvm, &vcpu->arch.apf.data, gpa, - sizeof(u64))) - return 1; vcpu->arch.apf.send_always = (data & KVM_ASYNC_PF_SEND_ALWAYS); vcpu->arch.apf.delivery_as_pf_vmexit = data & KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT; -- 2.47.3