KVM: x86: Defer non-architectural deliver of exception payload to userspace read

When attempting to play nice with userspace that hasn't enabled
KVM_CAP_EXCEPTION_PAYLOAD, defer KVM's non-architectural delivery of the
payload until userspace actually reads relevant vCPU state, and more
importantly, force delivery of the payload in *all* paths where userspace
saves relevant vCPU state, not just KVM_GET_VCPU_EVENTS.

Ignoring userspace save/restore for the moment, delivering the payload
before the exception is injected is wrong regardless of whether L1 or L2
is running.  To make matters even more confusing, the flaw *currently*
being papered over by the !is_guest_mode() check isn't even the same bug
that commit da998b46d244 ("kvm: x86: Defer setting of CR2 until #PF
delivery") was trying to avoid.

At the time of commit da998b46d244, KVM didn't correctly handle exception
intercepts, as KVM would wait until VM-Entry into L2 was imminent to check
if the queued exception should morph to a nested VM-Exit.  I.e. KVM would
deliver the payload to L2 and then synthesize a VM-Exit into L1.  But the
payload was only the most blatant issue, e.g. waiting to check exception
intercepts would also lead to KVM incorrectly escalating a
should-be-intercepted #PF into a #DF.

That underlying bug was eventually fixed by commit 7709aba8f716 ("KVM: x86:
Morph pending exceptions to pending VM-Exits at queue time"), but in the
interim, commit a06230b62b89 ("KVM: x86: Deliver exception payload on
KVM_GET_VCPU_EVENTS") came along and subtly added another dependency on
the !is_guest_mode() check.

While not recorded in the changelog, the motivation for deferring the
!exception_payload_enabled delivery was to fix a flaw where a synthesized
MTF (Monitor Trap Flag) VM-Exit would drop a pending #DB and clobber DR6.
On a VM-Exit, VMX CPUs save pending #DB information into the VMCS, which
is emulated by KVM in nested_vmx_update_pending_dbg() by grabbing the
payload from the queue/pending exception.  I.e. prematurely delivering the
payload would cause the pending #DB to not be recorded in the VMCS, and of
course, clobber L2's DR6 as seen by L1.

Jumping back to save+restore, the quirked behavior of forcing delivery of
the payload only works if userspace does KVM_GET_VCPU_EVENTS *before*
CR2 or DR6 is saved, i.e. before KVM_GET_SREGS{,2} and KVM_GET_DEBUGREGS.
E.g. if userspace does KVM_GET_SREGS before KVM_GET_VCPU_EVENTS, then the
CR2 saved by userspace won't contain the payload for the exception save by
KVM_GET_VCPU_EVENTS.

Deliberately deliver the payload in the store_regs() path, as it's the
least awful option even though userspace may not be doing save+restore.
Because if userspace _is_ doing save restore, it could elide KVM_GET_SREGS
knowing that SREGS were already saved when the vCPU exited.

Link: https://lore.kernel.org/all/20200207103608.110305-1-oupton@google.com
Cc: Yosry Ahmed <yosry.ahmed@linux.dev>
Cc: stable@vger.kernel.org
Reviewed-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Tested-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Link: https://patch.msgid.link/20260218005438.2619063-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 39 insertions, 23 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a03530795707..6e87ec52fa06 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -864,9 +864,6 @@ static void kvm_multiple_exception(struct kvm_vcpu *vcpu, unsigned int nr,
 		vcpu->arch.exception.error_code = error_code;
 		vcpu->arch.exception.has_payload = has_payload;
 		vcpu->arch.exception.payload = payload;
-		if (!is_guest_mode(vcpu))
-			kvm_deliver_exception_payload(vcpu,
-						      &vcpu->arch.exception);
 		return;
 	}
 
@@ -5531,18 +5528,8 @@ static int kvm_vcpu_ioctl_x86_set_mce(struct kvm_vcpu *vcpu,
 	return 0;
 }
 
-static void kvm_vcpu_ioctl_x86_get_vcpu_events(struct kvm_vcpu *vcpu,
-					       struct kvm_vcpu_events *events)
+static struct kvm_queued_exception *kvm_get_exception_to_save(struct kvm_vcpu *vcpu)
 {
-	struct kvm_queued_exception *ex;
-
-	process_nmi(vcpu);
-
-#ifdef CONFIG_KVM_SMM
-	if (kvm_check_request(KVM_REQ_SMI, vcpu))
-		process_smi(vcpu);
-#endif
-
 	/*
 	 * KVM's ABI only allows for one exception to be migrated.  Luckily,
 	 * the only time there can be two queued exceptions is if there's a
@@ -5553,21 +5540,46 @@ static void kvm_vcpu_ioctl_x86_get_vcpu_events(struct kvm_vcpu *vcpu,
 	if (vcpu->arch.exception_vmexit.pending &&
 	    !vcpu->arch.exception.pending &&
 	    !vcpu->arch.exception.injected)
-		ex = &vcpu->arch.exception_vmexit;
-	else
-		ex = &vcpu->arch.exception;
+		return &vcpu->arch.exception_vmexit;
+
+	return &vcpu->arch.exception;
+}
+
+static void kvm_handle_exception_payload_quirk(struct kvm_vcpu *vcpu)
+{
+	struct kvm_queued_exception *ex = kvm_get_exception_to_save(vcpu);
 
 	/*
-	 * In guest mode, payload delivery should be deferred if the exception
-	 * will be intercepted by L1, e.g. KVM should not modifying CR2 if L1
-	 * intercepts #PF, ditto for DR6 and #DBs.  If the per-VM capability,
-	 * KVM_CAP_EXCEPTION_PAYLOAD, is not set, userspace may or may not
-	 * propagate the payload and so it cannot be safely deferred.  Deliver
-	 * the payload if the capability hasn't been requested.
+	 * If KVM_CAP_EXCEPTION_PAYLOAD is disabled, then (prematurely) deliver
+	 * the pending exception payload when userspace saves *any* vCPU state
+	 * that interacts with exception payloads to avoid breaking userspace.
+	 *
+	 * Architecturally, KVM must not deliver an exception payload until the
+	 * exception is actually injected, e.g. to avoid losing pending #DB
+	 * information (which VMX tracks in the VMCS), and to avoid clobbering
+	 * state if the exception is never injected for whatever reason.  But
+	 * if KVM_CAP_EXCEPTION_PAYLOAD isn't enabled, then userspace may or
+	 * may not propagate the payload across save+restore, and so KVM can't
+	 * safely defer delivery of the payload.
 	 */
 	if (!vcpu->kvm->arch.exception_payload_enabled &&
 	    ex->pending && ex->has_payload)
 		kvm_deliver_exception_payload(vcpu, ex);
+}
+
+static void kvm_vcpu_ioctl_x86_get_vcpu_events(struct kvm_vcpu *vcpu,
+					       struct kvm_vcpu_events *events)
+{
+	struct kvm_queued_exception *ex = kvm_get_exception_to_save(vcpu);
+
+	process_nmi(vcpu);
+
+#ifdef CONFIG_KVM_SMM
+	if (kvm_check_request(KVM_REQ_SMI, vcpu))
+		process_smi(vcpu);
+#endif
+
+	kvm_handle_exception_payload_quirk(vcpu);
 
 	memset(events, 0, sizeof(*events));
 
@@ -5746,6 +5758,8 @@ static int kvm_vcpu_ioctl_x86_get_debugregs(struct kvm_vcpu *vcpu,
 	    vcpu->arch.guest_state_protected)
 		return -EINVAL;
 
+	kvm_handle_exception_payload_quirk(vcpu);
+
 	memset(dbgregs, 0, sizeof(*dbgregs));
 
 	BUILD_BUG_ON(ARRAY_SIZE(vcpu->arch.db) != ARRAY_SIZE(dbgregs->db));
@@ -12136,6 +12150,8 @@ static void __get_sregs_common(struct kvm_vcpu *vcpu, struct kvm_sregs *sregs)
 	if (vcpu->arch.guest_state_protected)
 		goto skip_protected_regs;
 
+	kvm_handle_exception_payload_quirk(vcpu);
+
 	kvm_get_segment(vcpu, &sregs->cs, VCPU_SREG_CS);
 	kvm_get_segment(vcpu, &sregs->ds, VCPU_SREG_DS);
 	kvm_get_segment(vcpu, &sregs->es, VCPU_SREG_ES);