KVM: SVM: Don't inject #UD if KVM attempts to skip SEV guest insn

Don't inject a #UD if KVM attempts to "emulate" to skip an instruction
for an SEV guest, and instead resume the guest and hope that it can make
forward progress.  When commit 04c40f3 ("KVM: SVM: Inject #UD on
attempted emulation for SEV guest w/o insn buffer") added the completely
arbitrary #UD behavior, there were no known scenarios where a well-behaved
guest would induce a VM-Exit that triggered emulation, i.e. it was thought
that injecting #UD would be helpful.

However, now that KVM (correctly) attempts to re-inject INT3/INTO, e.g. if
a #NPF is encountered when attempting to deliver the INT3/INTO, an SEV
guest can trigger emulation without a buffer, through no fault of its own.
Resuming the guest and retrying the INT3/INTO is architecturally wrong,
e.g. the vCPU will incorrectly re-hit code #DBs, but for SEV guests there
is literally no other option that has a chance of making forward progress.

Drop the #UD injection for all "skip" emulation, not just those related to
INT3/INTO, even though that means that the guest will likely end up in an
infinite loop instead of getting a #UD (the vCPU may also crash, e.g. if
KVM emulated everything about an instruction except for advancing RIP).
There's no evidence that suggests that an unexpected #UD is actually
better than hanging the vCPU, e.g. a soft-hung vCPU can still respond to
IRQs and NMIs to generate a backtrace.

Reported-by: Wu Zongyo <wuzongyo@mail.ustc.edu.cn>
Closes: https://lore.kernel.org/all/8eb933fd-2cf3-d7a9-32fe-2a1d82eac42a@mail.ustc.edu.cn
Fixes: 6ef88d6 ("KVM: SVM: Re-inject INT3/INTO instead of retrying the instruction")
Cc: stable@vger.kernel.org
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20230825013621.2845700-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/svm/svm.c

@@ -365,6 +365,8 @@ static void svm_set_interrupt_shadow(struct kvm_vcpu *vcpu, int mask)
 		svm->vmcb->control.int_state |= SVM_INTERRUPT_SHADOW_MASK;
 
 }
+static bool svm_can_emulate_instruction(struct kvm_vcpu *vcpu, int emul_type,
+					void *insn, int insn_len);
 
 static int __svm_skip_emulated_instruction(struct kvm_vcpu *vcpu,
 					   bool commit_side_effects)
@@ -385,6 +387,14 @@ static int __svm_skip_emulated_instruction(struct kvm_vcpu *vcpu,
 	}
 
 	if (!svm->next_rip) {
+		/*
+		 * FIXME: Drop this when kvm_emulate_instruction() does the
+		 * right thing and treats "can't emulate" as outright failure
+		 * for EMULTYPE_SKIP.
+		 */
+		if (!svm_can_emulate_instruction(vcpu, EMULTYPE_SKIP, NULL, 0))
+			return 0;
+
 		if (unlikely(!commit_side_effects))
 			old_rflags = svm->vmcb->save.rflags;
 
@@ -4677,16 +4687,25 @@ static bool svm_can_emulate_instruction(struct kvm_vcpu *vcpu, int emul_type,
 	 * and cannot be decrypted by KVM, i.e. KVM would read cyphertext and
 	 * decode garbage.
 	 *
-	 * Inject #UD if KVM reached this point without an instruction buffer.
-	 * In practice, this path should never be hit by a well-behaved guest,
-	 * e.g. KVM doesn't intercept #UD or #GP for SEV guests, but this path
-	 * is still theoretically reachable, e.g. via unaccelerated fault-like
-	 * AVIC access, and needs to be handled by KVM to avoid putting the
-	 * guest into an infinite loop.   Injecting #UD is somewhat arbitrary,
-	 * but its the least awful option given lack of insight into the guest.
+	 * If KVM is NOT trying to simply skip an instruction, inject #UD if
+	 * KVM reached this point without an instruction buffer.  In practice,
+	 * this path should never be hit by a well-behaved guest, e.g. KVM
+	 * doesn't intercept #UD or #GP for SEV guests, but this path is still
+	 * theoretically reachable, e.g. via unaccelerated fault-like AVIC
+	 * access, and needs to be handled by KVM to avoid putting the guest
+	 * into an infinite loop.   Injecting #UD is somewhat arbitrary, but
+	 * its the least awful option given lack of insight into the guest.
+	 *
+	 * If KVM is trying to skip an instruction, simply resume the guest.
+	 * If a #NPF occurs while the guest is vectoring an INT3/INTO, then KVM
+	 * will attempt to re-inject the INT3/INTO and skip the instruction.
+	 * In that scenario, retrying the INT3/INTO and hoping the guest will
+	 * make forward progress is the only option that has a chance of
+	 * success (and in practice it will work the vast majority of the time).
 	 */
 	if (unlikely(!insn)) {
-		kvm_queue_exception(vcpu, UD_VECTOR);
+		if (!(emul_type & EMULTYPE_SKIP))
+			kvm_queue_exception(vcpu, UD_VECTOR);
 		return false;
 	}