[PATCH] KVM: x86: Clamp the EOI vector if its OOB instead of bugging the kernel

[Sean Christopherson <seanjc@google.com>]

If KVM handles an I/O APIC EOI exit request with a bad vector, clamp the
vector to 255 and hope for the best instead of bugging the host.  In all
likelihood, a missed EOI is survivable for the guest, and it's most
definitely not remotely fatal to the host, i.e. potentially panicking the
host is completely unjustified.  Arbitrarily use 255 for the dummy vector,
the goal is purely to ensure the vector is covered by the bitmap.

Opportunistically ensure the EOI vector isn't negative, as it's a signed
integer, i.e. the "greater than 255" check won't guard against setting the
vector to a negative value (KVM uses -1 to say "no IRQ" in many flows).

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/x86.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index d9d51803b7b2..fda09e03b960 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11212,7 +11212,10 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 		if (kvm_check_request(KVM_REQ_NMI, vcpu))
 			process_nmi(vcpu);
 		if (kvm_check_request(KVM_REQ_IOAPIC_EOI_EXIT, vcpu)) {
-			BUG_ON(vcpu->arch.pending_ioapic_eoi > 255);
+			if (WARN_ON_ONCE(vcpu->arch.pending_ioapic_eoi < 0 ||
+					 vcpu->arch.pending_ioapic_eoi > 255))
+				vcpu->arch.pending_ioapic_eoi = 255;
+
 			if (test_bit(vcpu->arch.pending_ioapic_eoi,
 				     vcpu->arch.ioapic_handled_vectors)) {
 				vcpu->run->exit_reason = KVM_EXIT_IOAPIC_EOI;

base-commit: 9d4853b044beefa21c4ee3e18c40653601a64ced
-- 
2.55.0.rc0.738.g0c8ab3ebcc-goog