Ignore -EBUSY when checking nested events after exiting a blocking state
while L2 is active, as exiting to userspace will generate a spurious
userspace exit, usually with KVM_EXIT_UNKNOWN, and likely lead to the VM's
demise.  Continuing with the wakeup isn't perfect either, as *something*
has gone sideways if a vCPU is awakened in L2 with an injected event (or
worse, a nested run pending), but continuing on gives the VM a decent
chance of surviving without any major side effects.

As explained in the Fixes commits, it _should_ be impossible for a vCPU to
be put into a blocking state with an already-injected event (exception,
IRQ, or NMI).  Unfortunately, userspace can stuff MP_STATE and/or injected
events, and thus put the vCPU into what should be an impossible state.

Don't bother trying to preserve the WARN, e.g. with an anti-syzkaller
Kconfig, as WARNs can (hopefully) be added in paths where _KVM_ would be
violating x86 architecture, e.g. by WARNing if KVM attempts to inject an
exception or interrupt while the vCPU isn't running.

Cc: Alessandro Ratti <alessandro@0x65c.net>
Cc: stable@vger.kernel.org
Fixes: 26844fee6ade ("KVM: x86: never write to memory from kvm_vcpu_check_block()")
Fixes: 45405155d876 ("KVM: x86: WARN if a vCPU gets a valid wakeup that KVM can't yet inject")
Link: https://syzkaller.appspot.com/text?tag=ReproC&x=10d4261a580000
Reported-by: syzbot+1522459a74d26b0ac33a@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/671bc7a7.050a0220.455e8.022a.GAE@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/x86.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index ff8812f3a129..4bf9be1e17a7 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11596,8 +11596,7 @@ static inline int vcpu_block(struct kvm_vcpu *vcpu)
 	if (is_guest_mode(vcpu)) {
 		int r = kvm_check_nested_events(vcpu);
 
-		WARN_ON_ONCE(r == -EBUSY);
-		if (r < 0)
+		if (r < 0 && r != -EBUSY)
 			return 0;
 	}
 

base-commit: 9448598b22c50c8a5bb77a9103e2d49f134c9578
-- 
2.52.0.457.g6b5491de43-goog

On Thu, 08 Jan 2026 19:06:57 -0800, Sean Christopherson wrote:
> Ignore -EBUSY when checking nested events after exiting a blocking state
> while L2 is active, as exiting to userspace will generate a spurious
> userspace exit, usually with KVM_EXIT_UNKNOWN, and likely lead to the VM's
> demise.  Continuing with the wakeup isn't perfect either, as *something*
> has gone sideways if a vCPU is awakened in L2 with an injected event (or
> worse, a nested run pending), but continuing on gives the VM a decent
> chance of surviving without any major side effects.
> 
> [...]

Applied to kvm-x86 misc, thanks!

[1/1] KVM: x86: Ignore -EBUSY when checking nested events from vcpu_block()
      https://github.com/kvm-x86/linux/commit/ead63640d4e7

--
https://github.com/kvm-x86/linux/tree/next