[PATCH 6.12 054/265] KVM: x86: Ignore -EBUSY when checking nested events from vcpu_block()

Greg Kroah-Hartman
2026-03-12T20:17:09Z

From: Sean Christopherson <seanjc@google.com>

[ Upstream commit ead63640d4e72e6f6d464f4e31f7fecb79af8869 ]

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
Link: https://patch.msgid.link/20260109030657.994759-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 8f673aaa0490f..0d9035993ed36 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11285,8 +11285,7 @@ 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;
}

--
2.51.0