linux-kernel - [PATCH 2/2] KVM: selftests: Explicit set #UD when *potentially* injecting exception

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Date:   Thu, 17 Aug 2023 16:34:30 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Michal Luczaj <mhal@...x.co>
Subject: [PATCH 2/2] KVM: selftests: Explicit set #UD when *potentially*
 injecting exception

Explicitly set the exception vector to #UD when potentially injecting an
exception in sync_regs_test's subtests that try to detect TOCTOU bugs
in KVM's handling of exceptions injected by userspace.  A side effect of
the original KVM bug was that KVM would clear the vector, but relying on
KVM to clear the vector (i.e. make it #DE) makes it less likely that the
test would ever find *new* KVM bugs, e.g. because only the first iteration
would run with a legal vector to start.

Explicitly inject #UD for race_events_inj_pen() as well, e.g. so that it
doesn't inherit the illegal 255 vector from race_events_exc(), which
currently runs first.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 tools/testing/selftests/kvm/x86_64/sync_regs_test.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tools/testing/selftests/kvm/x86_64/sync_regs_test.c b/tools/testing/selftests/kvm/x86_64/sync_regs_test.c
index 21e99dae2ff2..00965ba33f73 100644
--- a/tools/testing/selftests/kvm/x86_64/sync_regs_test.c
+++ b/tools/testing/selftests/kvm/x86_64/sync_regs_test.c
@@ -91,6 +91,8 @@ static void *race_events_inj_pen(void *arg)
 	struct kvm_run *run = (struct kvm_run *)arg;
 	struct kvm_vcpu_events *events = &run->s.regs.events;

+	WRITE_ONCE(events->exception.nr, UD_VECTOR);
+
 	for (;;) {
 		WRITE_ONCE(run->kvm_dirty_regs, KVM_SYNC_X86_EVENTS);
 		WRITE_ONCE(events->flags, 0);
@@ -115,6 +117,7 @@ static void *race_events_exc(void *arg)
 	for (;;) {
 		WRITE_ONCE(run->kvm_dirty_regs, KVM_SYNC_X86_EVENTS);
 		WRITE_ONCE(events->flags, 0);
+		WRITE_ONCE(events->exception.nr, UD_VECTOR);
 		WRITE_ONCE(events->exception.pending, 1);
 		WRITE_ONCE(events->exception.nr, 255);

-- 
2.42.0.rc1.204.g551eb34607-goog