[PATCH v2 7/8] KVM: x86: Bug the VM if the emulator generates a bogus exception vector

Sean Christopherson
2022-05-26T21:24:48Z

Bug the VM if KVM's emulator attempts to inject a bogus exception vector.
The guest is likely doomed even if KVM continues on, and propagating a
bad vector to the rest of KVM runs the risk of breaking other assumptions
in KVM and thus triggering a more egregious bug.

All existing users of emulate_exception() have hardcoded vector numbers
(__load_segment_descriptor() uses a few different vectors, but they're
all hardcoded), and future users are likely to follow suit, i.e. the
change to emulate_exception() is a glorified nop.

As for the ctxt->exception.vector check in x86_emulate_insn(), the few
known times the WARN has been triggered in the past is when the field was
not set when synthesizing a fault, i.e. for all intents and purposes the
check protects against consumption of uninitialized data.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/emulate.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 70a8e0cd9fdc..2aa17462a9ac 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -624,7 +624,9 @@ static unsigned long seg_base(struct x86_emulate_ctxt *ctxt, int seg)
static int emulate_exception(struct x86_emulate_ctxt *ctxt, int vec,
u32 error, bool valid)
{
- WARN_ON(vec > 0x1f);
+ if (KVM_EMULATOR_BUG_ON(vec > 0x1f, ctxt))
+ return X86EMUL_UNHANDLEABLE;
+
ctxt->exception.vector = vec;
ctxt->exception.error_code = error;
ctxt->exception.error_code_valid = valid;
@@ -5728,7 +5730,8 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)

done:
if (rc == X86EMUL_PROPAGATE_FAULT) {
- WARN_ON(ctxt->exception.vector > 0x1f);
+ if (KVM_EMULATOR_BUG_ON(ctxt->exception.vector > 0x1f, ctxt))
+ return EMULATION_FAILED;
ctxt->have_exception = true;
}
if (rc == X86EMUL_INTERCEPTED)
--
2.36.1.255.ge46751e96f-goog