KVM: x86: Suppress pending MMIO write exits if emulator detects exception

Clear vcpu->mmio_needed when injecting an exception from the emulator to
squash a (legitimate) warning about vcpu->mmio_needed being true at the
start of KVM_RUN without a callback being registered to complete the
userspace MMIO exit.  Suppressing the MMIO write exit is inarguably wrong
from an architectural perspective, but it is the least awful hack-a-fix
due to shortcomings in KVM's uAPI, not to mention that KVM already
suppresses MMIO writes in this scenario.

Outside of REP string instructions, KVM doesn't provide a way to resume
an instruction at the exact point where it was "interrupted" if said
instruction partially completed before encountering an MMIO access.  For
MMIO reads, KVM immediately exits to userspace upon detecting MMIO as
userspace provides the to-be-read value in a buffer, and so KVM can safely
(more or less) restart the instruction from the beginning.  When the
emulator re-encounters the MMIO read, KVM will service the MMIO by getting
the value from the buffer instead of exiting to userspace, i.e. KVM won't
put the vCPU into an infinite loop.

On an emulated MMIO write, KVM finishes the instruction before exiting to
userspace, as exiting immediately would ultimately hang the vCPU due to
the aforementioned shortcoming of KVM not being able to resume emulation
in the middle of an instruction.

For the vast majority of _emulated_ instructions, deferring the userspace
exit doesn't cause problems as very few x86 instructions (again ignoring
string operations) generate multiple writes.  But for instructions that
generate multiple writes, e.g. PUSHA (multiple pushes onto the stack),
deferring the exit effectively results in only the final write triggering
an exit to userspace.  KVM does support multiple MMIO "fragments", but
only for page splits; if an instruction performs multiple distinct MMIO
writes, the number of fragments gets reset when the next MMIO write comes
along and any previous MMIO writes are dropped.

Circling back to the warning, if a deferred MMIO write coincides with an
exception, e.g. in this case a #SS due to PUSHA underflowing the stack
after queueing a write to an MMIO page on a previous push, KVM injects
the exceptions and leaves the deferred MMIO pending without registering a
callback, thus triggering the splat.

Sweep the problem under the proverbial rug as dropping MMIO writes is not
unique to the exception scenario (see above), i.e. instructions like PUSHA
are fundamentally broken with respect to MMIO, and have been since KVM's
inception.
Reported-by: zhangjianguo <zhangjianguo18@huawei.com>
Reported-by: syzbot+760a73552f47a8cd0fd9@syzkaller.appspotmail.com
Reported-by: syzbot+8accb43ddc6bd1f5713a@syzkaller.appspotmail.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20230322141220.2206241-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -8903,6 +8903,8 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
 	}
 
 	if (ctxt->have_exception) {
+		WARN_ON_ONCE(vcpu->mmio_needed && !vcpu->mmio_is_write);
+		vcpu->mmio_needed = false;
 		r = 1;
 		inject_emulated_exception(vcpu);
 	} else if (vcpu->arch.pio.count) {