[PATCH] KVM: x86: Suppress MMIO that is triggered during task switch emulation

[Sean Christopherson <seanjc@google.com>]

Explicitly suppress userspace emulated MMIO exits that are triggered when
emulating a task switch as KVM doesn't support userspace MMIO during
complex (multi-step) emulation.  Silently ignoring the exit request can
result in the WARN_ON_ONCE(vcpu->mmio_needed) firing if KVM exits to
userspace for some other reason prior to purging mmio_needed.

See commit 0dc902267cb3 ("KVM: x86: Suppress pending MMIO write exits if
emulator detects exception") for more details on KVM's limitations with
respect to emulated MMIO during complex emulator flows.

Reported-by: syzbot+2fb9f8ed752c01bc9a3f@syzkaller.appspotmail.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
---

This is from a syzkaller report on a Google-internal kernel, but it repros
on upstream (obviously).  There are unfortunately an absurd number of upstream
reports with "WARNING in kvm_arch_vcpu_ioctl_run" as the title, so I haven't
been able to hunt down an upstream report.

 arch/x86/kvm/x86.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 994743266480..47bd8a9fdb21 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11803,7 +11803,13 @@ int kvm_task_switch(struct kvm_vcpu *vcpu, u16 tss_selector, int idt_index,
 
 	ret = emulator_task_switch(ctxt, tss_selector, idt_index, reason,
 				   has_error_code, error_code);
-	if (ret) {
+
+	/*
+	 * Report an error userspace if MMIO is needed, as KVM doesn't support
+	 * MMIO during a task switch (or any other complex operation).
+	 */
+	if (ret || vcpu->mmio_needed) {
+		vcpu->mmio_needed = false;
 		vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
 		vcpu->run->internal.suberror = KVM_INTERNAL_ERROR_EMULATION;
 		vcpu->run->internal.ndata = 0;

base-commit: 771df9ffadb8204e61d3e98f36c5067102aab78f
-- 
2.45.2.993.g49e7a77208-goog