Commit 2a1fc7dc authored by Sean Christopherson's avatar Sean Christopherson Committed by Paolo Bonzini
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KVM: x86: Suppress MMIO that is triggered during task switch emulation 



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 0dc90226 ("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: default avatar <syzbot+2fb9f8ed752c01bc9a3f@syzkaller.appspotmail.com>
Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
Message-ID: <20240712144841.1230591-1-seanjc@google.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 9fe17d2a

arch/x86/kvm/x86.c

+7 −1
Original line number Diff line number Diff line
@@ -11800,7 +11800,13 @@ 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;