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

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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 0dc902267c ("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>
Message-ID: <20240712144841.1230591-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Sean Christopherson 2024-07-12 07:48:41 -07:00 committed by Paolo Bonzini
parent 9fe17d2ada
commit 2a1fc7dc36
1 changed files with 7 additions and 1 deletions
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8
arch/x86/kvm/x86.c
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@ -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;