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KVM: x86: Whitelist port 0x7e for pre-incrementing %rip
KVM's recent bug fix to update %rip after emulating I/O broke userspace
that relied on the previous behavior of incrementing %rip prior to
exiting to userspace. When running a Windows XP guest on AMD hardware,
Qemu may patch "OUT 0x7E" instructions in reaction to the OUT itself.
Because KVM's old behavior was to increment %rip before exiting to
userspace to handle the I/O, Qemu manually adjusted %rip to account for
the OUT instruction.
Arguably this is a userspace bug as KVM requires userspace to re-enter
the kernel to complete instruction emulation before taking any other
actions. That being said, this is a bit of a grey area and breaking
userspace that has worked for many years is bad.
Pre-increment %rip on OUT to port 0x7e before exiting to userspace to
hack around the issue.
Fixes: 45def77ebf
("KVM: x86: update %rip after emulating IO")
Reported-by: Simon Becherer <simon@becherer.de>
Reported-and-tested-by: Iakov Karpov <srid@rkmail.ru>
Reported-by: Gabriele Balducci <balducci@units.it>
Reported-by: Antti Antinoja <reader@fennosys.fi>
Cc: stable@vger.kernel.org
Cc: Takashi Iwai <tiwai@suse.com>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
parent
dbcdae185a
commit
8764ed55c9
2 changed files with 20 additions and 2 deletions
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@ -381,6 +381,7 @@ struct kvm_sync_regs {
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#define KVM_X86_QUIRK_LINT0_REENABLED (1 << 0)
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#define KVM_X86_QUIRK_CD_NW_CLEARED (1 << 1)
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#define KVM_X86_QUIRK_LAPIC_MMIO_HOLE (1 << 2)
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#define KVM_X86_QUIRK_OUT_7E_INC_RIP (1 << 3)
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#define KVM_STATE_NESTED_GUEST_MODE 0x00000001
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#define KVM_STATE_NESTED_RUN_PENDING 0x00000002
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@ -6539,6 +6539,12 @@ int kvm_emulate_instruction_from_buffer(struct kvm_vcpu *vcpu,
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}
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EXPORT_SYMBOL_GPL(kvm_emulate_instruction_from_buffer);
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static int complete_fast_pio_out_port_0x7e(struct kvm_vcpu *vcpu)
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{
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vcpu->arch.pio.count = 0;
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return 1;
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}
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static int complete_fast_pio_out(struct kvm_vcpu *vcpu)
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{
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vcpu->arch.pio.count = 0;
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@ -6555,12 +6561,23 @@ static int kvm_fast_pio_out(struct kvm_vcpu *vcpu, int size,
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unsigned long val = kvm_register_read(vcpu, VCPU_REGS_RAX);
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int ret = emulator_pio_out_emulated(&vcpu->arch.emulate_ctxt,
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size, port, &val, 1);
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if (ret)
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return ret;
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if (!ret) {
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/*
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* Workaround userspace that relies on old KVM behavior of %rip being
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* incremented prior to exiting to userspace to handle "OUT 0x7e".
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*/
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if (port == 0x7e &&
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kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_OUT_7E_INC_RIP)) {
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vcpu->arch.complete_userspace_io =
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complete_fast_pio_out_port_0x7e;
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kvm_skip_emulated_instruction(vcpu);
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} else {
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vcpu->arch.pio.linear_rip = kvm_get_linear_rip(vcpu);
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vcpu->arch.complete_userspace_io = complete_fast_pio_out;
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}
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return ret;
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return 0;
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}
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static int complete_fast_pio_in(struct kvm_vcpu *vcpu)
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