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KVM: x86: Whitelist port 0x7e for pre-incrementing %rip

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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:
Sean Christopherson 2019-04-29 07:04:15 -07:00 committed by Paolo Bonzini
parent dbcdae185a
commit 8764ed55c9
2 changed files with 20 additions and 2 deletions
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1
arch/x86/include/uapi/asm/kvm.h
View File

@ -381,6 +381,7 @@ struct kvm_sync_regs {
#define KVM_X86_QUIRK_LINT0_REENABLED (1 << 0)
#define KVM_X86_QUIRK_CD_NW_CLEARED (1 << 1)
#define KVM_X86_QUIRK_LAPIC_MMIO_HOLE (1 << 2)
#define KVM_X86_QUIRK_OUT_7E_INC_RIP (1 << 3)
#define KVM_STATE_NESTED_GUEST_MODE 0x00000001
#define KVM_STATE_NESTED_RUN_PENDING 0x00000002

21
arch/x86/kvm/x86.c
View File

@ -6539,6 +6539,12 @@ int kvm_emulate_instruction_from_buffer(struct kvm_vcpu *vcpu,
}
EXPORT_SYMBOL_GPL(kvm_emulate_instruction_from_buffer);
static int complete_fast_pio_out_port_0x7e(struct kvm_vcpu *vcpu)
{
vcpu->arch.pio.count = 0;
return 1;
}
static int complete_fast_pio_out(struct kvm_vcpu *vcpu)
{
vcpu->arch.pio.count = 0;
@ -6555,12 +6561,23 @@ static int kvm_fast_pio_out(struct kvm_vcpu *vcpu, int size,
unsigned long val = kvm_register_read(vcpu, VCPU_REGS_RAX);
int ret = emulator_pio_out_emulated(&vcpu->arch.emulate_ctxt,
size, port, &val, 1);
if (ret)
return ret;
if (!ret) {
/*
* Workaround userspace that relies on old KVM behavior of %rip being
* incremented prior to exiting to userspace to handle "OUT 0x7e".
*/
if (port == 0x7e &&
kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_OUT_7E_INC_RIP)) {
vcpu->arch.complete_userspace_io =
complete_fast_pio_out_port_0x7e;
kvm_skip_emulated_instruction(vcpu);
} else {
vcpu->arch.pio.linear_rip = kvm_get_linear_rip(vcpu);
vcpu->arch.complete_userspace_io = complete_fast_pio_out;
}
return ret;
return 0;
}
static int complete_fast_pio_in(struct kvm_vcpu *vcpu)