KVM: x86: Whitelist port 0x7e for pre-incrementing %rip [Linux 5.1]

KVM: x86: Whitelist port 0x7e for pre-incrementing %rip [Linux 5.1]

This Linux kernel change "KVM: x86: Whitelist port 0x7e for pre-incrementing %rip" is included in the Linux 5.1 release. This change is authored by Sean Christopherson <sean.j.christopherson [at] intel.com> on Mon Apr 29 07:04:15 2019 -0700. The commit for this change in Linux stable tree is 8764ed5 (patch).

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: 45def77ebf79e ("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>

There are 22 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 arch/x86/include/uapi/asm/kvm.h |  1 +
 arch/x86/kvm/x86.c              | 21 +++++++++++++++++++--
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
index dabfcf7..7a0e64c 100644
--- a/arch/x86/include/uapi/asm/kvm.h
+++ b/arch/x86/include/uapi/asm/kvm.h
@@ -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
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 9482cb3..5762131 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -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)

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