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* [PATCH] KVM: x86: Whitelist port 0x7e for pre-incrementing %rip
@ 2019-04-26 23:38 Sean Christopherson
  2019-04-27  4:19 ` Paolo Bonzini
  0 siblings, 1 reply; 3+ messages in thread
From: Sean Christopherson @ 2019-04-26 23:38 UTC (permalink / raw)
  To: Paolo Bonzini, Radim Krčmář
  Cc: kvm, Simon Becherer, Iakov Karpov, Gabriele Balducci,
	Antti Antinoja, Takashi Iwai, Jiri Slaby, Greg Kroah-Hartman

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>
---
 arch/x86/kvm/x86.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 9482cb36b92a..5cc22a593e6d 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 +6561,22 @@ 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) {
+		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)
-- 
2.21.0


^ permalink raw reply related	[flat|nested] 3+ messages in thread
end of thread, other threads:[~2019-04-29 14:16 UTC | newest]

Thread overview: 3+ messages (download: mbox.gz follow: Atom feed
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2019-04-26 23:38 [PATCH] KVM: x86: Whitelist port 0x7e for pre-incrementing %rip Sean Christopherson
2019-04-27  4:19 ` Paolo Bonzini
2019-04-29 14:16   ` Sean Christopherson

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