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 <[email protected]>
Reported-and-tested-by: Iakov Karpov <[email protected]>
Reported-by: Gabriele Balducci <[email protected]>
Reported-by: Antti Antinoja <[email protected]>
Cc: [email protected]
Cc: Takashi Iwai <[email protected]>
Cc: Jiri Slaby <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Sean Christopherson <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

0 files changed, 0 insertions, 0 deletions