KVM: x86: Don't WARN if userspace mucks with RCX during string I/O exit

BugLink: https://bugs.launchpad.net/bugs/1954931
commit d07898eaf39909806128caccb6ebd922ee3edd69 upstream.

Replace a WARN with a comment to call out that userspace can modify RCX
during an exit to userspace to handle string I/O.  KVM doesn't actually
support changing the rep count during an exit, i.e. the scenario can be
ignored, but the WARN needs to go as it's trivial to trigger from
userspace.

Cc: stable@vger.kernel.org
Fixes: 3b27de271839 ("KVM: x86: split the two parts of emulator_pio_in")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211025201311.1881846-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index fa3bbef1cf0b0941b9c66c9c10d86a216cee5f02..cd59465834ab5c5e5c6b11896e78ba5d28edc399 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7021,7 +7021,13 @@ static int emulator_pio_in(struct kvm_vcpu *vcpu, int size,
                           unsigned short port, void *val, unsigned int count)
 {
        if (vcpu->arch.pio.count) {
-               /* Complete previous iteration.  */
+               /*
+                * Complete a previous iteration that required userspace I/O.
+                * Note, @count isn't guaranteed to match pio.count as userspace
+                * can modify ECX before rerunning the vCPU.  Ignore any such
+                * shenanigans as KVM doesn't support modifying the rep count,
+                * and the emulator ensures @count doesn't overflow the buffer.
+                */
        } else {
                int r = __emulator_pio_in(vcpu, size, port, count);
                if (!r)
@@ -7030,7 +7036,6 @@ static int emulator_pio_in(struct kvm_vcpu *vcpu, int size,
                /* Results already available, fall through.  */
        }
 
-       WARN_ON(count != vcpu->arch.pio.count);
        complete_emulator_pio_in(vcpu, val);
        return 1;
 }