[PATCH v2 4/4] KVM: x86: Exit to userspace if RDMSR/WRMSR emulation returns X86EMUL_IO_NEEDED

[Hou Wenlong]

In em_{rd,wr}msr(), it returns X86EMUL_IO_NEEDED if MSR accesses needed
to exit to userspace. However, x86_emulate_insn() doesn't return
X86EMUL_*, so x86_emulate_instruction() doesn't directly act on
X86EMUL_IO_NEEDED, it instead looks for other signals to differentiate
between PIO, MMIO, etc... So RDMSR/WRMSR emulation won't exit to
userspace now.

The userspace_msr_exit_test testcase in seftests had tested RDMSR/WRMSR
emulation with kvm.force_enable_prefix enabled and it was passed.
Because x86_emulate_instruction() returns 1 and guest continues,
but RIP has been updated to point to RDMSR/WRMSR. Then guest would
execute RDMSR/WRMSR and exit to userspace by kvm_emulate_{rd,wr}msr()
finally. In such situation, instruction emulation didn't take effect but
userspace exit information had been filled, which was inappropriate.

Since X86EMUL_IO_NEEDED path would provide a complete_userspace_io
callback, x86_emulate_instruction() should return 0 if callback is
not NULL. Then RDMSR/WRMSR instruction emulation can exit to userspace
and skip RDMSR/WRMSR execution and inteception.

Fixes: 1ae099540e8c7 ("KVM: x86: Allow deflecting unknown MSR accesses to user space")
Signed-off-by: Hou Wenlong <houwenlong93@xxxxxxxxxxxxxxxxx>
---
 arch/x86/kvm/x86.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 5eadf5ddba3e..f5573f010a8e 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8214,6 +8214,9 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
 			writeback = false;
 		r = 0;
 		vcpu->arch.complete_userspace_io = complete_emulated_mmio;
+	} else if (vcpu->arch.complete_userspace_io) {
+		writeback = false;
+		r = 0;
 	} else if (r == EMULATION_RESTART)
 		goto restart;
 	else
-- 
2.31.1