[PATCH] KVM: x86: emulate: correct page fault error code for NoWrite instructions

[PATCH] KVM: x86: emulate: correct page fault error code for NoWrite instructions

[Paolo Bonzini]

NoWrite instructions (e.g. cmp or test) never set the "write access"
bit in the error code, even if one of the operands is treated as a
destination.

Fixes: c205fb7d7d4f81e46fc577b707ceb9e356af1456
Cc: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 arch/x86/kvm/emulate.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 81dcf7964701..a943bf0c06d0 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -4954,7 +4954,8 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
 		rc = segmented_read(ctxt, ctxt->dst.addr.mem,
 				   &ctxt->dst.val, ctxt->dst.bytes);
 		if (rc != X86EMUL_CONTINUE) {
-			if (rc == X86EMUL_PROPAGATE_FAULT &&
+			if (!(ctxt->d & NoWrite) &&
+			    rc == X86EMUL_PROPAGATE_FAULT &&
 			    ctxt->exception.vector == PF_VECTOR)
 				ctxt->exception.error_code |= PFERR_WRITE_MASK;
 			goto done;
-- 
1.8.3.1

Re: [PATCH] KVM: x86: emulate: correct page fault error code for NoWrite instructions

[Nadav Amit]

Sorry for that. I’ll try to run some few more tests next week.

Nadav

Paolo Bonzini <pbonzini@redhat.com> wrote:

> NoWrite instructions (e.g. cmp or test) never set the "write access"
> bit in the error code, even if one of the operands is treated as a
> destination.
> 
> Fixes: c205fb7d7d4f81e46fc577b707ceb9e356af1456
> Cc: Nadav Amit <namit@cs.technion.ac.il>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> arch/x86/kvm/emulate.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 81dcf7964701..a943bf0c06d0 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -4954,7 +4954,8 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
> 		rc = segmented_read(ctxt, ctxt->dst.addr.mem,
> 				   &ctxt->dst.val, ctxt->dst.bytes);
> 		if (rc != X86EMUL_CONTINUE) {
> -			if (rc == X86EMUL_PROPAGATE_FAULT &&
> +			if (!(ctxt->d & NoWrite) &&
> +			    rc == X86EMUL_PROPAGATE_FAULT &&
> 			    ctxt->exception.vector == PF_VECTOR)
> 				ctxt->exception.error_code |= PFERR_WRITE_MASK;
> 			goto done;
> -- 
> 1.8.3.1
> 
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Re: [PATCH] KVM: x86: emulate: correct page fault error code for NoWrite instructions

[Wanpeng Li]

Hi Paolo,
2015-02-09 17:03 GMT+08:00 Paolo Bonzini <pbonzini@redhat.com>:
> NoWrite instructions (e.g. cmp or test) never set the "write access"
> bit in the error code, even if one of the operands is treated as a
> destination.

Sorry to reply to an old commit, btw, could you point out where in SDM
describe above?

Regards,
Wanpeng Li

Re: [PATCH] KVM: x86: emulate: correct page fault error code for NoWrite instructions

[Wanpeng Li]

2015-11-20 10:52 GMT+08:00 Wanpeng Li <kernellwp@gmail.com>:
> Hi Paolo,
> 2015-02-09 17:03 GMT+08:00 Paolo Bonzini <pbonzini@redhat.com>:
>> NoWrite instructions (e.g. cmp or test) never set the "write access"
>> bit in the error code, even if one of the operands is treated as a
>> destination.
>
> Sorry to reply to an old commit, btw, could you point out where in SDM
> describe above?

I see.

Re: [PATCH] KVM: x86: emulate: correct page fault error code for NoWrite instructions

[Nadav Amit]

Wanpeng Li <kernellwp@gmail.com> wrote:

> 2015-11-20 10:52 GMT+08:00 Wanpeng Li <kernellwp@gmail.com>:
>> Hi Paolo,
>> 2015-02-09 17:03 GMT+08:00 Paolo Bonzini <pbonzini@redhat.com>:
>>> NoWrite instructions (e.g. cmp or test) never set the "write access"
>>> bit in the error code, even if one of the operands is treated as a
>>> destination.
>> 
>> Sorry to reply to an old commit, btw, could you point out where in SDM
>> describe above?
> 
> I see.

I don’t understand whether you still need my help, so to clarify: on a
page-fault the error code should indicate whether the access was due to a
write access. Previously KVM marked it as “write access” for instructions
such as test and cmp that do not perform write.

Nadav