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Subject: Re: WARNING in x86_emulate_insn
To: Tianyu Lan <lantianyu1986@gmail.com>,
        Jim Mattson <jmattson@google.com>
Cc: Wanpeng Li <kernellwp@gmail.com>, Dmitry Vyukov <dvyukov@google.com>,
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From: Paolo Bonzini <pbonzini@redhat.com>
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Date: Mon, 11 Dec 2017 23:45:57 +0100
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On 08/12/2017 09:28, Tianyu Lan wrote:
> I find this is pop instruction emulation issue. According "SDM VOL2,
> chapter INSTRUCTION
> SET REFERENCE. POP—Pop a Value from the Stack"
> 
> Protected Mode Exceptions
> #GP(0) If attempt is made to load SS register with NULL segment selector.

This is not what the testcase is testing; this is already covered by 
__load_segment_descriptor:

        if (null_selector) {
                if (seg == VCPU_SREG_CS || seg == VCPU_SREG_TR)
                        goto exception;

                if (seg == VCPU_SREG_SS) {
                        if (ctxt->mode != X86EMUL_MODE_PROT64 || rpl != cpl)
                                goto exception;
		...
	}

Is there a path that can return X86EMUL_PROPAGATE_FAULT without setting
ctxt->exception.vector and/or without going through emulate_exception?

I don't think it's possible to write a test in kvm-unit-tests, because the
state has "impossible" segment descriptor cache contents.

Paolo

> This test case hits it but current code doesn't check such case.
> The following patch can fix the issue.
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index abe74f7..e2ac5cc 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -1844,6 +1844,9 @@ static int emulate_pop(struct x86_emulate_ctxt *ctxt,
>         int rc;
>         struct segmented_address addr;
> 
> +       if ( !get_segment_selector(ctxt, VCPU_SREG_SS))
> +               return emulate_gp(ctxt, 0);
> +
>         addr.ea = reg_read(ctxt, VCPU_REGS_RSP) & stack_mask(ctxt);
>         addr.seg = VCPU_SREG_SS;
>         rc = segmented_read(ctxt, addr, dest, len);