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Re: [Xen-devel] [PATCH] x86emul: refine VME/PVI early #GP check


  • To: Jan Beulich <JBeulich@xxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Mon, 10 Dec 2018 13:28:17 +0000
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  • Cc: Wei Liu <wei.liu2@xxxxxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
  • Delivery-date: Mon, 10 Dec 2018 13:28:45 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 10/12/2018 11:32, Jan Beulich wrote:
> In commit efe9cba66c ("x86emul: VME and PVI modes require a #GP(0) check
> first thing") I neglected the fact that the retire flags get zapped only
> in x86_decode(), which hasn't been invoked yet at the point of the #GP(0)
> check added.
>
> Ahead of the other explicit return (rather than "goto") path use an
> explicit return here too, to make the flow more obvious.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

How did you discover this issue?  I ask because given the comment, its
quite likely that you'll hit the assert in x86_emul_hw_exception()

Why is the zeroing code in decode rather than emulate?  I'm tempted to
suggest that we fix this by requiring that callers pass in a suitably
initialised ctxt.

>
> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
> @@ -3400,10 +3400,15 @@ x86_emulate(
>  
>      ASSERT(ops->read);
>  
> -    generate_exception_if((mode_vif() &&
> -                           (_regs.eflags & X86_EFLAGS_VIF) &&
> -                           (_regs.eflags & X86_EFLAGS_VIP)),
> -                          EXC_GP, 0);
> +    if ( unlikely(mode_vif()) &&
> +         (_regs.eflags & X86_EFLAGS_VIF) &&
> +         (_regs.eflags & X86_EFLAGS_VIP) )
> +    {
> +        x86_emul_hw_exception(EXC_GP, 0, ctxt);
> +        /* x86_decode() was not called yet. */
> +        ctxt->retire.raw = 0;
> +        return X86EMUL_EXCEPTION;
> +    }
>  
>      rc = x86_decode(&state, ctxt, ops);
>      if ( rc != X86EMUL_OKAY )
>
>
>
>


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