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Re: [PATCH for-4.19] x86/entry: don't clear DF when raising #UD for lack of syscall handler


  • To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Mon, 1 Jul 2024 16:35:42 +0100
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  • Cc: Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
  • Delivery-date: Mon, 01 Jul 2024 15:35:57 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 06/03/2024 1:44 pm, Jan Beulich wrote:
> --- a/xen/arch/x86/x86_64/entry.S
> +++ b/xen/arch/x86/x86_64/entry.S
> @@ -47,12 +55,13 @@ UNLIKELY_START(z, syscall_no_callback) /
>          testb $4, X86_EXC_UD * TRAPINFO_sizeof + TRAPINFO_flags(%rdi)
>          setnz %cl
>          lea   TBF_EXCEPTION(, %rcx, TBF_INTERRUPT), %ecx
> +        or    $~0, %esi                 # don't clear DF

Our predominant comment style is /* */

>  UNLIKELY_END(syscall_no_callback)
>  
>          movq  %rax,TRAPBOUNCE_eip(%rdx)
>          movb  %cl,TRAPBOUNCE_flags(%rdx)
>          call  create_bounce_frame
> -        andl  $~X86_EFLAGS_DF,UREGS_eflags(%rsp)
> +        and   %esi, UREGS_eflags(%rsp)

Could we gain a /* Conditionally clear DF */ comment here?

Otherwise, Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

CC Oleksii for 4.19.  This fixes a subtle regression in the PV ABI
caused by a post-XSA fix a few years ago.

It's a low-risk fix to take; while I still don't have an XTF test
covering this, the corner case it's changing used to be completely fatal
to guests, so it's a corner unused in practice.

~Andrew



 


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