Re: [Xen-devel] [PATCH] x86-64: clear DF for kernel when forwarding syscall

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

Thanks. By the way, I've been thinking that rather than rev'ing the
paravirtual hypercall interface for specifying syscall/sysenter callback
points, since these have a direct native equivalent that we're trying to
emulate the semantics of as closely as possible then we may as well emulate
the MSRs for specifying RIP/RFLAGS_mask/etc too. Callers can catch the #GP
to detect whether the new MSR-based interface is supported, and/or we can
add a feature flag in our CPUID leaves.

 -- Keir

On 22/6/07 16:01, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:

> While this is not really matching native behavior, no guest seems to assume
> EFLAGS.DF being set or reflecting application state. Thus clear it for now,
> the syscall/sysenter patch that I'll hopefully be able to get to work will
> then introduce a more consistent solution. In any case, without this any
> app can easily force kernel data corruption.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>
> 
> Index: 2007-06-18/xen/arch/x86/x86_64/entry.S
> ===================================================================
> --- 2007-06-18.orig/xen/arch/x86/x86_64/entry.S 2007-06-22 16:35:55.000000000
> +0200
> +++ 2007-06-18/xen/arch/x86/x86_64/entry.S 2007-06-22 16:36:59.000000000 +0200
> @@ -34,6 +34,7 @@ switch_to_kernel:
>          jnc   1f
>          movb  $TBF_INTERRUPT,TRAPBOUNCE_flags(%rdx)
>  1:      call  create_bounce_frame
> +        andl  $~X86_EFLAGS_DF,UREGS_eflags(%rsp)
>          jmp   test_all_events
>  
>  /* %rbx: struct vcpu, interrupts disabled */
> 
> 
> 
> 
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