Re: [Xen-devel] [PATCH 2/2] x86/AMD: Fix handling of x87 exception pointers on Fam17h hardware

[Jan Beulich <jbeulich@xxxxxxxx>]

On 19.08.2019 20:26, Andrew Cooper wrote:
> AMD Pre-Fam17h CPUs "optimise" {F,}X{SAVE,RSTOR} by not saving/restoring
> FOP/FIP/FDP if an x87 exception isn't pending.  This causes an information
> leak, CVE-2006-1056, and worked around by several OSes, including Xen.  AMD
> Fam17h CPUs no longer have this leak, and advertise so in a CPUID bit.
> 
> Introduce the RSTR_FP_ERR_PTRS feature, as specified by AMD, and expose to all
> guests by default.  While adjusting libxl's cpuid table, add CLZERO which
> looks to have been omitted previously.
> 
> Also introduce an X86_BUG bit to trigger the (F)XRSTOR workaround, and set it
> on AMD hardware where RSTR_FP_ERR_PTRS is not advertised.  Optimise the
> workaround path by dropping the data-dependent unpredictable conditions which
> will evalute to true for all 64bit OSes and most 32bit ones.

I definitely don't buy the "all 64bit OSes" part here: Anyone doing
full 80-bit FP operations will have to use the FPU, and hence may
want to have some unmasked exceptions. I'm also not sure why you
call them "unpredictable": If all (or most) cases match, the branch
there could be pretty well predicted (subject of course to capacity).

All in all I'd prefer if the conditions remained in place; my minimal
request would be for there to be a comment why there's no evaluation
of FSW/FCW.

> --- a/xen/arch/x86/i387.c
> +++ b/xen/arch/x86/i387.c
> @@ -43,20 +43,17 @@ static inline void fpu_fxrstor(struct vcpu *v)
>      const typeof(v->arch.xsave_area->fpu_sse) *fpu_ctxt = v->arch.fpu_ctxt;
>  
>      /*
> -     * AMD CPUs don't save/restore FDP/FIP/FOP unless an exception
> +     * Some CPUs don't save/restore FDP/FIP/FOP unless an exception

Are there any non-AMD CPUs known to have this issue? If not, is
there a particular reason you don't say "Some AMD CPUs ..."?

>       * is pending. Clear the x87 state here by setting it to fixed
>       * values. The hypervisor data segment can be sometimes 0 and
>       * sometimes new user value. Both should be ok. Use the FPU saved
>       * data block as a safe address because it should be in L1.
>       */
> -    if ( !(fpu_ctxt->fsw & ~fpu_ctxt->fcw & 0x003f) &&
> -         boot_cpu_data.x86_vendor == X86_VENDOR_AMD )
> -    {
> +    if ( cpu_bug_fpu_ptr_leak )
>          asm volatile ( "fnclex\n\t"
>                         "ffree %%st(7)\n\t" /* clear stack tag */
>                         "fildl %0"          /* load to clear state */
>                         : : "m" (*fpu_ctxt) );

If here and in the respective xsave instance you'd use alternatives
patching, I wouldn't mind the use of a X86_BUG_* for this (as made
possible by patch 1). But as said before, just like for synthetic
features I strongly think we should use simple boolean variables
when using them only in if()-s. Use of the feature(/bug) machinery
is needed only to not further complicate alternatives patching.

> @@ -169,11 +166,10 @@ static inline void fpu_fxsave(struct vcpu *v)
>                         : "=m" (*fpu_ctxt) : "R" (fpu_ctxt) );
>  
>          /*
> -         * AMD CPUs don't save/restore FDP/FIP/FOP unless an exception
> -         * is pending.
> +         * Some CPUs don't save/restore FDP/FIP/FOP unless an exception is
> +         * pending.  The restore code fills in suitable defaults.
>           */
> -        if ( !(fpu_ctxt->fsw & 0x0080) &&
> -             boot_cpu_data.x86_vendor == X86_VENDOR_AMD )
> +        if ( cpu_bug_fpu_ptr_leak && !(fpu_ctxt->fsw & 0x0080) )
>              return;

The comment addition seems a little unmotivated: The code here isn't
about leaking data, but about having valid data to consume (down
from here). With this, keying the return to cpu_bug_* also doesn't
look very nice, but I admit I can't suggest a better alternative
(other than leaving the vendor check in place and checking the
X86_FEATURE_RSTR_FP_ERR_PTRS bit).

An option might be to give the construct a different name, without
"leak" in it (NO_FP_ERR_PTRS?).

Jan

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