Re: umip: AMD Ryzen 3900X, pagefault after emulate SLDT/SIDT instruction

[Andreas Rammhold <andi@notmuch.email>]

On 06:33 26.05.20, Ricardo Neri wrote:
> On Sat, May 23, 2020 at 04:17:39AM +0200, Andreas Rammhold wrote:
> > On 12:43 19.05.20, Ricardo Neri wrote:
> > > I have a patch for this already that I wrote for testing purposes:
> > > https://github.com/ricardon/tip/commit/1692889cb3f8accb523d44b682458e234b93be50
> > > Perhaps it can be used as a starting point? Not sure what the spoofing
> > > value should be, though. Perhaps 0?
> > 
> > I tried the above patch (in modified/rebased version; hope that didn't
> > kill it [0]). The results are negative, as without the patch.
> 
> Ah. My patch above is based on a rather old kernel. There is a check in
> fixup_umip_exception() for SLDT and STR. I think this causes the
> exception you see. Perhaps you can try by removing such check:
> 
> diff --git a/arch/x86/kernel/umip.c b/arch/x86/kernel/umip.c
> @@ -383,10 +389,6 @@ bool fixup_umip_exception(struct pt_regs *regs)
>  	umip_pr_warn(regs, "%s instruction cannot be used by applications.\n",
>  			umip_insns[umip_inst]);
> 
> -	/* Do not emulate (spoof) SLDT or STR. */
> -	if (umip_inst == UMIP_INST_STR || umip_inst == UMIP_INST_SLDT)
> -		return false;
> -
> 	umip_pr_warn(regs, "For now, expensive software emulation returns the result.\n");
> 
> 	if (emulate_umip_insn(&insn, umip_inst, dummy_data, &dummy_data_size,
> 
> You would still need my old patch.

Thanks, that works for my application.

Regards,

Andi