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SubjectRe: [RFC PATCH v2 18/27] x86/cet/shstk: Introduce WRUSS instruction
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On Wed, 2018-07-11 at 11:44 +0200, Peter Zijlstra wrote:
> On Tue, Jul 10, 2018 at 03:26:30PM -0700, Yu-cheng Yu wrote:
> >
> > WRUSS is a new kernel-mode instruction but writes directly
> > to user shadow stack memory.  This is used to construct
> > a return address on the shadow stack for the signal
> > handler.
> >
> > This instruction can fault if the user shadow stack is
> > invalid shadow stack memory.  In that case, the kernel does
> > fixup.
> >
> >
> > +static inline int write_user_shstk_64(unsigned long addr, unsigned
> > long val)
> > +{
> > + int err = 0;
> > +
> > + asm volatile("1: wrussq %[val], (%[addr])\n"
> > +      "xor %[err], %[err]\n"
> this XOR is superfluous, you already cleared @err above.

I will fix it.

>
> >
> > +      "2:\n"
> > +      ".section .fixup,\"ax\"\n"
> > +      "3: mov $-1, %[err]; jmp 2b\n"
> > +      ".previous\n"
> > +      _ASM_EXTABLE(1b, 3b)
> > +      : [err] "=a" (err)
> > +      : [val] "S" (val), [addr] "D" (addr));
> > +
> > + return err;
> > +}
> > +#endif /* CONFIG_X86_INTEL_CET */
> > +
> >  #define nop() asm volatile ("nop")
> What happened to:
>
>   https://lkml.kernel.org/r/1528729376.4526.0.camel@2b52.sc.intel.com

Yes, I put that in once and realized we only need to skip the
instruction and return err.  Do you think we still need a handler for
that?

Yu-cheng
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