Re: [RFC PATCH v2 18/27] x86/cet/shstk: Introduce WRUSS instruction

[Yu-cheng Yu]

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.ÂÂThis 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.ÂÂIn 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"
> > +		ÂÂÂÂÂ"xor %[err], %[err]\n"
> this XOR is superfluous, you already cleared @err above.

I will fix it.

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

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

Yu-cheng