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

[Peter Zijlstra]

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.

> +		     "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@xxxxxxxxxxxxxxxxx