Re: [PATCH v3 27/37] x86/shstk: Introduce routines modifying shstk

Edgecombe, Rick P
2022-11-15T23:42:58Z

On Tue, 2022-11-15 at 15:18 +0100, Peter Zijlstra wrote:
> On Fri, Nov 04, 2022 at 03:35:54PM -0700, Rick Edgecombe wrote:
>
> > +#ifdef CONFIG_X86_USER_SHADOW_STACK
> > +static inline int write_user_shstk_64(u64 __user *addr, u64 val)
> > +{
> > + asm_volatile_goto("1: wrussq %[val], (%[addr])\n"
> > + _ASM_EXTABLE(1b, %l[fail])
> > + :: [addr] "r" (addr), [val] "r" (val)
> > + :: fail);
> > + return 0;
> > +fail:
> > + return -EFAULT;
> > +}
> > +#endif /* CONFIG_X86_USER_SHADOW_STACK */
>
> Why isn't this modelled after put_user() ?

You mean as far as supporting multiple sizes? It just isn't really
needed yet. We are only writing single frames. I suppose it might make
more sense with the alt shadow stack support, but that is dropped for
now.

The other difference here is that WRUSS is a weird instruction that is
treated as a user access even if it comes from the kernel mode. So it's
doesn't need to stac/clac.

>
> Should you write a 64bit value even if the task receiving a signal is
> 32bit ?

32 bit support was also dropped.