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

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Message-ID: <be65a66baf94cebf0bc8d726a704238787195837.camel@intel.com>
Date:   Tue, 15 Nov 2022 23:42:46 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "peterz@...radead.org" <peterz@...radead.org>
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Subject: Re: [PATCH v3 27/37] x86/shstk: Introduce routines modifying shstk

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.