Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for shadow stack

[Yu, Yu-cheng]

On 9/9/2020 3:59 PM, Dave Hansen wrote:
> On 9/9/20 3:08 PM, Yu, Yu-cheng wrote:
> > After looking at this more, I found the changes are more similar to
> > mprotect() than madvise().  We are going to change an anonymous mapping
> > to a read-only mapping, and add the VM_SHSTK flag to it.  Would an
> > x86-specific mprotect(PROT_SHSTK) make more sense?
> >
> > One alternative would be requiring a read-only mapping for
> > madvise(MADV_SHSTK).  But that is inconvenient for the application.
>
> Why?  It's just:
>
> 	mmap()/malloc();
> 	mprotect(PROT_READ);
> 	madvise(MADV_SHSTK);
>
> vs.
>
> 	mmap()/malloc();
> 	mprotect(PROT_SHSTK);
>
> I'm not sure a single syscall counts as inconvenient.
>
> I don't quite think we should use a PROT_ bit for this.  It seems like
> the kind of thing that could be fragile and break existing expectations.
>   I don't care _that_ strongly though.
What if a writable mapping is passed to madvise(MADV_SHSTK)?  Should 
that be rejected?