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

[Yu, Yu-cheng]

On 9/9/2020 4:11 PM, Dave Hansen wrote:
> On 9/9/20 4:07 PM, Yu, Yu-cheng wrote:
> > What if a writable mapping is passed to madvise(MADV_SHSTK)?  Should
> > that be rejected?
>
> It doesn't matter to me.  Even if it's readable, it _stops_ being even
> directly readable after it's a shadow stack, right?  I don't think
> writes are special in any way.  If anything, we *want* it to be writable
> because that indicates that it can be written to, and we will want to
> write to it soon.
But in a PROT_WRITE mapping, all the pte's have _PAGE_BIT_RW set.  To 
change them to shadow stack, we need to clear that bit from the pte's.
That will be like mprotect_fixup()/change_protection_range().