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

From: Yu, Yu-cheng
Date: Wed Sep 09 2020 - 22:19:28 EST

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().