Re: [PATCH 21/35] mm/mprotect: Exclude shadow stack from preserve_write

Dave Hansen
2022-02-10T19:27:42Z

On 1/30/22 13:18, Rick Edgecombe wrote:
> In change_pte_range(), when a PTE is changed for prot_numa, _PAGE_RW is
> preserved to avoid the additional write fault after the NUMA hinting fault.
> However, pte_write() now includes both normal writable and shadow stack
> (RW=0, Dirty=1) PTEs, but the latter does not have _PAGE_RW and has no need
> to preserve it.

This series creates an interesting situation: it causes a logical
disconnection between things that were tightly coupled before. For
instance, before this series, _PAGE_RW=1 and "writable" really were
synonyms. They meant the same thing.

One of the complexities in this series is differentiating the two. For
instance, a shadow stack page can be written to, even though it has
_PAGE_RW=0.

This particular patch seems to be hacking around the problem that a
p*_mkwrite() doesn't work on shadow stack PTE/PMDs. First, that makes
me wonder what *actually* happens if we do a plain pte_mkwrite() on a
shadow stack PTE. I *think* it will take the [Write=0,Dirty=1] PTE and

pte = pte_set_flags(pte, _PAGE_RW);

so we'll end up with [Write=1,Dirty=1], which is bad.

Let's say pte_mkwrite() can't be fixed. We should probably make it
VM_BUG_ON() if it's ever asked to muck with a shadow stack PTE.

It's also weird because we have this pte_write()==1 PTE in a !VM_WRITE
VMA. Then, we're trying to pte_mkwrite() under this !VM_WRITE VMA.

pte_write() <-- returns true for on shadow stack PTE!
pte_mkwrite() <-- illegal on shadow stack PTE

I need to think about this a little more. I don't have a solution.
But, as-is, it seems untenable. The rules are just too counter
intuitive to live.