Re: [PATCH v2 10/39] x86/mm: Introduce _PAGE_COW

On Mon, 2022-10-03 at 23:54 +0200, Jann Horn wrote:

quoted

quoted

These X86_FEATURE_SHSTK checks make me uneasy. Maybe use the
_PAGE_COW
logic for all machines with 64-bit entries. It will get you much
more
coverage and more universal rules.

Yes, I didn't like them either at first. The reasoning originally
was
that _PAGE_COW is a bit more work and it might show up for some
benchmark.

Looking at this again though, it is just a few more operations on
memory that is already getting touched either way. It must be a
very
tiny amount of impact if any. I'm fine removing them. Having just
one
set of logic around this would make it easier to reason about.

Dave, any thoughts on this?

But the rules wouldn't actually be universal - you'd still have to
look at X86_FEATURE_SHSTK in code that wants to figure out whether a
PTE is shadow stack (on a newer CPU) or readonly dirty (on an older
CPU that can set dirty bits on non-present PTEs), right?

Good point. It still would need a check in pte_shstk() or pte_write(),
so pte_write() doesn't think CPU created Write=0,Dirty=1 memory is
writable. Which then percolates to most of the other checks anyway.