Re: [RFC PATCH v2 16/27] mm: Modify can_follow_write_pte/pmd for shadow stack

[Dave Hansen]

On 07/19/2018 10:06 AM, Yu-cheng Yu wrote:
> Which pte_write() do you think is right?

There isn't one that's right.

The problem is that the behavior right now is ambiguous.  Some callers
of pte_write() need to know about _PAGE_RW alone and others want to know
if (_PAGE_RW || is_shstk()).

The point is that you need both, plus a big audit of all the pte_write()
users to ensure they use the right one.

For instance, see spurious_fault_check().  We can get a shadowstack
fault that also has X86_PF_WRITE, but pte_write()==0.  That might make a
shadowstack write fault falsely appear spurious.