Re: [RFC PATCH v9 14/27] mm: Handle Shadow Stack page fault

[Yu-cheng Yu]

On Tue, 2020-04-07 at 15:21 -0700, Dave Hansen wrote:
> On 4/7/20 11:14 AM, Yu-cheng Yu wrote:
> > On Wed, 2020-02-26 at 16:08 -0800, Dave Hansen wrote:
> > > > diff --git a/mm/memory.c b/mm/memory.c
> > > > index 45442d9a4f52..6daa28614327 100644
> > > > --- a/mm/memory.c
> > > > +++ b/mm/memory.c
> > > > @@ -772,7 +772,8 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
> > > >  	 * If it's a COW mapping, write protect it both
> > > >  	 * in the parent and the child
> > > >  	 */
> > > > -	if (is_cow_mapping(vm_flags) && pte_write(pte)) {
> > > > +	if ((is_cow_mapping(vm_flags) && pte_write(pte)) ||
> > > > +	    arch_copy_pte_mapping(vm_flags)) {
> > > >  		ptep_set_wrprotect(src_mm, addr, src_pte);
> > > >  		pte = pte_wrprotect(pte);
> > > >  	}
> > > 
> > > You have to modify this because pte_write()==0 for shadow stack PTEs, right?
> > > 
> > > Aren't shadow stack ptes *logically* writable, even if they don't have
> > > the write bit set?  What would happen if we made pte_write()==1 for them?
> > 
> > Here the vm_flags needs to have VM_MAYWRITE, and the PTE needs to have
> > _PAGE_WRITE.  A shadow stack does not have either.
> 
> I literally mean taking pte_write(), and doing something l
> 
> static inline int pte_write(pte_t pte)
> {
> 	if (pte_present(pte) && pte_is_shadow_stack(pte))
> 		return 1;
> 
>         return pte_flags(pte) & _PAGE_RW;
> }
> 
> Then if is_cow_mapping() returns true for shadow stack VMAs, the above
> code doesn't need to change.

One benefit of this change is can_follow_write_pte() does not need any changes. 
A shadow stack PTE not in copy-on-write status is pte_write().

However, there are places that use pte_write() to determine if the PTE can be
made _PAGE_RW.  One such case is in change_pte_range(), where

	preserve_write = prot_numa && pte_write(oldpte);

and later,

	if (preserve_write)
		ptent = pte_mk_savedwrite(ptent);

Currently, there are other checks and shadow stack PTEs won't become _PAGE_RW. 
I am wondering if this can be overlooked later when the code is modified.

Another potential issue is, because pte_write()==1, a shadow stack PTE is made a
write migration entry, and can later accidentally become _PAGE_RW.  I think the
page fault handler would catch that, but still call it out in case I miss
anything.

Yu-cheng