Re: [PATCH v2 16/39] x86/mm: Update maybe_mkwrite() for shadow stack

Edgecombe, Rick P
2022-10-14T15:46:15Z

On Fri, 2022-10-14 at 17:32 +0200, Peter Zijlstra wrote:
> On Thu, Sep 29, 2022 at 03:29:13PM -0700, Rick Edgecombe wrote:
>
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 8cd413c5a329..fef14ab3abcb 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -981,13 +981,25 @@ void free_compound_page(struct page *page);
> > * servicing faults for write access. In the normal case, do
> > always want
> > * pte_mkwrite. But get_user_pages can cause write faults for
> > mappings
> > * that do not have writing enabled, when used by
> > access_process_vm.
> > + *
> > + * If a vma is shadow stack (a type of writable memory), mark the
> > pte shadow
> > + * stack.
> > */
> > +#ifndef maybe_mkwrite
> > static inline pte_t maybe_mkwrite(pte_t pte, struct
> > vm_area_struct *vma)
> > {
> > - if (likely(vma->vm_flags & VM_WRITE))
> > + if (!(vma->vm_flags & VM_WRITE))
> > + goto out;
> > +
> > + if (vma->vm_flags & VM_SHADOW_STACK)
> > + pte = pte_mkwrite_shstk(pte);
> > + else
> > pte = pte_mkwrite(pte);
> > +
> > +out:
> > return pte;
> > }
> > +#endif
>
> Why the #ifndef guard? There is no other implementation, nor does
> this
> patch introduce one.

Oh yea, this series used to add another one, but I forgot to remove the
guards. Thanks.

>
> Also, wouldn't it be simpler to write it like:
>
> static inline pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct
> *vma)
> {
> if (!(vma->vm_flags & VM_WRITE))
> return pte;
>
> if (vma->vm_flags & VM_SHADOW_STACK)
> return pte_mkwrite_shstk(pte);
>
> return pte_mkwrite(pte);
> }

Yep, that looks better. Thanks.