linux-kernel - Re: [PATCH v2 16/39] x86/mm: Update maybe

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Date:   Fri, 14 Oct 2022 15:45:44 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "peterz@...radead.org" <peterz@...radead.org>
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Subject: Re: [PATCH v2 16/39] x86/mm: Update maybe_mkwrite() for shadow stack

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.