Re: [PATCH v1] mm/gup: fix FOLL_FORCE COW security issue and remove FOLL_COW

[David Hildenbrand <david@redhat.com>]

On 09.08.22 22:00, Linus Torvalds wrote:
> On Mon, Aug 8, 2022 at 12:32 AM David Hildenbrand <david@redhat.com> wrote:
>>
> 
> So I've read through the patch several times, and it seems fine, but
> this function (and the pmd version of it) just read oddly to me.
> 
>> +static inline bool can_follow_write_pte(pte_t pte, struct page *page,
>> +                                       struct vm_area_struct *vma,
>> +                                       unsigned int flags)
>> +{
>> +       if (pte_write(pte))
>> +               return true;
>> +       if (!(flags & FOLL_FORCE))
>> +               return false;
>> +
>> +       /*
>> +        * See check_vma_flags(): only COW mappings need that special
>> +        * "force" handling when they lack VM_WRITE.
>> +        */
>> +       if (vma->vm_flags & VM_WRITE)
>> +               return false;
>> +       VM_BUG_ON(!is_cow_mapping(vma->vm_flags));
> 
> So apart from the VM_BUG_ON(), this code just looks really strange -
> even despite the comment. Just conceptually, the whole "if it's
> writable, return that you cannot follow it for a write" just looks so
> very very strange.
> 
> That doesn't make the code _wrong_, but considering how many times
> this has had subtle bugs, let's not write code that looks strange.
> 
> So I would suggest that to protect against future bugs, we try to make
> it be fairly clear and straightforward, and maybe even a bit overly
> protective.
> 
> For example, let's kill the "shared mapping that you don't have write
> permissions to" very explicitly and without any subtle code at all.
> The vm_flags tests are cheap and easy, and we could very easily just
> add some core ones to make any mistakes much less critical.
> 
> Now, making that 'is_cow_mapping()' check explicit at the very top of
> this would already go a long way:
> 
>         /* FOLL_FORCE for writability only affects COW mappings */
>         if (!is_cow_mapping(vma->vm_flags))
>                 return false;

I actually put the is_cow_mapping() mapping check in there because
check_vma_flags() should make sure that we cannot possibly end up here
in that case. But we can spell it out with comments, doesn't hurt.

> 
> but I'd actually go even further: in this case that "is_cow_mapping()"
> helper to some degree actually hides what is going on.
> 
> So I'd actually prefer for that function to be written something like
> 
>         /* If the pte is writable, we can write to the page */
>         if (pte_write(pte))
>                 return true;
> 
>         /* Maybe FOLL_FORCE is set to override it? */
>         if (flags & FOLL_FORCE)
>                 return false;
> 
>         /* But FOLL_FORCE has no effect on shared mappings */
>         if (vma->vm_flags & MAP_SHARED)
>                 return false;
> 
>         /* .. or read-only private ones */
>         if (!(vma->vm_flags & MAP_MAYWRITE))
>                 return false;
> 
>         /* .. or already writable ones that just need to take a write fault */
>         if (vma->vm_flags & MAP_WRITE)
>                 return false;
> 
> and the two first vm_flags tests above are basically doing tat
> "is_cow_mapping()", and maybe we could even have a comment to that
> effect, but wouldn't it be nice to just write it out that way?
> 
> And after you've written it out like the above, now that
> 
>         if (!page || !PageAnon(page) || !PageAnonExclusive(page))
>                 return false;
> 
> makes you pretty safe from a data sharing perspective: it's most
> definitely not a shared page at that point.
> 
> So if you write it that way, the only remaining issues are the magic
> special soft-dirty and uffd ones, but at that point it's purely about
> the semantics of those features, no longer about any possible "oh, we
> fooled some shared page to be writable".
> 
> And I think the above is fairly legible without any subtle cases, and
> the one-liner comments make it all fairly clear that it's testing.
> 
> Is any of this in any _technical_ way different from what your patch
> did? No. It's literally just rewriting it to be a bit more explicit in
> what it is doing, I think, and it makes that odd "it's not writable if
> VM_WRITE is set" case a bit more explicit.
> 
> Hmm?

No strong opinion. I'm happy as long as it's fixed, and the fix is robust.


-- 
Thanks,

David / dhildenb