Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings by default

[David Hildenbrand]

> > Security is the primary case where we have historically closed uAPI
> > items.
>
> As this patch
>
> 1) Does not tackle GUP-fast
> 2) Does not take care of !FOLL_LONGTERM
>
> I am not convinced by the security argument in regard to this patch.
>
>
> If we want to sells this as a security thing, we have to block it
> *completely* and then CC stable.

Regarding GUP-fast, to fix the issue there as well, I guess we could do 
something similar as I did in gup_must_unshare():

If we're in GUP-fast (no VMA), and want to pin a !anon page writable, 
fallback to ordinary GUP. IOW, if we don't know, better be safe.

Of course, this would prevent hugetlb/shmem from getting pinned writable 
during gup-fast. Unless we're able to whitelist them somehow in there.


For FOLL_LONGTERM it might fairly uncontroversial. For everything else 
I'm not sure if there could be undesired side-effects.

--
Thanks,

David / dhildenb