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

David Hildenbrand
2023-04-28T15:34:46Z

On 28.04.23 17:33, Lorenzo Stoakes wrote:
> On Fri, Apr 28, 2023 at 05:23:29PM +0200, David Hildenbrand wrote:
>>>>
>>>> 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.
>
> How do we determine it's non-anon in the first place? The check is on the
> VMA. We could do it by following page tables down to folio and checking
> folio->mapping for PAGE_MAPPING_ANON I suppose?

PageAnon(page) can be called from GUP-fast after grabbing a reference.
See gup_must_unshare().

>
>>
>> Of course, this would prevent hugetlb/shmem from getting pinned writable
>> during gup-fast. Unless we're able to whitelist them somehow in there.
>
> We could degrade those to non-fast assuming not FOLL_FAST_ONLY. But it'd be
> a pity.

--
Thanks,

David / dhildenb