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

David Hildenbrand
2023-04-28T15:45:23Z

On 28.04.23 17:34, David Hildenbrand wrote:
> 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().

IIRC, PageHuge() can also be called from GUP-fast and could special-case
hugetlb eventually, as it's table while we hold a (temporary) reference.
Shmem might be not so easy ...

--
Thanks,

David / dhildenb