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

David Hildenbrand
2023-04-28T16:03:45Z

On 28.04.23 17:56, Peter Xu wrote:
> On Fri, Apr 28, 2023 at 05:34:35PM +0200, 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().
>
> Hmm.. Is it a good idea at all to sacrifise all "!anon" fast-gups for this?
> People will silently got degrade even on legal pins on shmem/hugetlb, I
> think, which seems to be still a very major use case.
>

Right. Optimizing for hugetlb should be easy. Shmem is problematic.

I once raised to John that PageAnonExclusive is essentially a "anon page
is pinnable" flag. Too bad we don't have spare page flags ;)

--
Thanks,

David / dhildenb