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

David Hildenbrand
2023-04-28T16:14:15Z

On 28.04.23 18:09, Kirill A . Shutemov wrote:
> On Fri, Apr 28, 2023 at 05:43:52PM +0200, David Hildenbrand wrote:
>> 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 ...
>
> page->mapping->a_ops should be enough to whitelist whatever fs you want.
>

The issue is how to stabilize that from GUP-fast, such that we can
safely dereference the mapping. Any idea?

At least for anon page I know that page->mapping only gets cleared when
freeing the page, and we don't dereference the mapping but only check a
single flag stored alongside the mapping. Therefore, PageAnon() is fine
in GUP-fast context.

--
Thanks,

David / dhildenb