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

David Hildenbrand
2023-04-28T17:29:45Z

On 28.04.23 19:13, Lorenzo Stoakes wrote:
> On Fri, Apr 28, 2023 at 07:05:38PM +0200, David Hildenbrand wrote:
>> On 28.04.23 19:01, Lorenzo Stoakes wrote:
>>> On Fri, Apr 28, 2023 at 06:51:46PM +0200, David Hildenbrand wrote:
>>>> On 28.04.23 18:39, Peter Xu wrote:
>>>>> On Fri, Apr 28, 2023 at 07:22:07PM +0300, Kirill A . Shutemov wrote:
>>>>>> On Fri, Apr 28, 2023 at 06:13:03PM +0200, David Hildenbrand wrote:
>>>>>>> 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.
>>>>>>
>>>>>> What codepath you are worry about that clears ->mapping on pages with
>>>>>> non-zero refcount?
>>>>>>
>>>>>> I can only think of truncate (and punch hole). READ_ONCE(page->mapping)
>>>>>> and fail GUP_fast if it is NULL should be fine, no?
>>>>>>
>>>>>> I guess we should consider if the inode can be freed from under us and the
>>>>>> mapping pointer becomes dangling. But I think we should be fine here too:
>>>>>> VMA pins inode and VMA cannot go away from under GUP.
>>>>>
>>>>> Can vma still go away if during a fast-gup?
>>>>>
>>>>
>>>> So, after we grabbed the page and made sure the the PTE didn't change (IOW,
>>>> the PTE was stable while we processed it), the page can get unmapped (but
>>>> not freed, because we hold a reference) and the VMA can theoretically go
>>>> away (and as far as I understand, nothing stops the file from getting
>>>> deleted, truncated etc).
>>>>
>>>> So we might be looking at folio->mapping and the VMA is no longer there.
>>>> Maybe even the file is no longer there.
>>>>
>>>
>>> This shouldn't be an issue though right? Because after a pup call unlocks the
>>> mmap_lock we're in the same situation anyway. GUP doesn't generally guarantee
>>> the mapping remains valid, only pinning the underlying folio.
>>
>> Yes. But the issue here is rather dereferencing something that has already
>> been freed, eventually leading to undefined behavior.
>>
>
> Is that an issue with interrupts disabled though? Will block page tables being
> removed and as Kirill says (sorry I maybe misinterpreted you) we should be ok.

Let's rule out page table freeing. If our VMA only spans a single page
and falls into the same PMD as another VMA, an munmap() would not even
free a single page table.

However, if unmapping a page (flushing the TLB) would imply an IPI as
Kirill said, we'd be fine. I recall that that's not the case for all
architectures, but I might be just wrong.

... and now I'll stop reading mails until Tuesday :)

--
Thanks,

David / dhildenb