Message-ID: <3d7fcfab-e445-1dc7-f000-9fbe7bea04c0@redhat.com>
Date:   Fri, 28 Apr 2023 19:05:38 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Lorenzo Stoakes <lstoakes@...il.com>
Cc:     Peter Xu <peterx@...hat.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Jason Gunthorpe <jgg@...dia.com>, linux-mm@...ck.org,
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        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings
 by default

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.

Maybe de-referencing folio->mapping is fine ... but yes, we could handle 
that optimization in a separate patch.

-- 
Thanks,

David / dhildenb

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