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Date:   Fri, 28 Apr 2023 19:29:35 +0200
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Subject: Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings
 by default
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To:     Lorenzo Stoakes <lstoakes@gmail.com>
Cc:     Peter Xu <peterx@redhat.com>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Jason Gunthorpe <jgg@nvidia.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Jens Axboe <axboe@kernel.dk>,
        Matthew Wilcox <willy@infradead.org>,
        Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>,
        Leon Romanovsky <leon@kernel.org>,
        Christian Benvenuti <benve@cisco.com>,
        Nelson Escobar <neescoba@cisco.com>,
        Bernard Metzler <bmt@zurich.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Ian Rogers <irogers@google.com>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Bjorn Topel <bjorn@kernel.org>,
        Magnus Karlsson <magnus.karlsson@intel.com>,
        Maciej Fijalkowski <maciej.fijalkowski@intel.com>,
        Jonathan Lemon <jonathan.lemon@gmail.com>,
        "David S . Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Christian Brauner <brauner@kernel.org>,
        Richard Cochran <richardcochran@gmail.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        John Fastabend <john.fastabend@gmail.com>,
        linux-fsdevel@vger.kernel.org, linux-perf-users@vger.kernel.org,
        netdev@vger.kernel.org, bpf@vger.kernel.org,
        Oleg Nesterov <oleg@redhat.com>,
        John Hubbard <jhubbard@nvidia.com>, Jan Kara <jack@suse.cz>,
        Pavel Begunkov <asml.silence@gmail.com>,
        Mika Penttila <mpenttil@redhat.com>,
        David Howells <dhowells@redhat.com>,
        Christoph Hellwig <hch@lst.de>
References: <077c4b21-8806-455f-be98-d7052a584259@lucifer.local>
 <62ec50da-5f73-559c-c4b3-bde4eb215e08@redhat.com>
 <6ddc7ac4-4091-632a-7b2c-df2005438ec4@redhat.com>
 <20230428160925.5medjfxkyvmzfyhq@box.shutemov.name>
 <39cc0f26-8fc2-79dd-2e84-62238d27fd98@redhat.com>
 <20230428162207.o3ejmcz7rzezpt6n@box.shutemov.name> <ZEv2196tk5yWvgW5@x1n>
 <173337c0-14f4-3246-15ff-7fbf03861c94@redhat.com>
 <40fc128f-1978-42db-b9c1-77ac3c2cebfe@lucifer.local>
 <3d7fcfab-e445-1dc7-f000-9fbe7bea04c0@redhat.com>
 <bd470e63-e2e0-4532-8aab-cffe326688b6@lucifer.local>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <bd470e63-e2e0-4532-8aab-cffe326688b6@lucifer.local>
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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