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

From: David Hildenbrand <david@redhat.com>
To: Peter Xu <peterx@redhat.com>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>,
	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>,
	"Kirill A . Shutemov" <kirill@shutemov.name>,
	Pavel Begunkov <asml.silence@gmail.com>,
	Mika Penttila <mpenttil@redhat.com>,
	David Howells <dhowells@redhat.com>,
	Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings by default
Date: Fri, 28 Apr 2023 18:02:56 +0200	[thread overview]
Message-ID: <0e6ef85d-d53a-c847-c70b-900eb925a413@redhat.com> (raw)
In-Reply-To: <ZEvsx998gDFig/zq@x1n>

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