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

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Date:   Tue, 2 May 2023 10:00:16 +0200
From:   Jan Kara <jack@...e.cz>
To:     "Kirill A . Shutemov" <kirill@...temov.name>
Cc:     David Hildenbrand <david@...hat.com>, Peter Xu <peterx@...hat.com>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Jason Gunthorpe <jgg@...dia.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>,
        Matthew Wilcox <willy@...radead.org>,
        Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>,
        Leon Romanovsky <leon@...nel.org>,
        Christian Benvenuti <benve@...co.com>,
        Nelson Escobar <neescoba@...co.com>,
        Bernard Metzler <bmt@...ich.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Bjorn Topel <bjorn@...nel.org>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Christian Brauner <brauner@...nel.org>,
        Richard Cochran <richardcochran@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        linux-fsdevel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        Oleg Nesterov <oleg@...hat.com>,
        John Hubbard <jhubbard@...dia.com>, Jan Kara <jack@...e.cz>,
        Pavel Begunkov <asml.silence@...il.com>,
        Mika Penttila <mpenttil@...hat.com>,
        David Howells <dhowells@...hat.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings
 by default

On Sat 29-04-23 02:43:32, Kirill A . Shutemov wrote:
> I think I found relevant snippet of code that solves similar issue.
> get_futex_key() uses RCU to stabilize page->mapping after GUP_fast:
> 
> 
> 		/*
> 		 * The associated futex object in this case is the inode and
> 		 * the page->mapping must be traversed. Ordinarily this should
> 		 * be stabilised under page lock but it's not strictly
> 		 * necessary in this case as we just want to pin the inode, not
> 		 * update the radix tree or anything like that.
> 		 *
> 		 * The RCU read lock is taken as the inode is finally freed
> 		 * under RCU. If the mapping still matches expectations then the
> 		 * mapping->host can be safely accessed as being a valid inode.
> 		 */
> 		rcu_read_lock();
> 
> 		if (READ_ONCE(page->mapping) != mapping) {
> 			rcu_read_unlock();
> 			put_page(page);
> 
> 			goto again;
> 		}
> 
> 		inode = READ_ONCE(mapping->host);
> 		if (!inode) {
> 			rcu_read_unlock();
> 			put_page(page);
> 
> 			goto again;
> 		}
> 
> I think something similar can be used inside GUP_fast too.

Yeah, inodes (and thus struct address_space) is RCU protected these days so
grabbing RCU lock in gup_fast() will get you enough protection for checking
aops if you are careful (like the futex code is).

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR