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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Kirill Smelkov <kirr@nexedi.com>
Cc: Eric Dumazet <edumazet@google.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Alan Stern <stern@rowland.harvard.edu>,
	Marco Elver <elver@google.com>,
	Eric Dumazet <eric.dumazet@gmail.com>,
	syzbot <syzbot+3ef049d50587836c0606@syzkaller.appspotmail.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
	Andrea Parri <parri.andrea@gmail.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	LKMM Maintainers -- Akira Yokosawa <akiyks@gmail.com>
Subject: Re: KCSAN: data-race in __alloc_file / __alloc_file
Date: Tue, 12 Nov 2019 09:23:34 -0800
Message-ID: <CAHk-=witx+fY-no_UTNhsxXvZnOaFLM80Q8so6Mvm6hUTjZdGg@mail.gmail.com> (raw)
In-Reply-To: <20191112165033.GA7905@deco.navytux.spb.ru>

On Tue, Nov 12, 2019 at 8:50 AM Kirill Smelkov <kirr@nexedi.com> wrote:
>
> The same logic applies if it is not 2 processes, but 2 threads:
> thread T2 adjusts file position racily to thread T1 while T1 is doing
> read syscall with the end result that T1 read could access file range
> that it should not be allowed to access.

Well, I think we actually always copy the file position before we pass
it down. So everybody always _uses_ their own private pointer, and the
race is only in the "read original value" vs "write new value back".

You had a patch that passed the address of file->f_pos down in your
original series iirc, but I NAK'ed that one. Exactly because it made
me nervous.

> By the way on "1" topic I suspect there is a race of how
> `N(file-users) > 1` check is done: file_count(file) is
> atomic_long_read(&file->f_count), but let's think on how that atomic
> read is positioned wrt another process creation: I did not studied in
> detail, so I might be wrong here, but offhand it looks like there is no
> synchronization.

Well, that's one reason to add the test for threads - it also gets rid
of that race. Because without threads, there's nothing else that could
access - or fork - a "N(file-users) == 1" file but us.

> So talking about the kernel I would also review the possibility of
> file_count wrt clone race once again.

See above. That goes away with the test for FDPUT_FPUT.

> About "2": I generally agree with the direction, but I think the kernel
> is not yet ready for this switch. Let me quote myself:

Hmm. I thought we already then applied all the patches that marked
things that didn't use f_pos as FMODE_STREAM. Including pipes and
sockets etc.

But if we didn't - and no, I didn't double-check now either - then
obviously that part of the patch can't be applied now.

             Linus

  reply index

Thread overview: 67+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <CAHk-=wjB61GNmqpX0BLA5tpL4tsjWV7akaTc2Roth7uGgax+mw@mail.gmail.com>
2019-11-10 16:09 ` Alan Stern
2019-11-10 19:10   ` Marco Elver
2019-11-11 15:51     ` Alan Stern
2019-11-11 16:51       ` Linus Torvalds
2019-11-11 17:52         ` Eric Dumazet
2019-11-11 18:04           ` Linus Torvalds
2019-11-11 18:31             ` Eric Dumazet
2019-11-11 18:44               ` Eric Dumazet
2019-11-11 19:00                 ` Linus Torvalds
2019-11-11 19:13                   ` Eric Dumazet
2019-11-11 20:43                     ` Linus Torvalds
2019-11-11 20:46                       ` Linus Torvalds
2019-11-11 21:53                         ` Eric Dumazet
2019-11-11 23:51                   ` Linus Torvalds
2019-11-12 16:50                     ` Kirill Smelkov
2019-11-12 17:23                       ` Linus Torvalds [this message]
2019-11-12 17:36                         ` Linus Torvalds
2019-11-17 18:56                           ` Kirill Smelkov
2019-11-17 19:20                             ` Linus Torvalds
2019-11-11 18:50               ` Linus Torvalds
2019-11-11 18:59                 ` Marco Elver
2019-11-11 18:59                 ` Eric Dumazet
2019-11-10 19:12   ` Linus Torvalds
2019-11-10 19:20     ` Linus Torvalds
2019-11-10 20:44       ` Paul E. McKenney
2019-11-10 21:10         ` Linus Torvalds
2019-11-10 21:31           ` Paul E. McKenney
2019-11-11 14:17         ` Marco Elver
2019-11-11 14:31           ` Paul E. McKenney
2019-11-11 15:10             ` Marco Elver
2019-11-13  0:25               ` Paul E. McKenney
2019-11-12 19:14     ` Alan Stern
2019-11-12 19:47       ` Linus Torvalds
2019-11-12 20:29         ` Alan Stern
2019-11-12 20:58           ` Linus Torvalds
2019-11-12 21:13             ` Linus Torvalds
2019-11-12 22:05               ` Marco Elver
2019-11-12 21:48             ` Alan Stern
2019-11-12 22:07               ` Eric Dumazet
2019-11-12 22:44                 ` Alexei Starovoitov
2019-11-12 23:17                   ` Eric Dumazet
2019-11-12 23:40                     ` Linus Torvalds
2019-11-13 15:00                       ` Marco Elver
2019-11-13 16:57                         ` Linus Torvalds
2019-11-13 21:33                           ` Marco Elver
2019-11-13 21:50                             ` Alan Stern
2019-11-13 22:48                               ` Marco Elver
2019-11-08 13:16 syzbot
2019-11-08 13:28 ` Eric Dumazet
2019-11-08 17:01   ` Linus Torvalds
2019-11-08 17:22     ` Eric Dumazet
2019-11-08 17:38       ` Linus Torvalds
2019-11-08 17:53         ` Eric Dumazet
2019-11-08 17:55           ` Eric Dumazet
2019-11-08 18:02             ` Eric Dumazet
2019-11-08 18:12               ` Linus Torvalds
2019-11-08 20:30             ` Linus Torvalds
2019-11-08 20:53               ` Eric Dumazet
2019-11-08 21:36                 ` Linus Torvalds
2019-11-08 18:05           ` Linus Torvalds
2019-11-08 18:15             ` Marco Elver
2019-11-08 18:40               ` Linus Torvalds
2019-11-08 19:48                 ` Marco Elver
2019-11-08 20:26                   ` Linus Torvalds
2019-11-08 21:57                     ` Alan Stern
2019-11-08 22:06                       ` Linus Torvalds
2019-11-09 23:08                         ` Alan Stern

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