From: Nicolai Stange <nstange@suse.de>
To: Bart Van Assche <bvanassche@acm.org>
Cc: Eric Sandeen <sandeen@sandeen.net>,
Luis Chamberlain <mcgrof@kernel.org>,
axboe@kernel.dk, viro@zeniv.linux.org.uk,
gregkh@linuxfoundation.org, rostedt@goodmis.org,
mingo@redhat.com, jack@suse.cz, ming.lei@redhat.com,
nstange@suse.de, mhocko@suse.com, linux-block@vger.kernel.org,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
Omar Sandoval <osandov@fb.com>, Hannes Reinecke <hare@suse.com>,
Michal Hocko <mhocko@kernel.org>,
syzbot+603294af2d01acfdd6da@syzkaller.appspotmail.com
Subject: Re: [RFC 2/3] blktrace: fix debugfs use after free
Date: Mon, 06 Apr 2020 11:18:13 +0200 [thread overview]
Message-ID: <87d08lj7l6.fsf@suse.de> (raw)
In-Reply-To: <75aa4cff-1b90-ebd4-17a4-c1cb6d390b30@acm.org> (Bart Van Assche's message of "Sun, 5 Apr 2020 21:25:41 -0700")
Bart Van Assche <bvanassche@acm.org> writes:
> On 2020-04-05 18:27, Eric Sandeen wrote:
>> The thing I can't figure out from reading the change log is
>>
>> 1) what the root cause of the problem is, and
>> 2) how this patch fixes it?
>
> I think that the root cause is that do_blk_trace_setup() uses
> debugfs_lookup() and that debugfs_lookup() may return a pointer
> associated with a previous incarnation of the block device.
That's correct, the debugfs_lookup() can find a previous incarnation's
dir of the same name which is about to get removed from a not yet
schedule work.
I.e. something like the following is possible:
LOOP_CTL_DEL(loop0) /* schedule __blk_release_queue() work_struct */
LOOP_CTL_ADD(loop0) /* debugfs_create_dir() from
* blk_mq_debugfs_register() fails with EEXIST
*/
BLKTRACE_SETUP(loop0) /* debugfs_lookup() finds the directory about to
* get deleted and blktrace files will be created
* thereunder.
*/
The work_struct gets scheduled and the debugfs dir debugfs_remove()ed
recursively, which includes the blktrace files just created. blktrace's
dentry pointers are now dangling and there will be a UAF when it
attempts to delete those again.
Luis' patch [2/3] fixes the issue of the debugfs_lookup() from
blk_mq_debugfs_register() potentially returning an existing directory
associated with a previous block device incarnation of the same name and
thus, fixes the UAF.
However, the problem that the debugfs_create_dir() from
blk_mq_debugfs_register() in a sequence of
LOOP_CTL_DEL(loop0)
LOOP_CTL_ADD(loop0)
could silently fail still remains. The RFC patch [3/3] from Luis
attempts to address this issue by folding the delayed
__blk_release_queue() work back into blk_release_queue(), the release
handler associated with the queue kobject and executed from the final
blk_queue_put(). However, that's still no full solution, because the
kobject release handler can run asynchronously, long after
blk_unregister_queue() has returned (c.f. also
CONFIG_DEBUG_KOBJECT_RELEASE).
Note that I proposed this change here (internally) as a potential
cleanup, because I missed the kobject_del() from blk_unregister_queue()
and *wrongly* concluded that blk_queue_put() must be allowed to sleep
nowadays. However, that kobject_del() is in place and moreover, the
analysis requested by Bart (c.f. [1] in this thread) revealed that there
are indeed a couple of sites calling blk_queue_put() from atomic
context.
So I'd suggest to drop patch [3/3] from this series and modify this
patch [2/3] here to move the blk_q_debugfs_unregister(q) invocation from
__blk_release_queue() to blk_unregister_queue() instead.
> Additionally, I think the following changes fix that problem by using
> q->debugfs_dir in the blktrace code instead of debugfs_lookup():
That would fix the UAF, but !queue_is_mq() queues wouldn't get a debugfs
directory created for them by blktrace anymore?
Thanks,
Nicolai
[1] https://lkml.kernel.org/r/87o8saj62m.fsf@suse.de
> [ ... ]
> --- a/kernel/trace/blktrace.c
> +++ b/kernel/trace/blktrace.c
> @@ -311,7 +311,6 @@ static void blk_trace_free(struct blk_trace *bt)
> debugfs_remove(bt->msg_file);
> debugfs_remove(bt->dropped_file);
> relay_close(bt->rchan);
> - debugfs_remove(bt->dir);
> free_percpu(bt->sequence);
> free_percpu(bt->msg_data);
> kfree(bt);
> [ ... ]
> @@ -509,21 +510,19 @@ static int do_blk_trace_setup(struct request_queue
> *q, char *name, dev_t dev,
>
> ret = -ENOENT;
>
> - dir = debugfs_lookup(buts->name, blk_debugfs_root);
> - if (!dir)
> - bt->dir = dir = debugfs_create_dir(buts->name, blk_debugfs_root);
> -
> bt->dev = dev;
> atomic_set(&bt->dropped, 0);
> INIT_LIST_HEAD(&bt->running_list);
>
> ret = -EIO;
> - bt->dropped_file = debugfs_create_file("dropped", 0444, dir, bt,
> + bt->dropped_file = debugfs_create_file("dropped", 0444,
> + q->debugfs_dir, bt,
> &blk_dropped_fops);
> [ ... ]
>
> Bart.
--
SUSE Software Solutions Germany GmbH, Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg), GF: Felix Imendörffer
next prev parent reply other threads:[~2020-04-06 9:18 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-01 23:59 [RFC 0/3] block: address blktrace use-after-free Luis Chamberlain
2020-04-02 0:00 ` [RFC 1/3] block: move main block debugfs initialization to its own file Luis Chamberlain
2020-04-05 3:12 ` Bart Van Assche
2020-04-06 14:23 ` Luis Chamberlain
2020-04-02 0:00 ` [RFC 2/3] blktrace: fix debugfs use after free Luis Chamberlain
2020-04-02 1:57 ` Eric Sandeen
2020-04-02 16:14 ` Luis Chamberlain
2020-04-05 3:39 ` Bart Van Assche
2020-04-06 1:27 ` Eric Sandeen
2020-04-06 4:25 ` Bart Van Assche
2020-04-06 9:18 ` Nicolai Stange [this message]
2020-04-06 15:19 ` Luis Chamberlain
2020-04-07 8:15 ` Luis Chamberlain
2020-04-06 14:29 ` Eric Sandeen
2020-04-07 8:09 ` Luis Chamberlain
2020-04-06 15:14 ` Luis Chamberlain
2020-04-02 0:00 ` [RFC 3/3] block: avoid deferral of blk_release_queue() work Luis Chamberlain
2020-04-02 3:39 ` Bart Van Assche
2020-04-02 14:49 ` Nicolai Stange
2020-04-06 9:11 ` Nicolai Stange
2020-04-09 18:11 ` Luis Chamberlain
2020-04-02 7:44 ` [RFC 0/3] block: address blktrace use-after-free Greg KH
2020-04-03 8:19 ` Ming Lei
2020-04-03 14:06 ` Luis Chamberlain
2020-04-03 14:13 ` Bart Van Assche
2020-04-03 19:49 ` Luis Chamberlain
2020-04-07 2:47 ` yukuai (C)
2020-04-07 19:00 ` Luis Chamberlain
2020-04-09 20:59 ` Luis Chamberlain
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