Subject: Re: [PATCH] af_unix: Fix splice-bind deadlock

From: Rainer Weikusat <[email-masked]>
Date: Sun, 03 Jan 2016 18:03:24 +0000

Rainer Weikusat <[email-masked]> writes:

> Hannes Frederic Sowa <[email-masked]> writes:
>> On 27.12.2015 21:13, Rainer Weikusat wrote:
>>> -static int unix_mknod(const char *sun_path, umode_t mode, struct path *res)
>>> +static int unix_mknod(struct dentry *dentry, struct path *path, umode_t mode,
>>> +		      struct path *res)
>>>   {
>>> -	struct dentry *dentry;
>>> -	struct path path;
>>> -	int err = 0;
>>> -	/*
>>> -	 * Get the parent directory, calculate the hash for last
>>> -	 * component.
>>> -	 */
>>> -	dentry = kern_path_create(AT_FDCWD, sun_path, &path, 0);
>>> -	err = PTR_ERR(dentry);
>>> -	if (IS_ERR(dentry))
>>> -		return err;
>>> +	int err;
>>>
>>> -	/*
>>> -	 * All right, let's create it.
>>> -	 */
>>> -	err = security_path_mknod(&path, dentry, mode, 0);
>>> +	err = security_path_mknod(path, dentry, mode, 0);
>>>   	if (!err) {
>>> -		err = vfs_mknod(d_inode(path.dentry), dentry, mode, 0);
>>> +		err = vfs_mknod(d_inode(path->dentry), dentry, mode, 0);
>>>   		if (!err) {
>>> -			res->mnt = mntget(path.mnt);
>>> +			res->mnt = mntget(path->mnt);
>>>   			res->dentry = dget(dentry);
>>>   		}
>>>   	}
>>> -	done_path_create(&path, dentry);
>>> +
>>
>> The reordered call to done_path_create will change the locking
>> ordering between the i_mutexes and the unix readlock. Can you comment
>> on this? On a first sight this looks like a much more dangerous change
>> than the original deadlock report. Can't this also conflict with
>> splice code deep down in vfs layer?
>
> Practical consideration

[...]

> A deadlock was possible here if the thread doing the bind then blocked
> when trying to acquire the readlock while the thread holding the
> readlock is blocked on another lock held by a thread trying to perform
> an operation on the same directory as the bind (possibly with some
> indirection).

Since this was probably pretty much a "write only" sentence, I think I
should try this again (with apologies in case a now err on the other
side and rather explain to much --- my abilities to express myself such
that people understand what I mean to express instead of just getting
mad at me are not great).

For a deadlock to happen here, there needs to be a cycle (circle?) of
threads each holding one lock and blocking while trying to acquire
another lock which ultimatively ends with a thread trying to acquire the
i_mutex of the directory where the socket name is to be created. The
binding thread would need to block when trying to acquire the
readlock. But (contrary to what I originally wrote[*]) this cannot happen
because the af_unix code doesn't lock anything non-socket related while
holding the readlock. The only instance of that was in _bind and caused
the deadlock.

[*] I misread

static ssize_t skb_unix_socket_splice(struct sock *sk,
				      struct pipe_inode_info *pipe,
				      struct splice_pipe_desc *spd)
{
	int ret;
	struct unix_sock *u = unix_sk(sk);

	mutex_unlock(&u->readlock);
	ret = splice_to_pipe(pipe, spd);
	mutex_lock(&u->readlock);

	return ret;
}

as 'lock followed by unlock' instead of 'unlock followed by lock'.