[PATCH 6.1.y 3/4] net: rfkill: reduce data->mtx scope in rfkill_fop_open

[Sasha Levin <sashal@kernel.org>]

From: Johannes Berg <johannes.berg@intel.com>

[ Upstream commit f2ac54ebf85615a6d78f5eb213a8bbeeb17ebe5d ]

In syzbot runs, lockdep reports that there's a (potential)
deadlock here of data->mtx being locked recursively. This
isn't really a deadlock since they are different instances,
but lockdep cannot know, and teaching it would be far more
difficult than other fixes.

At the same time we don't even really _need_ the mutex to
be locked in rfkill_fop_open(), since we're modifying only
a completely fresh instance of 'data' (struct rfkill_data)
that's not yet added to the global list.

However, to avoid any reordering etc. within the globally
locked section, and to make the code look more symmetric,
we should still lock the data->events list manipulation,
but also need to lock _only_ that. So do that.

Reported-by: syzbot+509238e523e032442b80@syzkaller.appspotmail.com
Fixes: 2c3dfba4cf84 ("rfkill: sync before userspace visibility/changes")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Stable-dep-of: ea245d78dec5 ("net: rfkill: prevent unlimited numbers of rfkill events from being created")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/rfkill/core.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/net/rfkill/core.c b/net/rfkill/core.c
index 981771598f1eb..7ec100adf5945 100644
--- a/net/rfkill/core.c
+++ b/net/rfkill/core.c
@@ -1180,7 +1180,6 @@ static int rfkill_fop_open(struct inode *inode, struct file *file)
 	init_waitqueue_head(&data->read_wait);
 
 	mutex_lock(&rfkill_global_mutex);
-	mutex_lock(&data->mtx);
 	/*
 	 * start getting events from elsewhere but hold mtx to get
 	 * startup events added first
@@ -1192,10 +1191,11 @@ static int rfkill_fop_open(struct inode *inode, struct file *file)
 			goto free;
 		rfkill_sync(rfkill);
 		rfkill_fill_event(&ev->ev, rfkill, RFKILL_OP_ADD);
+		mutex_lock(&data->mtx);
 		list_add_tail(&ev->list, &data->events);
+		mutex_unlock(&data->mtx);
 	}
 	list_add(&data->list, &rfkill_fds);
-	mutex_unlock(&data->mtx);
 	mutex_unlock(&rfkill_global_mutex);
 
 	file->private_data = data;
@@ -1203,7 +1203,6 @@ static int rfkill_fop_open(struct inode *inode, struct file *file)
 	return stream_open(inode, file);
 
  free:
-	mutex_unlock(&data->mtx);
 	mutex_unlock(&rfkill_global_mutex);
 	mutex_destroy(&data->mtx);
 	list_for_each_entry_safe(ev, tmp, &data->events, list)
-- 
2.53.0