[PATCH 2/2] blktrace: fix dereference after null check

[Benjamin M Romer]

From: Cengiz Can <cengiz at kernel.wtf>

There was a recent change in blktrace.c that added a RCU protection to
`q->blk_trace` in order to fix a use-after-free issue during access.

However the change missed an edge case that can lead to dereferencing of
`bt` pointer even when it's NULL:

Coverity static analyzer marked this as a FORWARD_NULL issue with CID
1460458.

```
/kernel/trace/blktrace.c: 1904 in sysfs_blk_trace_attr_store()
1898            ret = 0;
1899            if (bt == NULL)
1900                    ret = blk_trace_setup_queue(q, bdev);
1901
1902            if (ret == 0) {
1903                    if (attr == &dev_attr_act_mask)
>>>     CID 1460458:  Null pointer dereferences  (FORWARD_NULL)
>>>     Dereferencing null pointer "bt".
1904                            bt->act_mask = value;
1905                    else if (attr == &dev_attr_pid)
1906                            bt->pid = value;
1907                    else if (attr == &dev_attr_start_lba)
1908                            bt->start_lba = value;
1909                    else if (attr == &dev_attr_end_lba)
```

Added a reassignment with RCU annotation to fix the issue.

Fixes: c780e86dd48 ("blktrace: Protect q->blk_trace with RCU")
Cc: stable at vger.kernel.org
Reviewed-by: Ming Lei <ming.lei at redhat.com>
Reviewed-by: Bob Liu <bob.liu at oracle.com>
Reviewed-by: Steven Rostedt (VMware) <rostedt at goodmis.org>
Signed-off-by: Cengiz Can <cengiz at kernel.wtf>
Signed-off-by: Jens Axboe <axboe at kernel.dk>

CVE-2019-19768
(cherry picked from commit 153031a301bb07194e9c37466cfce8eacb977621)
Signed-off-by: Benjamin M Romer <benjamin.romer at canonical.com>
---
 kernel/trace/blktrace.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index a222697fd108..bd9ecb64479a 100644
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -1781,8 +1781,11 @@ static ssize_t sysfs_blk_trace_attr_store(struct device *dev,
 	}
 
 	ret = 0;
-	if (bt == NULL)
+	if (bt == NULL) {
 		ret = blk_trace_setup_queue(q, bdev);
+		bt = rcu_dereference_protected(q->blk_trace,
+				lockdep_is_held(&q->blk_trace_mutex));
+	}
 
 	if (ret == 0) {
 		if (attr == &dev_attr_act_mask)
-- 
2.20.1