[PATCH] blktrace: fix dereference after null check

[Cengiz Can <cengiz@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:

```
        bt->act_mask = value; // bt can still be NULL here
```

Added a reassignment into the NULL check block to fix the issue.

Fixes: c780e86dd48 ("blktrace: Protect q->blk_trace with RCU")

Signed-off-by: Cengiz Can <cengiz@kernel.wtf>
---
 Huge thanks goes to Steven Rostedt for his assistance.

 kernel/trace/blktrace.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index 4560878f0bac..29ea88f10b87 100644
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -1896,8 +1896,10 @@ 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 = q->blk_trace;
+	}

 	if (ret == 0) {
 		if (attr == &dev_attr_act_mask)
--
2.25.1