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Subject[PATCH 4.14 042/114] blktrace: fix dereference after null check
Date
From: Cengiz Can <cengiz@kernel.wtf>

commit 153031a301bb07194e9c37466cfce8eacb977621 upstream.

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")
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Bob Liu <bob.liu@oracle.com>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Cengiz Can <cengiz@kernel.wtf>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/trace/blktrace.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -1911,8 +1911,11 @@ static ssize_t sysfs_blk_trace_attr_stor
}

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)

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