This is a note to let you know that I've just added the patch titled blktrace: fix dereference after null check to the 4.4-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: blktrace-fix-dereference-after-null-check.patch and it can be found in the queue-4.4 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. commit af666a41a5823e30f88387ff0dc85cd3d6d0cbe4 Author: Cengiz Can <cengiz@xxxxxxxxxx> Date: Wed Mar 4 13:58:19 2020 +0300 blktrace: fix dereference after null check 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@xxxxxxxxxx> Reviewed-by: Bob Liu <bob.liu@xxxxxxxxxx> Reviewed-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx> Signed-off-by: Cengiz Can <cengiz@xxxxxxxxxx> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx> Signed-off-by: Ben Hutchings <ben.hutchings@xxxxxxxxxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c index bf9bf3f22edb5..6737564680193 100644 --- a/kernel/trace/blktrace.c +++ b/kernel/trace/blktrace.c @@ -1822,8 +1822,11 @@ 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)