3.16.83-rc1 review patch. If anyone has any objections, please let me know.
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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@decadent.org.uk --- kernel/trace/blktrace.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)
--- a/kernel/trace/blktrace.c +++ b/kernel/trace/blktrace.c @@ -1834,8 +1834,11 @@ 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)