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    Subject[PATCH 4.4 27/86] 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
    @@ -1822,8 +1822,11 @@ 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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