[PATCH] gfs2: Fix use-after-free in glockfd iterator

gfs2_glockfd_seq_stop() drops the file and task references held by
the iterator, but leaves both pointers unchanged. seq_file can invoke the
iterator again after stop, for example after a bounded read followed by a
rewind.

The next start then drops the stale task reference again in
gfs2_glockfd_next_task(), and can similarly drop the stale file reference
in gfs2_glockfd_next_file(). A duplicate task put can queue the task_struct
for RCU freeing before fget_task_next() uses it.

Clear both pointers when releasing their references so later iterator
callbacks cannot put them again.

Fixes: 4480c27ca3ea ("gfs2: Add glockfd debugfs file")
Cc: [email protected]
Signed-off-by: Shuangpeng Bai <[email protected]>
---
 fs/gfs2/glock.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/fs/gfs2/glock.c b/fs/gfs2/glock.c
index b8a144d3a73b..283c5701dc1a 100644
--- a/fs/gfs2/glock.c
+++ b/fs/gfs2/glock.c
@@ -2734,10 +2734,14 @@ static void gfs2_glockfd_seq_stop(struct seq_file *seq, void *iter_ptr)
 {
 	struct gfs2_glockfd_iter *i = seq->private;
 
-	if (i->file)
+	if (i->file) {
 		fput(i->file);
-	if (i->task)
+		i->file = NULL;
+	}
+	if (i->task) {
 		put_task_struct(i->task);
+		i->task = NULL;
+	}
 }
 
 static void gfs2_glockfd_seq_show_flock(struct seq_file *seq,
-- 
2.43.0