GFS2: Fix unsafe dereference in dump_holder()

GLOCK_BUG_ON() might call this function without RCU read lock. Make sure that
RCU read lock is held when using task_struct returned from pid_task().

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/fs/gfs2/glock.c b/fs/gfs2/glock.c
index c8420f7e4db6..6f7a47c05259 100644
--- a/fs/gfs2/glock.c
+++ b/fs/gfs2/glock.c
@@ -1655,6 +1655,7 @@ static int dump_holder(struct seq_file *seq, const struct gfs2_holder *gh)
         struct task_struct *gh_owner = NULL;
         char flags_buf[32];
 
+        rcu_read_lock();
         if (gh->gh_owner_pid)
                 gh_owner = pid_task(gh->gh_owner_pid, PIDTYPE_PID);
         gfs2_print_dbg(seq, " H: s:%s f:%s e:%d p:%ld [%s] %pS\n",
@@ -1664,6 +1665,7 @@ static int dump_holder(struct seq_file *seq, const struct gfs2_holder *gh)
                        gh->gh_owner_pid ? (long)pid_nr(gh->gh_owner_pid) : -1,
                        gh_owner ? gh_owner->comm : "(ended)",
                        (void *)gh->gh_ip);
+        rcu_read_unlock();
         return 0;
 }