[PATCH v2] fs/proc: Fix NULL pointer dereference in pid_delete_dentry

[Gao Yahu <yahu.gao@xxxxxxxxxxxxx>]

Get the staus of task from the pointer of proc inode directly is not
safe. Make it happen in RCU protection.

Signed-off-by: Yahu Gao <yahu.gao@xxxxxxxxxxxxx>
---
v2 changes:
  - Use RCU lock to avoid NULL dereference of pid.
---
 fs/proc/base.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index b3422cda2a91..d44b5f2414a6 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1,3 +1,4 @@
+
 // SPDX-License-Identifier: GPL-2.0
 /*
  *  linux/fs/proc/base.c
@@ -1994,7 +1995,16 @@ static int pid_revalidate(struct dentry *dentry, unsigned int flags)
 
 static inline bool proc_inode_is_dead(struct inode *inode)
 {
-	return !proc_pid(inode)->tasks[PIDTYPE_PID].first;
+	struct pid *pid = NULL;
+	struct hlist_node *first = NULL;
+
+	rcu_read_lock();
+	pid = proc_pid(inode);
+	if (likely(pid))
+		first = rcu_dereference_check(hlist_first_rcu(&pid->tasks[PIDTYPE_PID]),
+					      lockdep_tasklist_lock_is_held());
+	rcu_read_unlock();
+	return !first;
 }
 
 int pid_delete_dentry(const struct dentry *dentry)
-- 
2.25.1