[SRU][J][PATCH v4 10/11] kernfs: fix potential NULL dereference in __kernfs_remove

[Ghadi Elie Rahme]

From: Yushan Zhou <katrinzhou at tencent.com>

BugLink: https://bugs.launchpad.net/bugs/2125142

When lockdep is enabled, lockdep_assert_held_write would
cause potential NULL pointer dereference.

Fix the following smatch warnings:

fs/kernfs/dir.c:1353 __kernfs_remove() warn: variable dereferenced before check 'kn' (see line 1346)

Fixes: 393c3714081a ("kernfs: switch global kernfs_rwsem lock to per-fs lock")
Signed-off-by: Yushan Zhou <katrinzhou at tencent.com>
Link: https://lore.kernel.org/r/20220630082512.3482581-1-zys.zljxml@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
(cherry picked from commit 72b5d5aef246a0387cefa23121dd90901c7a691a)
Signed-off-by: Ghadi Elie Rahme <ghadi.rahme at canonical.com>
---
 fs/kernfs/dir.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 2a791d590c70..32dc8435630d 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -1362,14 +1362,17 @@ static void __kernfs_remove(struct kernfs_node *kn)
 {
 	struct kernfs_node *pos;
 
+	/* Short-circuit if non-root @kn has already finished removal. */
+	if (!kn)
+		return;
+
 	lockdep_assert_held_write(&kernfs_root(kn)->kernfs_rwsem);
 
 	/*
-	 * Short-circuit if non-root @kn has already finished removal.
 	 * This is for kernfs_remove_self() which plays with active ref
 	 * after removal.
 	 */
-	if (!kn || (kn->parent && RB_EMPTY_NODE(&kn->rb)))
+	if (kn->parent && RB_EMPTY_NODE(&kn->rb))
 		return;
 
 	pr_debug("kernfs %s: removing\n", kn->name);
-- 
2.34.1