[PATCH] ext4: Fix possible NULL pointer dereference in ext4_group_desc_free()

[Zqiang]

This can happen if the kvmalloc_objs() fails and sbi->s_group_desc pointer
is NULL in the ext4_group_desc_init(), and then the ext4_group_desc_free()
is called, leading to a NULL group_desc pointer dereference.

This commit therefore adds a NULL check for sbi->s_group_desc before
accessing its internal members.

Signed-off-by: Zqiang <qiang.zhang@xxxxxxxxx>
---
 fs/ext4/super.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 43f680c750ae..c4307dc04687 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1256,9 +1256,11 @@ static void ext4_group_desc_free(struct ext4_sb_info *sbi)
 
 	rcu_read_lock();
 	group_desc = rcu_dereference(sbi->s_group_desc);
-	for (i = 0; i < sbi->s_gdb_count; i++)
-		brelse(group_desc[i]);
-	kvfree(group_desc);
+	if (group_desc) {
+		for (i = 0; i < sbi->s_gdb_count; i++)
+			brelse(group_desc[i]);
+		kvfree(group_desc);
+	}
 	rcu_read_unlock();
 }
 
-- 
2.17.1