ext4: fix possible use-after-free in ext4_remove_li_request()

We need to take reference to the s_li_request after we take a mutex,
because it might be freed since then, hence result in accessing old
already freed memory. Also we should protect the whole
ext4_remove_li_request() because ext4_li_info might be in the process of
being freed in ext4_lazyinit_thread().

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>

1 files changed, 6 insertions, 4 deletions

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index fd51a4a266b6..ed5e80ef48c4 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -2735,14 +2735,16 @@ static void ext4_remove_li_request(struct ext4_li_request *elr)
 
 static void ext4_unregister_li_request(struct super_block *sb)
 {
-        struct ext4_li_request *elr = EXT4_SB(sb)->s_li_request;
-
-        if (!ext4_li_info)
+        mutex_lock(&ext4_li_mtx);
+        if (!ext4_li_info) {
+                mutex_unlock(&ext4_li_mtx);
                 return;
+        }
 
         mutex_lock(&ext4_li_info->li_list_mtx);
-        ext4_remove_li_request(elr);
+        ext4_remove_li_request(EXT4_SB(sb)->s_li_request);
         mutex_unlock(&ext4_li_info->li_list_mtx);
+        mutex_unlock(&ext4_li_mtx);
 }
 
 static struct task_struct *ext4_lazyinit_task;