[PATCH v2] fs: f2fs: fix UAF in f2fs_available_free_memory

[Dongliang Mu]

if2fs_fill_super
-> f2fs_build_segment_manager
   -> create_discard_cmd_control
      -> f2fs_start_discard_thread

It invokes kthread_run to create a thread and run issue_discard_thread.

However, if f2fs_build_node_manager fails, the control flow goes to
free_nm and calls f2fs_destroy_node_manager. This function will free
sbi->nm_info. However, if issue_discard_thread accesses sbi->nm_info
after the deallocation, but before the f2fs_stop_discard_thread, it will
cause UAF(Use-after-free).

-> f2fs_destroy_segment_manager
   -> destroy_discard_cmd_control
      -> f2fs_stop_discard_thread

Fix this by stopping discard thread before f2fs_destroy_node_manager.

Note that, the commit d6d2b491a82e1 introduces the call of
f2fs_available_free_memory into issue_discard_thread.

Fixes: d6d2b491a82e ("f2fs: allow to change discard policy based on cached discard cmds")
Signed-off-by: Dongliang Mu <mudongliangabcd@xxxxxxxxx>
---
 fs/f2fs/super.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index 78ebc306ee2b..dbe040b66802 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -4352,6 +4352,8 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
 free_stats:
 	f2fs_destroy_stats(sbi);
 free_nm:
+	/* stop discard thread before destroying node manager */
+	f2fs_stop_discard_thread(sbi);
 	f2fs_destroy_node_manager(sbi);
 free_sm:
 	f2fs_destroy_segment_manager(sbi);
-- 
2.25.1