f2fs: fix race in concurrent f2fs_stop_gc_thread

In my test case, concurrent calls to f2fs shutdown report the following
stack trace:

 Oops: general protection fault, probably for non-canonical address 0xc6cfff63bb5513fc: 0000 [#1] PREEMPT SMP PTI
 CPU: 0 UID: 0 PID: 678 Comm: f2fs_rep_shutdo Not tainted 6.12.0-rc5-next-20241029-g6fb2fa9805c5-dirty torvalds#85
 Call Trace:
  <TASK>
  ? show_regs+0x8b/0xa0
  ? __die_body+0x26/0xa0
  ? die_addr+0x54/0x90
  ? exc_general_protection+0x24b/0x5c0
  ? asm_exc_general_protection+0x26/0x30
  ? kthread_stop+0x46/0x390
  f2fs_stop_gc_thread+0x6c/0x110
  f2fs_do_shutdown+0x309/0x3a0
  f2fs_ioc_shutdown+0x150/0x1c0
  __f2fs_ioctl+0xffd/0x2ac0
  f2fs_ioctl+0x76/0xe0
  vfs_ioctl+0x23/0x60
  __x64_sys_ioctl+0xce/0xf0
  x64_sys_call+0x2b1b/0x4540
  do_syscall_64+0xa7/0x240
  entry_SYSCALL_64_after_hwframe+0x76/0x7e

The root cause is a race condition in f2fs_stop_gc_thread() called from
different f2fs shutdown paths:

  [CPU0]                       [CPU1]
  ----------------------       -----------------------
  f2fs_stop_gc_thread          f2fs_stop_gc_thread
                                 gc_th = sbi->gc_thread
    gc_th = sbi->gc_thread
    kfree(gc_th)
    sbi->gc_thread = NULL
                                 < gc_th != NULL >
                                 kthread_stop(gc_th->f2fs_gc_task) //UAF

The commit c7f114d ("f2fs: fix to avoid use-after-free in
f2fs_stop_gc_thread()") attempted to fix this issue by using a read
semaphore to prevent races between shutdown and remount threads, but
itfails to prevent all race conditions.

While upgrading to s_umount write lock in f2fs_do_shutdown() would fix
the current issue, however, using s_umount lock requires extreme caution
to avoid lock recursion. A better solution is to introduce a semaphore
to prevent races between concurrent f2fs_stop_gc_thread calls.

Fixes: 7950e9a ("f2fs: stop gc/discard thread after fs shutdown")
Signed-off-by: Long Li <leo.lilong@huawei.com>

fs/f2fs/f2fs.h

@@ -1679,6 +1679,7 @@ struct f2fs_sb_info {
 						 * race between GC and GC or CP
 						 */
 	struct f2fs_gc_kthread	*gc_thread;	/* GC thread */
+	struct semaphore gc_clean_lock;		/* semaphore for clean GC thread */
 	struct atgc_management am;		/* atgc management */
 	unsigned int cur_victim_sec;		/* current victim section num */
 	unsigned int gc_mode;			/* current GC state */

fs/f2fs/gc.c

@@ -232,14 +232,19 @@ int f2fs_start_gc_thread(struct f2fs_sb_info *sbi)
 
 void f2fs_stop_gc_thread(struct f2fs_sb_info *sbi)
 {
-	struct f2fs_gc_kthread *gc_th = sbi->gc_thread;
+	struct f2fs_gc_kthread *gc_th;
 
-	if (!gc_th)
+	down(&sbi->gc_clean_lock);
+	gc_th = sbi->gc_thread;
+	if (!gc_th) {
+		up(&sbi->gc_clean_lock);
 		return;
+	}
 	kthread_stop(gc_th->f2fs_gc_task);
 	wake_up_all(&gc_th->fggc_wq);
 	kfree(gc_th);
 	sbi->gc_thread = NULL;
+	up(&sbi->gc_clean_lock);
 }
 
 static int select_gc_type(struct f2fs_sb_info *sbi, int gc_type)

fs/f2fs/super.c

@@ -4420,6 +4420,7 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
 
 	/* initialize locks within allocated memory */
 	init_f2fs_rwsem(&sbi->gc_lock);
+	sema_init(&sbi->gc_clean_lock, 1);
 	mutex_init(&sbi->writepages);
 	init_f2fs_rwsem(&sbi->cp_global_sem);
 	init_f2fs_rwsem(&sbi->node_write);