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 raspberrypi#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
it fails to prevent all race conditions.

Fix it by converting to write lock of s_umount in f2fs_do_shutdown().

Fixes: 7950e9a ("f2fs: stop gc/discard thread after fs shutdown")
Signed-off-by: Long Li <leo.lilong@huawei.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/file.c

@@ -2365,9 +2365,12 @@ int f2fs_do_shutdown(struct f2fs_sb_info *sbi, unsigned int flag,
 	if (readonly)
 		goto out;
 
-	/* grab sb->s_umount to avoid racing w/ remount() */
+	/*
+	 * grab sb->s_umount to avoid racing w/ remount() and other shutdown
+	 * paths.
+	 */
 	if (need_lock)
-		down_read(&sbi->sb->s_umount);
+		down_write(&sbi->sb->s_umount);
 
 	f2fs_stop_gc_thread(sbi);
 	f2fs_stop_discard_thread(sbi);
@@ -2376,7 +2379,7 @@ int f2fs_do_shutdown(struct f2fs_sb_info *sbi, unsigned int flag,
 	clear_opt(sbi, DISCARD);
 
 	if (need_lock)
-		up_read(&sbi->sb->s_umount);
+		up_write(&sbi->sb->s_umount);
 
 	f2fs_update_time(sbi, REQ_TIME);
 out: