f2fs: fix to avoid use-after-free in f2fs_stop_gc_thread()

syzbot reports a f2fs bug as below:

 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0x241/0x360 lib/dump_stack.c:114
 print_report+0xe8/0x550 mm/kasan/report.c:491
 kasan_report+0x143/0x180 mm/kasan/report.c:601
 kasan_check_range+0x282/0x290 mm/kasan/generic.c:189
 instrument_atomic_read_write include/linux/instrumented.h:96 [inline]
 atomic_fetch_add_relaxed include/linux/atomic/atomic-instrumented.h:252 [inline]
 __refcount_add include/linux/refcount.h:184 [inline]
 __refcount_inc include/linux/refcount.h:241 [inline]
 refcount_inc include/linux/refcount.h:258 [inline]
 get_task_struct include/linux/sched/task.h:118 [inline]
 kthread_stop+0xca/0x630 kernel/kthread.c:704
 f2fs_stop_gc_thread+0x65/0xb0 fs/f2fs/gc.c:210
 f2fs_do_shutdown+0x192/0x540 fs/f2fs/file.c:2283
 f2fs_ioc_shutdown fs/f2fs/file.c:2325 [inline]
 __f2fs_ioctl+0x443a/0xbe60 fs/f2fs/file.c:4325
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:907 [inline]
 __se_sys_ioctl+0xfc/0x170 fs/ioctl.c:893
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

The root cause is below race condition, it may cause use-after-free
issue in sbi->gc_th pointer.

- remount
 - f2fs_remount
  - f2fs_stop_gc_thread
   - kfree(gc_th)
				- f2fs_ioc_shutdown
				 - f2fs_do_shutdown
				  - f2fs_stop_gc_thread
				   - kthread_stop(gc_th->f2fs_gc_task)
   : sbi->gc_thread = NULL;

We will call f2fs_do_shutdown() in two paths:
- for f2fs_ioc_shutdown() path, we should grab sb->s_umount semaphore
for fixing.
- for f2fs_shutdown() path, it's safe since caller has already grabbed
sb->s_umount semaphore.

Reported-by: syzbot+1a8e2b31f2ac9bd3d148@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/linux-f2fs-devel/0000000000005c7ccb061e032b9b@google.com
Fixes: 7950e9ac ("f2fs: stop gc/discard thread after fs shutdown")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/f2fs.h

@@ -3508,7 +3508,7 @@ int f2fs_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
 int f2fs_truncate_hole(struct inode *inode, pgoff_t pg_start, pgoff_t pg_end);
 void f2fs_truncate_data_blocks_range(struct dnode_of_data *dn, int count);
 int f2fs_do_shutdown(struct f2fs_sb_info *sbi, unsigned int flag,
-							bool readonly);
+						bool readonly, bool need_lock);
 int f2fs_precache_extents(struct inode *inode);
 int f2fs_fileattr_get(struct dentry *dentry, struct fileattr *fa);
 int f2fs_fileattr_set(struct mnt_idmap *idmap,

fs/f2fs/file.c

@@ -2297,7 +2297,7 @@ static int f2fs_ioc_abort_atomic_write(struct file *filp)
 }
 
 int f2fs_do_shutdown(struct f2fs_sb_info *sbi, unsigned int flag,
-							bool readonly)
+						bool readonly, bool need_lock)
 {
 	struct super_block *sb = sbi->sb;
 	int ret = 0;
@@ -2344,12 +2344,19 @@ 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() */
+	if (need_lock)
+		down_read(&sbi->sb->s_umount);
+
 	f2fs_stop_gc_thread(sbi);
 	f2fs_stop_discard_thread(sbi);
 
 	f2fs_drop_discard_cmd(sbi);
 	clear_opt(sbi, DISCARD);
 
+	if (need_lock)
+		up_read(&sbi->sb->s_umount);
+
 	f2fs_update_time(sbi, REQ_TIME);
 out:
 
@@ -2386,7 +2393,7 @@ static int f2fs_ioc_shutdown(struct file *filp, unsigned long arg)
 		}
 	}
 
-	ret = f2fs_do_shutdown(sbi, in, readonly);
+	ret = f2fs_do_shutdown(sbi, in, readonly, true);
 
 	if (need_drop)
 		mnt_drop_write_file(filp);

fs/f2fs/super.c

@@ -2560,7 +2560,7 @@ static int f2fs_remount(struct super_block *sb, int *flags, char *data)
 
 static void f2fs_shutdown(struct super_block *sb)
 {
-	f2fs_do_shutdown(F2FS_SB(sb), F2FS_GOING_DOWN_NOSYNC, false);
+	f2fs_do_shutdown(F2FS_SB(sb), F2FS_GOING_DOWN_NOSYNC, false, false);
 }
 
 #ifdef CONFIG_QUOTA