f2fs: fix to avoid potential deadlock

There is a potential deadlock reported by syzbot as below:

F2FS-fs (loop2): invalid crc value
F2FS-fs (loop2): Found nat_bits in checkpoint
F2FS-fs (loop2): Mounted with checkpoint version = 48b305e4
======================================================
WARNING: possible circular locking dependency detected
6.1.0-rc8-syzkaller-33330-ga5541c0811a0 #0 Not tainted
------------------------------------------------------
syz-executor.2/32123 is trying to acquire lock:
ffff0000c0e1a608 (&mm->mmap_lock){++++}-{3:3}, at: __might_fault+0x54/0xb4 mm/memory.c:5644

but task is already holding lock:
ffff0001317c6088 (&sbi->sb_lock){++++}-{3:3}, at: f2fs_down_write fs/f2fs/f2fs.h:2205 [inline]
ffff0001317c6088 (&sbi->sb_lock){++++}-{3:3}, at: f2fs_ioc_get_encryption_pwsalt fs/f2fs/file.c:2334 [inline]
ffff0001317c6088 (&sbi->sb_lock){++++}-{3:3}, at: __f2fs_ioctl+0x1370/0x3318 fs/f2fs/file.c:4151

which lock already depends on the new lock.

Chain exists of:
  &mm->mmap_lock --> &nm_i->nat_tree_lock --> &sbi->sb_lock

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&sbi->sb_lock);
                               lock(&nm_i->nat_tree_lock);
                               lock(&sbi->sb_lock);
  lock(&mm->mmap_lock);

Let's try to avoid above deadlock condition by moving __might_fault()
out of sbi->sb_lock coverage.

Fixes: 95fa90c9 ("f2fs: support recording errors into superblock")
Link: https://lore.kernel.org/linux-f2fs-devel/000000000000cd5fe305ef617fe2@google.com/T/#u
Reported-by: syzbot+4793f6096d174c90b4f7@syzkaller.appspotmail.com
Signed-off-by: Chao Yu <chao@kernel.org>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/file.c

@@ -2336,6 +2336,7 @@ static int f2fs_ioc_get_encryption_pwsalt(struct file *filp, unsigned long arg)
 {
 	struct inode *inode = file_inode(filp);
 	struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
+	u8 encrypt_pw_salt[16];
 	int err;
 
 	if (!f2fs_sb_has_encrypt(sbi))
@@ -2360,12 +2361,14 @@ static int f2fs_ioc_get_encryption_pwsalt(struct file *filp, unsigned long arg)
 		goto out_err;
 	}
 got_it:
-	if (copy_to_user((__u8 __user *)arg, sbi->raw_super->encrypt_pw_salt,
-									16))
-		err = -EFAULT;
+	memcpy(encrypt_pw_salt, sbi->raw_super->encrypt_pw_salt, 16);
 out_err:
 	f2fs_up_write(&sbi->sb_lock);
 	mnt_drop_write_file(filp);
+
+	if (!err && copy_to_user((__u8 __user *)arg, encrypt_pw_salt, 16))
+		err = -EFAULT;
+
 	return err;
 }