[PATCH] ext4: validate donor file superblock early in EXT4_IOC_MOVE_EXT

[Yun Zhou]

Reject the EXT4_IOC_MOVE_EXT ioctl early if the donor file does not
belong to the same superblock as the original file.  Currently, this
validation is performed inside ext4_move_extents() by
mext_check_validity(), but only after lock_two_nondirectories() has
already acquired the inode locks.  When the donor fd refers to a file
on a different filesystem (e.g., overlayfs), this late validation
creates a circular lock dependency:

  CPU0 (overlayfs write)            CPU1 (ext4 ioctl)
  ----                              ----
  inode_lock(ovl_inode)
                                    mnt_want_write_file(filp)
                                      sb_start_write(ext4_sb)   [sb_writers]
    backing_file_write_iter()
      vfs_iter_write(real_file)
        file_start_write(real_file)
          sb_start_write(ext4_sb)   [blocked by freeze]
                                    lock_two_nondirectories()
                                      inode_lock(ovl_inode)     [blocked]

With a concurrent freeze operation holding sb_writers write side, this
forms a deadlock cycle: CPU0 waits for freeze to complete, freeze waits
for CPU1's sb_writers reader to exit, CPU1 waits for CPU0's inode lock.

Since EXT4_IOC_MOVE_EXT exchanges physical extents between two files,
it fundamentally requires both files to reside on the same ext4
filesystem.  Moving the superblock check before any lock acquisition
is both semantically correct and eliminates the circular dependency
by ensuring that cross-filesystem donor fds are rejected before
sb_writers or inode locks are taken.

Fixes: fcf6b1b729bc ("ext4: refactor ext4_move_extents code base")
Reported-by: syzbot+ad6118a7584b607c67f2@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=ad6118a7584b607c67f2
Signed-off-by: Yun Zhou <yun.zhou@xxxxxxxxxxxxx>
---
 fs/ext4/ioctl.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
index 1d0c3d4bdf47..f7cd419a3218 100644
--- a/fs/ext4/ioctl.c
+++ b/fs/ext4/ioctl.c
@@ -1650,6 +1650,9 @@ static long __ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 		if (!(fd_file(donor)->f_mode & FMODE_WRITE))
 			return -EBADF;
 
+		if (file_inode(filp)->i_sb != file_inode(fd_file(donor))->i_sb)
+			return -EXDEV;
+
 		err = mnt_want_write_file(filp);
 		if (err)
 			return err;
-- 
2.43.0