From: Yun Zhou Date: Mon, 8 Jun 2026 15:25:21 +0000 (+0800) Subject: ext4: validate donor file superblock early in EXT4_IOC_MOVE_EXT X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=c143957520c6c9b5cd72e0de8b52b814f0c576fe;p=thirdparty%2Fkernel%2Flinux.git ext4: validate donor file superblock early in EXT4_IOC_MOVE_EXT 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@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=ad6118a7584b607c67f2 Signed-off-by: Yun Zhou Reviewed-by: Jan Kara Reviewed-by: Andreas Dilger Link: https://patch.msgid.link/20260608152521.1292656-1-yun.zhou@windriver.com Signed-off-by: Theodore Ts'o --- diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c index 110e3fb194ec6..c8387e6a2c6e9 100644 --- a/fs/ext4/ioctl.c +++ b/fs/ext4/ioctl.c @@ -1656,6 +1656,9 @@ group_extend_out: 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;