CIFS: fix type confusion in copy offload ioctl

This might lead to local privilege escalation (code execution as
kernel) for systems where the following conditions are met:

 - CONFIG_CIFS_SMB2 and CONFIG_CIFS_POSIX are enabled
 - a cifs filesystem is mounted where:
  - the mount option "vers" was used and set to a value >=2.0
  - the attacker has write access to at least one file on the filesystem

To attack this, an attacker would have to guess the target_tcon
pointer (but guessing wrong doesn't cause a crash, it just returns an
error code) and win a narrow race.

CC: Stable <stable@vger.kernel.org>
Signed-off-by: Jann Horn <jann@thejh.net>
Signed-off-by: Steve French <smfrench@gmail.com>

diff --git a/fs/cifs/ioctl.c b/fs/cifs/ioctl.c
index c63f5227b68181ed772d1a999686a852a23d41e9..28a77bf1d55924693d27d1c701571e1b1fef2d49 100644 (file)
--- a/fs/cifs/ioctl.c
+++ b/fs/cifs/ioctl.c
@@ -67,6 +67,12 @@ static long cifs_ioctl_clone(unsigned int xid, struct file *dst_file,
                goto out_drop_write;
        }
 
+       if (src_file.file->f_op->unlocked_ioctl != cifs_ioctl) {
+               rc = -EBADF;
+               cifs_dbg(VFS, "src file seems to be from a different filesystem type\n");
+               goto out_fput;
+       }
+
        if ((!src_file.file->private_data) || (!dst_file->private_data)) {
                rc = -EBADF;
                cifs_dbg(VFS, "missing cifsFileInfo on copy range src file\n");