[PATCH 3.16.y-ckt 002/104] CIFS: fix type confusion in copy offload ioctl

[Luis Henriques]

3.16.7-ckt19 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jann Horn <jann at thejh.net>

commit 4c17a6d56bb0cad3066a714e94f7185a24b40f49 upstream.

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.

Signed-off-by: Jann Horn <jann at thejh.net>
Signed-off-by: Steve French <smfrench at gmail.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 fs/cifs/ioctl.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/cifs/ioctl.c b/fs/cifs/ioctl.c
index 8b7898b7670f..64a9bca976d0 100644
--- 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");