sg_start_req(): make sure that there's not too many elements in iovec

commit 451a2886 upstream.

unfortunately, allowing an arbitrary 16bit value means a possibility of
overflow in the calculation of total number of pages in bio_map_user_iov() -
we rely on there being no more than PAGE_SIZE members of sum in the
first loop there.  If that sum wraps around, we end up allocating
too small array of pointers to pages and it's easy to overflow it in
the second loop.

X-Coverup: TINC (and there's no lumber cartel either)
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
[bwh: s/MAX_UIOVEC/UIO_MAXIOV/. This was fixed upstream by commit
 fdc81f45 ("sg_start_req(): use import_iovec()"), but we don't have
 that function.]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Reference: CVE-2015-5707
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

drivers/scsi/sg.c

@@ -1759,6 +1759,9 @@ sg_start_req(Sg_request *srp, unsigned char *cmd)
 			md->from_user = 0;
 	}
 
+	if (unlikely(iov_count > UIO_MAXIOV))
+		return -EINVAL;
+
 	if (iov_count) {
 		int len, size = sizeof(struct sg_iovec) * iov_count;
 		struct iovec *iov;