usb: misc: ftdi-elan: Fix off-by-one memory corruptions

This patch fixes fives off-by-one bugs in the ftdi-elan driver code. The
bug can be triggered by plugging a USB adapter for CardBus 3G cards (model
U132 manufactured by Elan Digital Systems, Ltd), causing a kernel panic.
The fix was tested on Ubuntu 14.04.4 with 4.7.0-rc14.2.0-27-generic+ and
4.4.0-22-generic+ kernel. In the ftdi_elan_synchronize function, an
off-by-one memory corruption occurs when packet_bytes is equal or bigger
than m. After having read m bytes, that is bytes_read is equal to m, "
..\x00" is still copied to the stack variable causing an out bounds write
of 4 bytes, which overwrites the stack canary and results in a kernel
panic.

This off-by-one requires physical access to the machine. It is not
exploitable since we have no control on the overwritten data.  Similar
off-by-one bugs have been observed in 4 other functions:
ftdi_elan_stuck_waiting, ftdi_elan_read, ftdi_elan_edset_output and
ftdi_elan_flush_input_fifo.
Reported-by: Alex Palesandro <palexster@gmail.com>
Signed-off-by: Xiao Han <xiao.han@orange.fr>
Tested-by: Paul Chaignon <pchaigno@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/misc/ftdi-elan.c

@@ -665,7 +665,7 @@ static ssize_t ftdi_elan_read(struct file *file, char __user *buffer,
 {
 	char data[30 *3 + 4];
 	char *d = data;
-	int m = (sizeof(data) - 1) / 3;
+	int m = (sizeof(data) - 1) / 3 - 1;
 	int bytes_read = 0;
 	int retry_on_empty = 10;
 	int retry_on_timeout = 5;
@@ -1684,7 +1684,7 @@ wait:if (ftdi->disconnected > 0) {
 			int i = 0;
 			char data[30 *3 + 4];
 			char *d = data;
-			int m = (sizeof(data) - 1) / 3;
+			int m = (sizeof(data) - 1) / 3 - 1;
 			int l = 0;
 			struct u132_target *target = &ftdi->target[ed];
 			struct u132_command *command = &ftdi->command[
@@ -1876,7 +1876,7 @@ more:{
 		if (packet_bytes > 2) {
 			char diag[30 *3 + 4];
 			char *d = diag;
-			int m = (sizeof(diag) - 1) / 3;
+			int m = (sizeof(diag) - 1) / 3 - 1;
 			char *b = ftdi->bulk_in_buffer;
 			int bytes_read = 0;
 			diag[0] = 0;
@@ -2053,7 +2053,7 @@ static int ftdi_elan_synchronize(struct usb_ftdi *ftdi)
 			if (packet_bytes > 2) {
 				char diag[30 *3 + 4];
 				char *d = diag;
-				int m = (sizeof(diag) - 1) / 3;
+				int m = (sizeof(diag) - 1) / 3 - 1;
 				char *b = ftdi->bulk_in_buffer;
 				int bytes_read = 0;
 				unsigned char c = 0;
@@ -2155,7 +2155,7 @@ more:{
 		if (packet_bytes > 2) {
 			char diag[30 *3 + 4];
 			char *d = diag;
-			int m = (sizeof(diag) - 1) / 3;
+			int m = (sizeof(diag) - 1) / 3 - 1;
 			char *b = ftdi->bulk_in_buffer;
 			int bytes_read = 0;
 			diag[0] = 0;