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External USB fuzzing for Linux kernel
=====================================
Syzkaller supports fuzzing the Linux kernel USB subsystem externally
(as it would be done by plugging in a physical USB device with e.g. [Facedancer](https://github.com/usb-tools/Facedancer)).
This allowed to find over [200 bugs](/docs/linux/found_bugs_usb.md) in the Linux kernel USB stack so far.
This is still in development and things might change.
USB fuzzing support consists of 3 parts:
1. Syzkaller changes that are now in the mainline, see the [Internals](/docs/linux/external_fuzzing_usb.md#Internals) section for details.
2. Kernel interface for USB device emulation called [Raw Gadget](https://github.com/xairy/raw-gadget), which is now in the mainline.
3. KCOV changes that allow to collect coverage from background threads and interrupts, that are now in the mainline.
Some (partly outdated) details can be found in the OffensiveCon 2019 "Coverage-Guided USB Fuzzing with Syzkaller" talk
([slides](https://docs.google.com/presentation/d/1z-giB9kom17Lk21YEjmceiNUVYeI6yIaG5_gZ3vKC-M/edit?usp=sharing), [video](https://www.youtube.com/watch?v=1MD5JV6LfxA)).
As USB fuzzing requires kernel side support, for non-mainline kernels you need all mainline patches that touch `drivers/usb/gadget/udc/dummy_hcd.c`, `drivers/usb/gadget/legacy/raw_gadget.c` and `kernel/kcov.c`.
## Internals
Currently syzkaller defines 6 USB pseudo-syscalls (see [this](/sys/linux/vusb.txt) for syzkaller descriptions of these pseudo-syscalls and [this](/executor/common_usb.h) for their implementation; the implementation relies on the Raw Gadget interface linked above):
1. `syz_usb_connect` - connects a USB device. Handles all requests to the control endpoint until a `SET_CONFIGURATION` request is received.
2. `syz_usb_connect_ath9k` - connects an `ath9k` USB device. Compared to `syz_usb_connect` this syscalls also handles firmware download requests that happen after the `SET_CONFIGURATION` for the `ath9k` driver.
3. `syz_usb_disconnect` - disconnects a USB device.
4. `syz_usb_control_io` - sends or receives a control message over endpoint 0.
5. `syz_usb_ep_write` - sends a message to a non-control endpoint.
6. `syz_usb_ep_read` - receives a message from a non-control endpoint.
Current USB descriptions are targeted at a few different layers:
1. USB core enumeration process is targeted by the generic `syz_usb_connect` pseudo-syscall. As the USB device descriptor fields for this pseudo-syscall get [patched](/sys/linux/init_vusb.go) by syzkaller runtime, `syz_usb_connect` also briefly targets enumaration process of various USB drivers.
2. Enumeration process for class-specific drivers is targeted by `syz_usb_connect$hid`, `syz_usb_connect$cdc_ecm`, etc. pseudo-syscalls (the device descriptors provided to them have fixed identifying USB IDs to always match to the same USB class driver) accompanied by matching `syz_usb_control_io$*` pseudo-syscalls.
3. Subsequent communication through non-control endpoints for class-specific drivers is not targeted by existing descriptions yet for any of the supported classes, but can be triggered through generic `syz_usb_ep_write` and `syz_usb_ep_read` pseudo-syscalls.
4. Enumeration process for device-specific drivers is not covered by existing descriptions yet.
5. Subsequent communication through non-control endpoints for device-specific drivers is partially described only for `ath9k` driver via `syz_usb_connect_ath9k`, `syz_usb_ep_write$ath9k_ep1` and `syz_usb_ep_write$ath9k_ep2` pseudo-syscalls.
syzkaller USB runtests are [here](/sys/linux/test/) and start with `vusb` prefix. To run:
```
./bin/syz-runtest -config=usb-manager.cfg -tests=vusb
```
## TODO list
The core support for USB fuzzing is now in place, but there are still some things that could be done:
1. Remove device from `usb_devices` on disconnect in executor.
2. Add descriptions for more relevant USB classes and drivers.
3. Look for TODOs in [sys/linux/vusb.txt](/sys/linux/vusb.txt).
4. Implement a proper way for dynamically extracting relevant USB ids from the kernel (a related [discussion](https://www.spinics.net/lists/linux-usb/msg187915.html)).
5. Add a mode for standalone fuzzing of physical USB hosts (by using e.g. Raspberry Pi Zero, see below).
This includes at least: a. making sure that current USB emulation implementation works properly on different OSes (there are some [differences](https://github.com/RoganDawes/LOGITacker/blob/USB_host_enum/fingerprint_os.md#derive-the-os-from-the-fingerprint) in protocol implementation);
b. using USB requests coming from the host as a signal (like coverage) to enable "signal-driven" fuzzing,
c. making UDC driver name configurable for `syz-execprog` and `syz-prog2c`.
6. Generate syzkaller programs from usbmon trace that is produced by actual USB devices (this should make the fuzzer to go significantly deeper into the USB drivers code).
## Setting up
1. Checkout the `usb-fuzzer` branch from https://github.com/google/kasan
2. Configure the kernel (at the very least `CONFIG_USB_RAW_GADGET=y` and `CONFIG_USB_DUMMY_HCD=y` need to be enabled).
The easiest option is to use the [config](/dashboard/config/linux/upstream-usb.config) from the syzbot USB fuzzing instance.
3. Build the kernel.
4. Optionally update syzkaller descriptions by extracting USB IDs using the instructions below.
5. Enable `syz_usb_connect`, `syz_usb_disconnect`, `syz_usb_control_io`, `syz_usb_ep_write` and `syz_usb_ep_read` pseudo-syscalls in the manager config.
6. Set `sandbox` to `none` in the manager config.
7. Pass `dummy_hcd.num=8` (or whatever number you use for `procs`) to the kernel command line in the maganer config.
8. Run.
## Updating syzkaller USB IDs
Syzkaller uses a list of hardcoded [USB IDs](/sys/linux/init_vusb_ids.go) that are [patched](/sys/linux/init_vusb.go) into the `syz_usb_connect` pseudo-syscall by syzkaller runtime.
One of the ways to make syzkaller target only particular USB drivers is to alter that list.
The instructions below describe a hackish way to generate syzkaller USB IDs for all USB drivers enabled in your .config.
1. Apply [this](/tools/syz-usbgen/usb_ids.patch) kernel patch.
2. Build and boot the kernel.
3. Connect a USB HID device. In case you're using a `CONFIG_USB_RAW_GADGET=y` kernel, use the
[keyboard emulation program](https://raw.githubusercontent.com/xairy/raw-gadget/master/examples/keyboard.c).
4. Use [syz-usbgen](/tools/syz-usbgen/usbgen.go) script to update [syzkaller descriptions](/sys/linux/init_vusb_ids.go):
```
./bin/syz-usbgen $KERNEL_LOG ./sys/linux/init_vusb_ids.go
```
5. Don't forget to revert the applied patch and rebuild the kernel before doing actual fuzzing.
## Running reproducers with Raspberry Pi Zero W
It's possible to run syzkaller USB reproducers by using a Linux board plugged into a physical USB host.
These instructions describe how to set this up on a Raspberry Pi Zero W, but any other board that has a working USB UDC driver can be used as well.
1. Download `raspbian-stretch-lite.img` from [here](https://www.raspberrypi.org/downloads/raspbian/).
2. Flash the image into an SD card as described [here](https://www.raspberrypi.org/documentation/installation/installing-images/linux.md).
3. Enable UART as described [here](https://www.raspberrypi.org/documentation/configuration/uart.md).
4. Boot the board and get a shell over UART as described [here](https://learn.adafruit.com/raspberry-pi-zero-creation/give-it-life). You'll need a USB-UART module for that. The default login credentials are `pi` and `raspberry`.
5. Get the board connected to the internet (plug in a USB Ethernet adapter or follow [this](https://www.raspberrypi.org/documentation/configuration/wireless/wireless-cli.md)).
6. Update: `sudo apt-get update && sudo apt-get dist-upgrade && sudo rpi-update && sudo reboot`.
7. Install useful packages: `sudo apt-get install vim git`.
8. Download and install Go:
``` bash
curl https://dl.google.com/go/go1.14.2.linux-armv6l.tar.gz -o go.linux-armv6l.tar.gz
tar -xf go.linux-armv6l.tar.gz
mv go goroot
mkdir gopath
export GOPATH=~/gopath
export GOROOT=~/goroot
export PATH=~/goroot/bin:$PATH
export PATH=~/gopath/bin:$PATH
```
9. Download syzkaller, apply the patch below and build `syz-executor`:
``` c
diff --git a/executor/common_usb.h b/executor/common_usb.h
index e342d808..278c2f4e 100644
--- a/executor/common_usb.h
+++ b/executor/common_usb.h
@@ -269,9 +269,7 @@ static volatile long syz_usb_connect(volatile long a0, volatile long a1, volatil
// TODO: consider creating two dummy_udc's per proc to increace the chance of
// triggering interaction between multiple USB devices within the same program.
- char device[32];
- sprintf(&device[0], "dummy_udc.%llu", procid);
- rv = usb_raw_init(fd, speed, "dummy_udc", &device[0]);
+ rv = usb_raw_init(fd, speed, "20980000.usb", "20980000.usb");
if (rv < 0) {
debug("syz_usb_connect: usb_raw_init failed with %d\n", rv);
return rv;
diff --git a/executor/executor.cc b/executor/executor.cc
index 34949a01..1afcb288 100644
--- a/executor/executor.cc
+++ b/executor/executor.cc
@@ -604,8 +604,8 @@ retry:
call_extra_cover = true;
}
if (strncmp(syscalls[call_num].name, "syz_usb_connect", strlen("syz_usb_connect")) == 0) {
- prog_extra_timeout = 2000;
- call_extra_timeout = 2000;
+ prog_extra_timeout = 5000;
+ call_extra_timeout = 5000;
}
if (strncmp(syscalls[call_num].name, "syz_usb_control_io", strlen("syz_usb_control_io")) == 0)
call_extra_timeout = 300;
```
``` bash
go get -u -d github.com/google/syzkaller/prog
cd ~/gopath/src/github.com/google/syzkaller
# Put the patch above into ./syzkaller.patch
git apply ./syzkaller.patch
make executor
mkdir ~/syz-bin
cp bin/linux_arm/syz-executor ~/syz-bin/
```
10. Build `syz-execprog` on your host machine for arm32 with `make TARGETARCH=arm execprog` and copy to `~/syz-bin` onto the SD card. You may try building syz-execprog on the Raspberry Pi itself, but that worked poorly for me due to large memory consumption during the compilation process.
11. Make sure that ou can now execute syzkaller programs:
``` bash
cat socket.log
r0 = socket$inet_tcp(0x2, 0x1, 0x0)
sudo ./syz-bin/syz-execprog -executor ./syz-bin/syz-executor -threaded=0 -collide=0 -procs=1 -enable='' -debug socket.log
```
12. Setup the dwc2 USB gadget driver:
```
echo "dtoverlay=dwc2" | sudo tee -a /boot/config.txt
echo "dwc2" | sudo tee -a /etc/modules
sudo reboot
```
13. Get Linux kernel headers following [this](https://github.com/notro/rpi-source/wiki).
14. Download and build the USB Raw Gadget module following [this](https://github.com/xairy/raw-gadget/tree/master/raw_gadget).
15. Insert the module with `sudo insmod raw_gadget.ko`.
16. [Download](https://raw.githubusercontent.com/xairy/raw-gadget/master/examples/keyboard.c), build and run the [keyboard emulator program](https://github.com/xairy/raw-gadget/tree/master/examples):
``` bash
# Get keyboard.c
gcc keyboard.c -o keyboard
sudo ./keyboard 20980000.usb 20980000.usb
# Make sure you see the letter 'x' being entered on the host.
```
17. You should now be able to execute syzkaller USB programs:
``` bash
$ cat usb.log
r0 = syz_usb_connect(0x0, 0x24, &(0x7f00000001c0)={{0x12, 0x1, 0x0, 0x8e, 0x32, 0xf7, 0x20, 0xaf0, 0xd257, 0x4e87, 0x0, 0x0, 0x0, 0x1, [{{0x9, 0x2, 0x12, 0x1, 0x0, 0x0, 0x0, 0x0, [{{0x9, 0x4, 0xf, 0x0, 0x0, 0xff, 0xa5, 0x2c}}]}}]}}, 0x0)
$ sudo ./syz-bin/syz-execprog -executor ./syz-bin/syz-executor -threaded=0 -collide=0 -procs=1 -enable='' -debug usb.log
```
18. Steps 19 through 21 are optional. You may use a UART console and a normal USB cable instead of ssh and Zero Stem.
19. Follow [this](https://www.raspberrypi.org/documentation/configuration/wireless/access-point.md) to setup Wi-Fi hotspot.
20. Follow [this](https://www.raspberrypi.org/documentation/remote-access/ssh/) to enable ssh.
21. Optionally solder [Zero Stem](https://zerostem.io/) onto your Raspberry Pi Zero W.
21. You can now connect the board to an arbitrary USB port, wait for it to boot, join its Wi-Fi network, ssh onto it, and run arbitrary syzkaller USB programs.
|
