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Update external_fuzzing_usb.md

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docs/linux/external_fuzzing_usb.md
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@ -9,17 +9,9 @@ This is still in development and things might change.
USB fuzzing consists of 3 parts:
1. Syzkaller changes that are now upstream.
2. Kernel interface for USB device emulation, which can be found [here](https://github.com/google/kasan/commits/usb-fuzzer).
2. Kernel interface for USB device emulation, which can be found [here](https://github.com/google/kasan/commits/usb-fuzzer) and is now being upstreamed.
3. KCOV changes that allow to collect coverage from background threads and interrupts
(the former can be found [here](https://github.com/google/kasan/commits/usb-fuzzer), the latter in still in development).
Currently syzkaller defines 5 USB syzcalls (see [this](/sys/linux/vusb.txt) and [this](/executor/common_usb.h)):
1. `syz_usb_connect` - connects a USB device.
2. `syz_usb_disconnect` - disconnects a USB device.
3. `syz_usb_control_io` - sends or receives a control message over endpoint 0.
4. `syz_usb_ep_write` - sends a message to an endpoint.
4. `syz_usb_ep_read` - receives a message from an endpoint.
(the former can be found [here](https://github.com/google/kasan/commits/usb-fuzzer) and is now being upstreamed, the latter is still in development).
More details can be found:
@ -29,12 +21,13 @@ More details can be found:
A few major things that need to be done:
1. Collect coverage from interrupts (this is required to enable better fuzzing of USB drivers after enumeration completes).
2. Add descriptions for all main USB classes.
3. Upstream KCOV changes.
4. Upstream the kernel interface for USB device emulation.
1. Upstream KCOV changes that allow to collect coverage from background threads.
2. Upstream the kernel interface for USB device emulation.
3. Implement a proper way for extracting relevant USB ids from the kernel ([discussion](https://www.spinics.net/lists/linux-usb/msg187915.html) is ongoing).
4. Add descriptions for all main USB classes.
5. Collect coverage from interrupts (this is required to enable better fuzzing of USB drivers after enumeration completes).
The work on points 3 and 4 has started:
The work on points 1 and 2 has started:
Kernel patches in mainline:
@ -53,6 +46,17 @@ b. using USB requests coming from the host as a signal (like coverage) to enable
c. making UDC driver name configurable for syz-execprog and syz-prog2c.
2. 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).
## Internals
Currently syzkaller defines 5 USB syzcalls (see [this](/sys/linux/vusb.txt) and [this](/executor/common_usb.h)):
1. `syz_usb_connect` - connects a USB device.
2. `syz_usb_disconnect` - disconnects a USB device.
3. `syz_usb_control_io` - sends or receives a control message over endpoint 0.
4. `syz_usb_ep_write` - sends a message to an endpoint.
4. `syz_usb_ep_read` - receives a message from an endpoint.
Syzkaller descriptions for USB fuzzing can be found [here](/sys/linux/vusb.txt).
@ -91,7 +95,7 @@ Syzkaller descriptions for USB fuzzing can be found [here](/sys/linux/vusb.txt).
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` syzcall 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 way to generate syzkaller USB IDs for all USB drivers enabled in your .config.
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.