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Many bugs are easy to detect: they might cause assertions failures, crash our system, or cause other forms of undefined behaviour detectable by various dynamic analysis tools. However, certain classes of bugs, referred to as semantic bugs, cause none of these while still resulting in a misbehaving faulty system.

To find semantic bugs, one needs to establish a specification of the system’s intended behaviour. Depending on the complexity of the system, creating and centralising such specifications can be difficult. For example, the “specification” of the Linux kernel is not found in one place, but is rather a collection of documentation, man pages, and the implied expectations of a vast collection of user space programs. As such, detecting semantic bugs in the Linux kernel is significantly harder than other classes of bugs. Indeed, many test suites are meant to detect regressions, but creating and maintaining test cases, as well as covering new features requires significant amounts of engineering effort.

Differential fuzzing is a way to automate detection of semantic bugs by providing the same input to different implementations of the same systems and then cross-comparing the resulting behaviour to determine whether it is identical. In case the systems disagree, at least one of them is assumed to be wrong.

syz-verifier is a differential fuzzing tool that cross-compares the execution of programs on different versions of the Linux kernel to detect semantic bugs.

The architecture of syz-verifier is shown in the following diagram.

Architecture overview

The syz-verifier process starts and manages VM instances with the kernels to be cross-compared. It also starts the syz-runner process on the VMs. Communication between the host and the guest is done via RPCs.

syz-verifier generates and sends a continuous stream of programs to syz-runner via RPCs while syz-runner is responsible for starting syz-executor processes and turning the program into input for those. syz-executor processes the input, which triggers a sequence of syscalls in the kernel. Then, syz-runner collects the results and sends them back to the host.

At the moment, the results contain the errnos returned by each system call. When syz-verifier has received results from all the kernels for a specific program, it verifies them to ensure they are identical. If a mismatch is found, the program is rerun on all the kernels to ensure the mismatch is not flaky (i.e. it didn’t occur because of some background activity or external state). If the mismatch occurs in all reruns, syz-verifier creates a report for the program and write it to persistent storage.

How to use syz-verifier

After cloning the repository (see how here), build the tool as:

make verifier runner executor

To start using the tool, separate configuration files need to be created for each kernel you want to include in the verification. An example of Linux configs can be found here. The configuration files are identical to those used by syz-manager.

If you want to generate programs from a specific set of system calls, these can be listed in the kernel config files using the enable_syscalls option. If you want to disable some system calls, use the disable_syscalls option.

Start syz-verifier as:

./bin/syz-verifier -configs=kernel0.cfg,kernel1.cfg

syz-verifier will also gather statistics throughout execution. They will be printed to stdout by default, but an alternative file can be specified using the stat flag.

How to interpret the results

Results can be found in workdir/results.

When syz-verifier finds a mismatch in a program, it will create a report for that program. The report lists the results returned for each system call, by each of the cross-compared kernels, highlighting the ones were a mismatch was found. The system calls are listed in the order they appear in the program.

An extract of such a report is shown below:

ERRNO mismatches found for program:

[=] io_uring_register$IORING_REGISTER_PERSONALITY(0xffffffffffffffff, 0x9, 0x0, 0x0)
        ↳ Pool: 0, Flags: 3, Errno: 9 (bad file descriptor)
        ↳ Pool: 1, Flags: 3, Errno: 9 (bad file descriptor)

[=] syz_genetlink_get_family_id$devlink(&(0x7f0000000000), 0xffffffffffffffff)
        ↳ Pool: 0, Flags: 3, Errno: 2 (no such file or directory)
        ↳ Pool: 1, Flags: 3, Errno: 2 (no such file or directory)

[!] r1 = io_uring_setup(0x238e, &(0x7f0000000240)={0x0, 0xf39a, 0x20, 0x0, 0x146})
        ↳ Pool: 0, Flags: 3, Errno: 6 (no such device or address)
        ↳ Pool: 1, Flags: 3, Errno: 9 (bad file descriptor)

The order of the results is given by the order in which configuration files were passed so Pool: 0 reports results for the kernel created using kernel0.cfg and so on.

The Flags can be used to determine the state reached by the system call: