From: Ethan Graham Add Documentation/dev-tools/kfuzztest.rst and reference it in the dev-tools index. Signed-off-by: Ethan Graham --- v2: - Add documentation for kfuzztest-bridge tool introduced in patch 4. --- --- Documentation/dev-tools/index.rst | 1 + Documentation/dev-tools/kfuzztest.rst | 371 ++++++++++++++++++++++++++ 2 files changed, 372 insertions(+) create mode 100644 Documentation/dev-tools/kfuzztest.rst diff --git a/Documentation/dev-tools/index.rst b/Documentation/dev-tools/index.rst index 65c54b27a60b..00ccc4da003b 100644 --- a/Documentation/dev-tools/index.rst +++ b/Documentation/dev-tools/index.rst @@ -32,6 +32,7 @@ Documentation/process/debugging/index.rst kfence kselftest kunit/index + kfuzztest ktap checkuapi gpio-sloppy-logic-analyzer diff --git a/Documentation/dev-tools/kfuzztest.rst b/Documentation/dev-tools/kfuzztest.rst new file mode 100644 index 000000000000..aeaf433a320e --- /dev/null +++ b/Documentation/dev-tools/kfuzztest.rst @@ -0,0 +1,371 @@ +.. SPDX-License-Identifier: GPL-2.0 +.. Copyright 2025 Google LLC + +========================================= +Kernel Fuzz Testing Framework (KFuzzTest) +========================================= + +Overview +======== + +The Kernel Fuzz Testing Framework (KFuzzTest) is a framework designed to expose +internal kernel functions to a userspace fuzzing engine. + +It is intended for testing stateless or low-state functions that are difficult +to reach from the system call interface, such as routines involved in file +format parsing or complex data transformations. This provides a method for +in-situ fuzzing of kernel code without requiring that it be built as a separate +userspace library or that its dependencies be stubbed out. + +The framework consists of four main components: + +1. An API, based on the ``FUZZ_TEST`` macro, for defining test targets + directly in the kernel tree. +2. A binary serialization format for passing complex, pointer-rich data + structures from userspace to the kernel. +3. A ``debugfs`` interface through which a userspace fuzzer submits + serialized test inputs. +4. Metadata embedded in dedicated ELF sections of the ``vmlinux`` binary to + allow for the discovery of available fuzz targets by external tooling. + +.. warning:: + KFuzzTest is a debugging and testing tool. It exposes internal kernel + functions to userspace with minimal sanitization and is designed for + use in controlled test environments only. It must **NEVER** be enabled + in production kernels. + +Supported Architectures +======================= + +KFuzzTest is currently only supported for x86_64. + +Usage +===== + +To enable KFuzzTest, configure the kernel with:: + + CONFIG_KFUZZTEST=y + +which depends on ``CONFIG_DEBUGFS`` for receiving userspace inputs, and +``CONFIG_DEBUG_KERNEL`` as an additional guardrail for preventing KFuzzTest +from finding its way into a production build accidentally. + +The KFuzzTest sample fuzz targets can be built in with +``CONFIG_SAMPLE_KFUZZTEST``. + +KFuzzTest currently only supports code that is built into the kernel, as the +core module's startup process discovers fuzz targets, constraints, and +annotations from a dedicated ELF section during startup. + +Declaring a KFuzzTest target +---------------------------- + +A fuzz target is defined directly in a .c file, typically alongside the function +being tested. This process involves three main parts: defining an input +structure, writing the test body using the ``FUZZ_TEST`` macro, and optionally +adding metadata for the fuzzer. + +The following example illustrates how to create a fuzz target for a function +``int process_data(const char *data, size_t len)``. + +.. code-block:: c + + /* + * 1. Define a struct to model the inputs for the function under test. + * Each field corresponds to an argument needed by the function. + */ + struct process_data_inputs { + const char *data; + size_t len; + }; + + /* + * 2. Define the fuzz target using the FUZZ_TEST macro. + * The first parameter is a unique name for the target. + * The second parameter is the input struct defined above. + */ + FUZZ_TEST(test_process_data, struct process_data_inputs) + { + /* + * Within this body, the 'arg' variable is a pointer to a + * fully initialized 'struct process_data_inputs'. + */ + + /* + * 3. (Optional) Add constraints to define preconditions. + * This check ensures 'arg->data' is not NULL. If the condition + * is not met, the test exits early. This also creates metadata + * to inform the fuzzer. + */ + KFUZZTEST_EXPECT_NOT_NULL(process_data_inputs, data); + + /* + * 4. (Optional) Add annotations to provide semantic hints. + * This annotation informs the fuzzer that the 'len' field + * is the length of the buffer pointed to by 'data'. + * Annotations do not add any runtime checks. + */ + KFUZZTEST_ANNOTATE_LEN(process_data_inputs, len, data); + + /* + * 5. Call the kernel function with the provided inputs. + * Memory errors like out-of-bounds accesses on 'arg->data' will + * be detected by KASAN or other memory error detection tools. + */ + process_data(arg->data, arg->len); + } + +KFuzzTest provides two families of macros to improve the quality of fuzzing: + +- ``KFUZZTEST_EXPECT_*``: These macros define constraints, which are + preconditions that must be true for the test to proceed. They are enforced + with a runtime check in the kernel. If a check fails, the current test run is + aborted. This metadata helps the userspace fuzzer avoid generating invalid + inputs. + +- ``KFUZZTEST_ANNOTATE_*``: These macros define annotations, which are purely + semantic hints for the fuzzer. They do not add any runtime checks and exist + only to help the fuzzer generate more intelligent and structurally correct + inputs. For example, KFUZZTEST_ANNOTATE_LEN links a size field to a pointer + field, which is a common pattern in C APIs. + +Metadata +-------- + +Macros ``FUZZ_TEST``, `KFUZZTEST_EXPECT_*`` and ``KFUZZTEST_ANNOTATE_*`` embed +metadata into several sections within the main ``.data`` section of the final +``vmlinux`` binary; ``.kfuzztest_target``, ``.kfuzztest_constraint`` and +``.kfuzztest_annotation`` respectively. + +This serves two purposes: + +1. The core module uses the ``.kfuzztest_target`` section at boot to discover + every ``FUZZ_TEST`` instance and create its ``debugfs`` directory and + ``input`` file. +2. Userspace fuzzers can read this metadata from the ``vmlinux`` binary to + discover targets and learn about their rules and structure in order to + generate correct and effective inputs. + +The metadata in the ``.kfuzztest_*`` sections consists of arrays of fixed-size C +structs (e.g., ``struct kfuzztest_target``). Fields within these structs that +are pointers, such as ``name`` or ``arg_type_name``, contain addresses that +point to other locations in the ``vmlinux`` binary. A userspace tool that +parsing the ELF file must resolve these pointers to read the data that they +reference. For example, to get a target's name, a tool must: + +1. Read the ``struct kfuzztest_target`` from the ``.kfuzztest_target`` section. +2. Read the address in the ``.name`` field. +3. Use that address to locate and read null-terminated string from its position + elsewhere in the binary (e.g., ``.rodata``). + +Tooling Dependencies +-------------------- + +For userspace tools to parse the ``vmlinux`` binary and make use of emitted +KFuzzTest metadata, the kernel must be compiled with DWARF debug information. +This is required for tools to understand the layout of C structs, resolve type +information, and correctly interpret constraints and annotations. + +When using KFuzzTest with automated fuzzing tools, either +``CONFIG_DEBUG_INFO_DWARF4`` or ``CONFIG_DEBUG_INFO_DWARF5`` should be enabled. + +Input Format +============ + +KFuzzTest targets receive their inputs from userspace via a write to a dedicated +debugfs ``/sys/kernel/debug/kfuzztest//input``. + +The data written to this file must be a single binary blob that follows a +specific serialization format. This format is designed to allow complex, +pointer-rich C structures to be represented in a flat buffer, requiring only a +single kernel allocation and copy from userspace. + +An input is first prefixed by an 8-byte header containing a magic value in the +first four bytes, defined as ``KFUZZTEST_HEADER_MAGIC`` in +```, and a version number in the subsequent four +bytes. + +Version 0 +--------- + +In version 0 (i.e., when the version number in the 8-byte header is equal to 0), +the input format consists of three main parts laid out sequentially: a region +array, a relocation table, and the payload.:: + + +----------------+---------------------+-----------+----------------+ + | region array | relocation table | padding | payload | + +----------------+---------------------+-----------+----------------+ + +Region Array +^^^^^^^^^^^^ + +This component is a header that describes how the raw data in the Payload is +partitioned into logical memory regions. It consists of a count of regions +followed by an array of ``struct reloc_region``, where each entry defines a +single region with its size and offset from the start of the payload. + +.. code-block:: c + + struct reloc_region { + uint32_t offset; + uint32_t size; + }; + + struct reloc_region_array { + uint32_t num_regions; + struct reloc_region regions[]; + }; + +By convention, region 0 represents the top-level input struct that is passed +as the arg variable to the FUZZ_TEST body. Subsequent regions typically +represent data buffers pointed to by fields within that struct. Region array +entries must be ordered by offset ascending, and must not overlap with one +another. + +To satisfy C language alignment requirements and prevent potential hardware +faults, the memory address of each region's data must be correctly aligned for +the type it represents. The framework allocates a base buffer that is suitably +aligned for any C type. Therefore, the userspace tool that generates the input +is responsible for calculating each region's offset within the payload to ensure +this alignment is maintained. + +Relocation Table +^^^^^^^^^^^^^^^^ + +The relocation table provides the instructions for the kernel to "hydrate" the +payload by patching pointer fields. It contains an array of +``struct reloc_entry`` items. Each entry acts as a linking instruction, +specifying: + +- The location of a pointer that needs to be patched (identified by a region + ID and an offset within that region). + +- The target region that the pointer should point to (identified by the + target's region ID) or ``KFUZZTEST_REGIONID_NULL`` if the pointer is ``NULL``. + +This table also specifies the amount of padding between its end and the start +of the payload, which should be at least 8 bytes. + +.. code-block:: c + + struct reloc_entry { + uint32_t region_id; + uint32_t region_offset; + uint32_t value; + }; + + struct reloc_table { + uint32_t num_entries; + uint32_t padding_size; + struct reloc_entry entries[]; + }; + +Payload +^^^^^^^ + +The payload contains the raw binary data for all regions, concatenated together +according to their specified offsets. + +- Alignment: The start of the payload must be aligned to the most restrictive + alignment requirement of all its constituent regions. The framework ensures + that each region within the payload is then placed at an offset that respects + its own type's alignment. + +- Padding and Poisoning: The space between the end of one region's data and the + beginning of the next must be sufficient for padding. In KASAN builds, + KFuzzTest poisons this unused padding, allowing for precise detection of + out-of-bounds memory accesses between adjacent buffers. This padding should + be at least ``KFUZZTEST_POISON_SIZE`` bytes as defined in + `include/linux/kfuzztest.h``. + +KFuzzTest Bridge Tool +===================== + +The kfuzztest-bridge program is a userspace utility that encodes a random byte +stream into the structured binary format expected by a KFuzzTest harness. It +allows users to describe the target's input structure textually, making it easy +to perform smoke tests or connect harnesses to blob-based fuzzing engines. + +This tool is intended to be simple, both in usage and implementation. Its +structure and DSL are sufficient for simpler use-cases. For more advanced +coverage-guided fuzzing it is recommended to use syzkaller which implements +deeper support for KFuzzTest targets. + +Usage +----- + +The tool can be built with ``make tools/kfuzztest-bridge``. In the case of libc +incompatibilities, the tool may have to be built on the target system. + +Example: + +.. code-block:: sh + + ./kfuzztest-bridge \ + "foo { u32 ptr[bar] }; bar { ptr[data] len[data, u64]}; data { arr[u8, 42] };" \ + "my-fuzz-target" /dev/urandom + +The command takes three arguments + +1. A string describing the input structure (see `Textual Format`_ sub-section). +2. The name of the target test, which corresponds to its directory in + ``/sys/kernel/debug/kfuzztest/``. +3. A path to a file providing a stream of random data, such as + ``/dev/urandom``. + +The structure string in the example corresponds to the following C data +structures: + +.. code-block:: c + + struct foo { + u32 a; + struct bar *b; + }; + + struct bar { + struct data *d; + u64 data_len; /* Equals 42. */ + }; + + struct data { + char arr[42]; + }; + +Textual Format +-------------- + +The textual format is a human-readable representation of the region-based binary +format used by KFuzzTest. It is described by the following grammar: + +.. code-block:: text + + schema ::= region ( ";" region )* [";"] + region ::= identifier "{" type+ "}" + type ::= primitive | pointer | array | length | string + primitive ::= "u8" | "u16" | "u32" | "u64" + pointer ::= "ptr" "[" identifier "]" + array ::= "arr" "[" primitive "," integer "]" + length ::= "len" "[" identifier "," primitive "]" + string ::= "str" "[" integer "]" + identifier ::= [a-zA-Z_][a-zA-Z1-9_]* + integer ::= [0-9]+ + +Pointers must reference a named region. To fuzz a raw buffer, the buffer must be +defined in its own region, as shown below: + +.. code-block:: c + + struct my_struct { + char *buf; + size_t buflen; + }; + +This would correspond to the following textual description: + +.. code-block:: text + + my_struct { ptr[buf] len[buf, u64] }; buf { arr[u8, n] }; + +Where ``n`` is some integer value defining the size of the byte array inside of +the ``buf`` region. -- 2.51.0.318.gd7df087d1a-goog