Subject: [PATCH v2] fuzz: add fuzzer for config parsing

From: "Brian Tracy via GitGitGadget" <[email-masked]>
Date: Fri, 15 Mar 2024 05:47:31 +0000

From: Brian C Tracy <[email-masked]>

Add a new fuzz target that exercises the parsing of git configs.
The existing git_config_from_mem function is a perfect entry point
for fuzzing as it exercises the same code paths as the rest of the
config parsing functions and offers an easily fuzzable interface.

Config parsing is a useful thing to fuzz because it operates on user
controlled data and is a central component of many git operations.

Signed-off-by: Brian C Tracy <[email-masked]>
---
    fuzz: add fuzzer for config parsing

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1692%2Fbriantracy%2Fconfig-fuzzer-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1692/briantracy/config-fuzzer-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/1692

Range-diff vs v1:

 1:  a43a2e3a07c ! 1:  be6aa5efb1c fuzz: add fuzzer for config parsing
     @@ Commit message
          Signed-off-by: Brian C Tracy <[email-masked]>
      
       ## Makefile ##
     -@@ Makefile: FUZZ_OBJS += oss-fuzz/fuzz-commit-graph.o
     +@@ Makefile: ETAGS_TARGET = TAGS
     + # runs in the future.
     + FUZZ_OBJS += oss-fuzz/dummy-cmd-main.o
     + FUZZ_OBJS += oss-fuzz/fuzz-commit-graph.o
     ++FUZZ_OBJS += oss-fuzz/fuzz-config.o
       FUZZ_OBJS += oss-fuzz/fuzz-date.o
       FUZZ_OBJS += oss-fuzz/fuzz-pack-headers.o
       FUZZ_OBJS += oss-fuzz/fuzz-pack-idx.o
     -+FUZZ_OBJS += oss-fuzz/fuzz-config.o
     - .PHONY: fuzz-objs
     - fuzz-objs: $(FUZZ_OBJS)
     - 
      
       ## ci/run-build-and-minimal-fuzzers.sh ##
      @@ ci/run-build-and-minimal-fuzzers.sh: group "Build fuzzers" make \
     @@ ci/run-build-and-minimal-fuzzers.sh: group "Build fuzzers" make \
       	fuzz-all
       
      -for fuzzer in commit-graph date pack-headers pack-idx ; do
     -+for fuzzer in commit-graph date pack-headers pack-idx config ; do
     ++for fuzzer in commit-graph config date pack-headers pack-idx ; do
       	begin_group "fuzz-$fuzzer"
       	./oss-fuzz/fuzz-$fuzzer -verbosity=0 -runs=1 || exit 1
       	end_group "fuzz-$fuzzer"
      
       ## oss-fuzz/.gitignore ##
     -@@ oss-fuzz/.gitignore: fuzz-commit-graph
     +@@
     + fuzz-commit-graph
     ++fuzz-config
       fuzz-date
       fuzz-pack-headers
       fuzz-pack-idx
     -+fuzz-config
      
       ## oss-fuzz/fuzz-config.c (new) ##
      @@
      +#include "git-compat-util.h"
      +#include "config.h"
      +
     -+#include <stdio.h>
     -+#include <string.h>
     -+
      +int LLVMFuzzerTestOneInput(const uint8_t *, size_t);
      +static int config_parser_callback(const char *, const char *,
      +					const struct config_context *, void *);
     @@ oss-fuzz/fuzz-config.c (new)
      +					const struct config_context *ctx UNUSED,
      +					void *data UNUSED)
      +{
     -+	/* Visit every byte of memory we are given to make sure the parser
     -+	 * gave it to us appropriately. Ensure a return of 0 to indicate
     -+	 * success so the parsing continues. */
     -+	int c = strlen(key);
     ++	/*
     ++	 * Visit every byte of memory we are given to make sure the parser
     ++	 * gave it to us appropriately. We need to unconditionally return 0,
     ++	 * but we also want to prevent the strlen from being optimized away.
     ++	 */
     ++	size_t c = strlen(key);
     ++
      +	if (value)
      +		c += strlen(value);
     -+	return c < 0;
     ++	return c == SIZE_MAX;
      +}
      +
      +int LLVMFuzzerTestOneInput(const uint8_t *data, const size_t size)
      +{
      +	struct config_options config_opts = { 0 };
     ++
      +	config_opts.error_action = CONFIG_ERROR_SILENT;
      +	git_config_from_mem(config_parser_callback, CONFIG_ORIGIN_BLOB,
      +				"fuzztest-config", (const char *)data, size, NULL,

 Makefile                            |  1 +
 ci/run-build-and-minimal-fuzzers.sh |  2 +-
 oss-fuzz/.gitignore                 |  1 +
 oss-fuzz/fuzz-config.c              | 33 +++++++++++++++++++++++++++++
 4 files changed, 36 insertions(+), 1 deletion(-)
 create mode 100644 oss-fuzz/fuzz-config.c

diff --git a/Makefile b/Makefile
index 4e255c81f22..af32028b18f 100644
--- a/Makefile
+++ b/Makefile
@@ -757,6 +757,7 @@ ETAGS_TARGET = TAGS
 # runs in the future.
 FUZZ_OBJS += oss-fuzz/dummy-cmd-main.o
 FUZZ_OBJS += oss-fuzz/fuzz-commit-graph.o
+FUZZ_OBJS += oss-fuzz/fuzz-config.o
 FUZZ_OBJS += oss-fuzz/fuzz-date.o
 FUZZ_OBJS += oss-fuzz/fuzz-pack-headers.o
 FUZZ_OBJS += oss-fuzz/fuzz-pack-idx.o
diff --git a/ci/run-build-and-minimal-fuzzers.sh b/ci/run-build-and-minimal-fuzzers.sh
index 8ba486f6598..a51076d18df 100755
--- a/ci/run-build-and-minimal-fuzzers.sh
+++ b/ci/run-build-and-minimal-fuzzers.sh
@@ -12,7 +12,7 @@ group "Build fuzzers" make \
 	LIB_FUZZING_ENGINE="-fsanitize=fuzzer,address" \
 	fuzz-all
 
-for fuzzer in commit-graph date pack-headers pack-idx ; do
+for fuzzer in commit-graph config date pack-headers pack-idx ; do
 	begin_group "fuzz-$fuzzer"
 	./oss-fuzz/fuzz-$fuzzer -verbosity=0 -runs=1 || exit 1
 	end_group "fuzz-$fuzzer"
diff --git a/oss-fuzz/.gitignore b/oss-fuzz/.gitignore
index 5b954088254..a877c11f42b 100644
--- a/oss-fuzz/.gitignore
+++ b/oss-fuzz/.gitignore
@@ -1,4 +1,5 @@
 fuzz-commit-graph
+fuzz-config
 fuzz-date
 fuzz-pack-headers
 fuzz-pack-idx
diff --git a/oss-fuzz/fuzz-config.c b/oss-fuzz/fuzz-config.c
new file mode 100644
index 00000000000..94027f5b97e
--- /dev/null
+++ b/oss-fuzz/fuzz-config.c
@@ -0,0 +1,33 @@
+#include "git-compat-util.h"
+#include "config.h"
+
+int LLVMFuzzerTestOneInput(const uint8_t *, size_t);
+static int config_parser_callback(const char *, const char *,
+					const struct config_context *, void *);
+
+static int config_parser_callback(const char *key, const char *value,
+					const struct config_context *ctx UNUSED,
+					void *data UNUSED)
+{
+	/*
+	 * Visit every byte of memory we are given to make sure the parser
+	 * gave it to us appropriately. We need to unconditionally return 0,
+	 * but we also want to prevent the strlen from being optimized away.
+	 */
+	size_t c = strlen(key);
+
+	if (value)
+		c += strlen(value);
+	return c == SIZE_MAX;
+}
+
+int LLVMFuzzerTestOneInput(const uint8_t *data, const size_t size)
+{
+	struct config_options config_opts = { 0 };
+
+	config_opts.error_action = CONFIG_ERROR_SILENT;
+	git_config_from_mem(config_parser_callback, CONFIG_ORIGIN_BLOB,
+				"fuzztest-config", (const char *)data, size, NULL,
+				CONFIG_SCOPE_UNKNOWN, &config_opts);
+	return 0;
+}

base-commit: 945115026aa63df4ab849ab14a04da31623abece
-- 
gitgitgadget