sandbox: Add libfuzzer integration

Add an implementation of LLVMFuzzerTestOneInput() that starts the
sandbox on a secondary thread and exposes a function to synchronize the
generation of fuzzing inputs with their consumption by the sandbox.

Signed-off-by: Andrew Scull <ascull@google.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

3 files changed, 98 insertions, 0 deletions

diff --git a/arch/sandbox/config.mk b/arch/sandbox/config.mk
index 4106032..3e2c7f9 100644
--- a/arch/sandbox/config.mk
+++ b/arch/sandbox/config.mk
@@ -19,6 +19,9 @@ SANITIZERS :=
 ifdef CONFIG_ASAN
 SANITIZERS	+= -fsanitize=address
 endif
+ifdef CONFIG_FUZZ
+SANITIZERS	+= -fsanitize=fuzzer
+endif
 KBUILD_CFLAGS	+= $(SANITIZERS)
 
 cmd_u-boot__ = $(CC) -o $@ -Wl,-T u-boot.lds $(u-boot-init) \
diff --git a/arch/sandbox/cpu/os.c b/arch/sandbox/cpu/os.c
index f229d16..3b23060 100644
--- a/arch/sandbox/cpu/os.c
+++ b/arch/sandbox/cpu/os.c
@@ -8,6 +8,7 @@
 #include <dirent.h>
 #include <errno.h>
 #include <fcntl.h>
+#include <pthread.h>
 #include <getopt.h>
 #include <setjmp.h>
 #include <signal.h>
@@ -26,6 +27,7 @@
 #include <linux/compiler_attributes.h>
 #include <linux/types.h>
 
+#include <asm/fuzzing_engine.h>
 #include <asm/getopt.h>
 #include <asm/main.h>
 #include <asm/sections.h>
@@ -1003,7 +1005,75 @@ void os_relaunch(char *argv[])
 	os_exit(1);
 }
 
+
+#ifdef CONFIG_FUZZ
+static void *fuzzer_thread(void * ptr)
+{
+	char cmd[64];
+	char *argv[5] = {"./u-boot", "-T", "-c", cmd, NULL};
+	const char *fuzz_test;
+
+	/* Find which test to run from an environment variable. */
+	fuzz_test = getenv("UBOOT_SB_FUZZ_TEST");
+	if (!fuzz_test)
+		os_abort();
+
+	snprintf(cmd, sizeof(cmd), "fuzz %s", fuzz_test);
+
+	sandbox_main(4, argv);
+	os_abort();
+	return NULL;
+}
+
+static bool fuzzer_initialized = false;
+static pthread_mutex_t fuzzer_mutex = PTHREAD_MUTEX_INITIALIZER;
+static pthread_cond_t fuzzer_cond = PTHREAD_COND_INITIALIZER;
+static const uint8_t *fuzzer_data;
+static size_t fuzzer_size;
+
+int sandbox_fuzzing_engine_get_input(const uint8_t **data, size_t *size)
+{
+	if (!fuzzer_initialized)
+		return -ENOSYS;
+
+	/* Tell the main thread we need new inputs then wait for them. */
+	pthread_mutex_lock(&fuzzer_mutex);
+	pthread_cond_signal(&fuzzer_cond);
+	pthread_cond_wait(&fuzzer_cond, &fuzzer_mutex);
+	*data = fuzzer_data;
+	*size = fuzzer_size;
+	pthread_mutex_unlock(&fuzzer_mutex);
+	return 0;
+}
+
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
+{
+	static pthread_t tid;
+
+	pthread_mutex_lock(&fuzzer_mutex);
+
+	/* Initialize the sandbox on another thread. */
+	if (!fuzzer_initialized) {
+		fuzzer_initialized = true;
+		if (pthread_create(&tid, NULL, fuzzer_thread, NULL))
+			os_abort();
+		pthread_cond_wait(&fuzzer_cond, &fuzzer_mutex);
+	}
+
+	/* Hand over the input. */
+	fuzzer_data = data;
+	fuzzer_size = size;
+	pthread_cond_signal(&fuzzer_cond);
+
+	/* Wait for the inputs to be finished with. */
+	pthread_cond_wait(&fuzzer_cond, &fuzzer_mutex);
+	pthread_mutex_unlock(&fuzzer_mutex);
+
+	return 0;
+}
+#else
 int main(int argc, char *argv[])
 {
 	return sandbox_main(argc, argv);
 }
+#endif
diff --git a/arch/sandbox/include/asm/fuzzing_engine.h b/arch/sandbox/include/asm/fuzzing_engine.h
new file mode 100644
index 0000000..cf63963
--- /dev/null
+++ b/arch/sandbox/include/asm/fuzzing_engine.h
@@ -0,0 +1,25 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Copyright (c) 2022 Google, Inc.
+ * Written by Andrew Scull <ascull@google.com>
+ */
+
+#ifndef __ASM_FUZZING_ENGINE_H
+#define __ASM_FUZZING_ENGINE_H
+
+/** Function to get fuzzing engine input data. */
+/**
+ * sandbox_fuzzing_engine_get_input() - get an input from the sandbox fuzzing
+ * 					engine
+ *
+ * The function will return a pointer to the input data and the size of the
+ * data pointed to. The pointer will remain valid until the next invocation of
+ * this function.
+ *
+ * @data:	output pointer to input data
+ * @size	output size of input data
+ * Return:	0 if OK, -ve on error
+ */
+int sandbox_fuzzing_engine_get_input(const uint8_t **data, size_t *size);
+
+#endif /* __ASM_FUZZING_ENGINE_H */