[FFmpeg-devel] [PATCH] doc/examples: add fuzz target for individual ffmpeg APIs for in-process fuzzing with libFuzzer, AFL, and similar fuzzing engines.

[Michael Niedermayer]

From: Thomas Garnier <thgarnie at google.com>

Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
---
 doc/examples/decoder_targeted.c | 183 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 183 insertions(+)
 create mode 100644 doc/examples/decoder_targeted.c

diff --git a/doc/examples/decoder_targeted.c b/doc/examples/decoder_targeted.c
new file mode 100644
index 0000000..72e2c51
--- /dev/null
+++ b/doc/examples/decoder_targeted.c
@@ -0,0 +1,183 @@
+/* Targeted fuzzer that targets specific codecs depending on two
+   compile-time flags.
+  INSTRUCTIONS:
+
+  * Get the very fresh clang, e.g. see http://libfuzzer.info#versions
+  * Get and build libFuzzer:
+     svn co http://llvm.org/svn/llvm-project/llvm/trunk/lib/Fuzzer
+     ./Fuzzer/build.sh
+  * build ffmpeg for fuzzing:
+    FLAGS="-fsanitize=address -fsanitize-coverage=trace-pc-guard,trace-cmp -g" CC="clang $FLAGS" CXX="clang++ $FLAGS" ./configure  --disable-yasm
+    make clean && make -j
+  * build the fuzz target.
+    Choose the value of FFMPEG_CODEC (e.g. AV_CODEC_ID_DVD_SUBTITLE) and
+    choose one of FUZZ_FFMPEG_VIDEO, FUZZ_FFMPEG_AUDIO, FUZZ_FFMPEG_SUBTITLE.
+    clang -fsanitize=address -fsanitize-coverage=trace-pc-guard,trace-cmp doc/examples/decoder_targeted.c -o decoder_targeted -I.   -DFFMPEG_CODEC=AV_CODEC_ID_MPEG1VIDEO -DFUZZ_FFMPEG_VIDEO ../../libfuzzer/libFuzzer.a   -Llibavcodec -Llibavdevice -Llibavfilter -Llibavformat -Llibavresample -Llibavutil -Llibpostproc -Llibswscale -Llibswresample -Wl,--as-needed -Wl,-z,noexecstack -Wl,--warn-common -Wl,-rpath-link=libpostproc:libswresample:libswscale:libavfilter:libavdevice:libavformat:libavcodec:libavutil:libavresample -lavdevice -lavfilter -lavformat -lavcodec -lswresample -lswscale -lavutil -ldl -lxcb -lxcb-shm -lxcb -lxcb-xfixes  -lxcb -lxcb-shape -lxcb -lX11 -lasound -lm -lbz2 -lz -pthread
+  * create a corpus directory and put some samples there (empty dir is ok too):
+    mkdir CORPUS && cp some-files CORPUS
+
+  * Run fuzzing:
+    ./decoder_targeted -max_len=100000 CORPUS
+
+   More info:
+   http://libfuzzer.info
+   http://tutorial.libfuzzer.info
+   https://github.com/google/oss-fuzz
+   http://lcamtuf.coredump.cx/afl/
+   https://security.googleblog.com/2016/08/guided-in-process-fuzzing-of-chrome.html
+*/
+
+#include "libavutil/avassert.h"
+
+#include "libavcodec/avcodec.h"
+#include "libavformat/avformat.h"
+
+static void error(const char *err)
+{
+    fprintf(stderr, "%s", err);
+    exit(1);
+}
+
+static AVCodec *c = NULL;
+static AVCodec *AVCodecInitialize(enum AVCodecID codec_id)
+{
+    AVCodec *res;
+    av_register_all();
+    av_log_set_level(AV_LOG_PANIC);
+    res = avcodec_find_decoder(codec_id);
+    if (!res)
+        error("Failed to find decoder");
+    return res;
+}
+
+#if defined(FUZZ_FFMPEG_VIDEO)
+#define decode_handler avcodec_decode_video2
+#elif defined(FUZZ_FFMPEG_AUDIO)
+#define decode_handler avcodec_decode_audio4
+#elif defined(FUZZ_FFMPEG_SUBTITLE)
+static int subtitle_handler(AVCodecContext *avctx, void *frame,
+                            int *got_sub_ptr, AVPacket *avpkt)
+{
+    AVSubtitle sub;
+    int ret = avcodec_decode_subtitle2(avctx, &sub, got_sub_ptr, avpkt);
+    if (ret >= 0 && *got_sub_ptr)
+        avsubtitle_free(&sub);
+    return ret;
+}
+
+#define decode_handler subtitle_handler
+#else
+#error "Specify encoder type"  // To catch mistakes
+#endif
+
+// Class to handle buffer allocation and resize for each frame
+typedef struct FuzzDataBuffer {
+    size_t size_;
+    uint8_t *data_;
+} FuzzDataBuffer;
+
+void FDBCreate(FuzzDataBuffer *FDB) {
+    FDB->size_ = 0x1000;
+    FDB->data_ = av_malloc(FDB->size_);
+    if (!FDB->data_)
+        error("Failed memory allocation");
+}
+
+void FDBDesroy(FuzzDataBuffer *FDB) { av_free(FDB->data_); }
+
+void FDBRealloc(FuzzDataBuffer *FDB, size_t size) {
+    size_t needed = size + FF_INPUT_BUFFER_PADDING_SIZE;
+    av_assert0(needed > size);
+    if (needed > FDB->size_) {
+        av_free(FDB->data_);
+        FDB->size_ = needed;
+        FDB->data_ = av_malloc(FDB->size_);
+        if (!FDB->data_)
+            error("Failed memory allocation");
+    }
+}
+
+void FDBPrepare(FuzzDataBuffer *FDB, AVPacket *dst, const uint8_t *data,
+                size_t size)
+{
+    FDBRealloc(FDB, size);
+    memcpy(FDB->data_, data, size);
+    size_t padd = FDB->size_ - size;
+    if (padd > FF_INPUT_BUFFER_PADDING_SIZE)
+        padd = FF_INPUT_BUFFER_PADDING_SIZE;
+    memset(FDB->data_ + size, 0, padd);
+    av_init_packet(dst);
+    dst->data = FDB->data_;
+    dst->size = size;
+}
+
+// Ensure we don't loop forever
+const uint32_t maxiteration = 8096;
+
+static const uint64_t FUZZ_TAG = 0x4741542D5A5A5546ULL;
+
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+    const uint64_t fuzz_tag = FUZZ_TAG;
+    FuzzDataBuffer buffer;
+    const uint8_t *last = data;
+    const uint8_t *end = data + size;
+    uint32_t it = 0;
+
+    if (!c)
+        c = AVCodecInitialize(FFMPEG_CODEC);  // Done once.
+
+    AVCodecContext* ctx = avcodec_alloc_context3(NULL);
+    if (!ctx)
+        error("Failed memory allocation");
+    int res = avcodec_open2(ctx, c, NULL);
+    if (res < 0)
+        return res;
+
+    FDBCreate(&buffer);
+    int got_frame;
+    AVFrame *frame = av_frame_alloc();
+    if (!frame)
+        error("Failed memory allocation");
+
+    // Read very simple container
+    AVPacket avpkt;
+    while (data < end && it < maxiteration) {
+        // Search for the TAG
+        while (data + sizeof(fuzz_tag) < end) {
+            if (data[0] == (fuzz_tag & 0xFF) && *(const uint64_t *)(data) == fuzz_tag)
+                break;
+            data++;
+        }
+        if (data + sizeof(fuzz_tag) > end)
+            data = end;
+
+        FDBPrepare(&buffer, &avpkt, last, data - last);
+        data += sizeof(fuzz_tag);
+        last = data;
+
+        // Iterate through all data
+        while (avpkt.size > 0 && it++ < maxiteration) {
+            av_frame_unref(frame);
+            int ret = decode_handler(ctx, frame, &got_frame, &avpkt);
+            if (ret <= 0 || ret > avpkt.size)
+               break;
+            avpkt.data += ret;
+            avpkt.size -= ret;
+        }
+    }
+
+    av_init_packet(&avpkt);
+    avpkt.data = NULL;
+    avpkt.size = 0;
+
+    do {
+        got_frame = 0;
+        decode_handler(ctx, frame, &got_frame, &avpkt);
+    } while (got_frame == 1 && it++ < maxiteration);
+
+    av_frame_free(&frame);
+    avcodec_free_context(&ctx);
+    av_freep(&ctx);
+    FDBDesroy(&buffer);
+    return 0;
+}
-- 
2.10.1