Commit message: avcodec/cbs_jpeg: Check length for SOS

Fixes: out of array access
Fixes: 19734/clusterfuzz-testcase-minimized-ffmpeg_BSF_TRACE_HEADERS_fuzzer-5673507031875584
Fixes: 19353/clusterfuzz-testcase-minimized-ffmpeg_BSF_TRACE_HEADERS_fuzzer-5703944462663680

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
CVE: CVE-2020-12284
CWE: CWE-787
Vulnerability description: cbs_jpeg_split_fragment in libavcodec/cbs_jpeg.c in FFmpeg 4.1 and 4.2.2 has a heap-based buffer overflow during JPEG_MARKER_SOS handling because of a missing length check.

Project: FFmpeg
File: cbs_jpeg.c

static int cbs_jpeg_split_fragment(CodedBitstreamContext *ctx,
                                   CodedBitstreamFragment *frag,
                                   int header)
{
    AVBufferRef *data_ref;
    uint8_t *data;
    size_t data_size;
    int unit, start, end, marker, next_start, next_marker;
    int err, i, j, length;

    if (frag->data_size < 4) {
        // Definitely too short to be meaningful.
        return AVERROR_INVALIDDATA;
    }

    for (i = 0; i + 1 < frag->data_size && frag->data[i] != 0xff; i++);
    if (i > 0) {
        av_log(ctx->log_ctx, AV_LOG_WARNING, "Discarding %d bytes at "
               "beginning of image.\n", i);
    }
    for (++i; i + 1 < frag->data_size && frag->data[i] == 0xff; i++);
    if (i + 1 >= frag->data_size && frag->data[i]) {
        av_log(ctx->log_ctx, AV_LOG_ERROR, "Invalid JPEG image: "
               "no SOI marker found.\n");
        return AVERROR_INVALIDDATA;
    }
    marker = frag->data[i];
    if (marker != JPEG_MARKER_SOI) {
        av_log(ctx->log_ctx, AV_LOG_ERROR, "Invalid JPEG image: first "
               "marker is %02x, should be SOI.\n", marker);
        return AVERROR_INVALIDDATA;
    }
    for (++i; i + 1 < frag->data_size && frag->data[i] == 0xff; i++);
    if (i + 1 >= frag->data_size) {
        av_log(ctx->log_ctx, AV_LOG_ERROR, "Invalid JPEG image: "
               "no image content found.\n");
        return AVERROR_INVALIDDATA;
    }
    marker = frag->data[i];
    start  = i + 1;

    for (unit = 0;; unit++) {
        if (marker == JPEG_MARKER_EOI) {
            break;
        } else if (marker == JPEG_MARKER_SOS) {
            for (i = start; i + 1 < frag->data_size; i++) {
                if (frag->data[i] != 0xff)
                    continue;
                end = i;
                for (++i; i + 1 < frag->data_size &&
                          frag->data[i] == 0xff; i++);
                if (i + 1 >= frag->data_size) {
                    next_marker = -1;
                } else {
                    if (frag->data[i] == 0x00)
                        continue;
                    next_marker = frag->data[i];
                    next_start  = i + 1;
                }
                break;
            }
        } else {
            i = start;
            if (i + 2 > frag->data_size) {
                av_log(ctx->log_ctx, AV_LOG_ERROR, "Invalid JPEG image: "
                       "truncated at %02x marker.\n", marker);
                return AVERROR_INVALIDDATA;
            }
            length = AV_RB16(frag->data + i);
            if (i + length > frag->data_size) {
                av_log(ctx->log_ctx, AV_LOG_ERROR, "Invalid JPEG image: "
                       "truncated at %02x marker segment.\n", marker);
                return AVERROR_INVALIDDATA;
            }
            end = start + length;

            i = end;
            if (frag->data[i] != 0xff) {
                next_marker = -1;
            } else {
                for (++i; i + 1 < frag->data_size &&
                          frag->data[i] == 0xff; i++);
                if (i + 1 >= frag->data_size) {
                    next_marker = -1;
                } else {
                    next_marker = frag->data[i];
                    next_start  = i + 1;
                }
            }
        }

        if (marker == JPEG_MARKER_SOS) {
            length = AV_RB16(frag->data + start);

            data_ref = NULL;
            data     = av_malloc(end - start +
                                 AV_INPUT_BUFFER_PADDING_SIZE);
            if (!data)
                return AVERROR(ENOMEM);

            memcpy(data, frag->data + start, length);
            for (i = start + length, j = length; i < end; i++, j++) {
                if (frag->data[i] == 0xff) {
                    while (frag->data[i] == 0xff)
                        ++i;
                    data[j] = 0xff;
                } else {
                    data[j] = frag->data[i];
                }
            }
            data_size = j;

            memset(data + data_size, 0, AV_INPUT_BUFFER_PADDING_SIZE);

        } else {
            data      = frag->data + start;
            data_size = end - start;
            data_ref  = frag->data_ref;
        }

        err = ff_cbs_insert_unit_data(ctx, frag, unit, marker,
                                      data, data_size, data_ref);
        if (err < 0)
            return err;

        if (next_marker == -1)
            break;
        marker = next_marker;
        start  = next_start;
    }

    return 0;
}