[FFmpeg-devel] [PATCH] libavcodec/g723_1enc: Fix crash

["Kovacs, Zsolt via ffmpeg-devel" <ffmpeg-devel@ffmpeg.org>]

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Hi All,

We had crashes in prod while compressing audio with G.723.1 using ffmpeg as a library. The callstack was:

fcb_search, line 1027
ff_encode_encode_cb, line 253
encode_simple_internal, line 339
avcodec_send_frame, line 530

After debugging the issue I found that the crash is caused by FCBParam optim; being uninitialized in fcb_search().

The context of fcb_search(), line 1027 in libavcodec\g723_1enc.c:
    /* Reconstruct the excitation */
    memset(buf, 0, sizeof(int16_t) * SUBFRAME_LEN);
    for (i = 0; i < pulse_cnt; i++)
        buf[optim.pulse_pos[i]] = optim.pulse_sign[i];

The last line is 1027, the crash is caused by out of bounds indexing buf with the values in optim.pulse_pos (pulse_cnt is either 5 or 6, and the size of FCBParam::pulse_sign and pulse_pos is PULSE_MAX (6)).

The local variable optim is not initialized in fcb_search(). In get_fcb_param() it's assigned at the end of the function in the /* Minimize */ part, but only if (err < optim->min_err), where optim.min_err = 1 << 30;. err is calculated above that in the /* Compute square of error */ part, by clamping a 64 bit int to 32 bits, so it can easily be larger than 1 << 30. If this happens in all the iterations in get_fcb_param(), then optim is not initialized, and buf is indexed by an uninitialized variable, which caused the crashes.

The fix is to initialize optim in fcb_search(). After we applied the patch to ffmpeg, the compressions did not crash anymore.

Note: this only fixes the crash by ensuring the indices are in the valid range, it doesn't make them correct.

Thanks,
Zsolt


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