CVE-2019-7577: Fix a buffer overread in MS_ADPCM_nibble and MS_ADPCM_decode

If a chunk of RIFF/WAV file with MS ADPCM encoding contains an invalid
predictor (a valid predictor's value is between 0 and 6 inclusive),
a buffer overread can happen when the predictor is used as an index
into an array of MS ADPCM coefficients.

The overead happens when indexing MS_ADPCM_state.aCoeff[] array in
MS_ADPCM_decode() and later when dereferencing a coef pointer in
MS_ADPCM_nibble().

This patch fixes it by checking the MS ADPCM predictor values fit
into the valid range.

CVE-2019-7577
Reproducer: https://bugzilla.libsdl.org/show_bug.cgi?id=4492

Signed-off-by: Petr P?sa? <ppisar@redhat.com>

Author: ppisar

src/audio/SDL_wave.c

@@ -147,6 +147,9 @@ static int MS_ADPCM_decode(Uint8 **audio_buf, Uint32 *audio_len)
 		if ( stereo ) {
 			state[1]->hPredictor = *encoded++;
 		}
+		if (state[0]->hPredictor >= 7 || state[1]->hPredictor >= 7) {
+			goto invalid_predictor;
+		}
 		state[0]->iDelta = ((encoded[1]<<8)|encoded[0]);
 		encoded += sizeof(Sint16);
 		if ( stereo ) {
@@ -217,6 +220,10 @@ static int MS_ADPCM_decode(Uint8 **audio_buf, Uint32 *audio_len)
 	SDL_SetError("Too short chunk for a MS ADPCM decoder");
 	SDL_free(freeable);
 	return(-1);
+invalid_predictor:
+	SDL_SetError("Invalid predictor value for a MS ADPCM decoder");
+	SDL_free(freeable);
+	return(-1);
 }
 
 struct IMA_ADPCM_decodestate {