h264: disallow constrained intra prediction modes for luma.

Conversion of the luma intra prediction mode to one of the constrained
("alzheimer") ones can happen by crafting special bitstreams, causing
a crash because we'll call a NULL function pointer for 16x16 block intra
prediction, since constrained intra prediction functions are only
implemented for chroma (8x8 blocks).

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

libavcodec/h264.c

@@ -105,7 +105,7 @@ int ff_h264_check_intra4x4_pred_mode(H264Context *h){
  * Check if the top & left blocks are available if needed and
  * change the dc mode so it only uses the available blocks.
  */
-int ff_h264_check_intra_pred_mode(H264Context *h, int mode){
+int ff_h264_check_intra_pred_mode(H264Context *h, int mode, int is_chroma){
     MpegEncContext * const s = &h->s;
     static const int8_t top [7]= {LEFT_DC_PRED8x8, 1,-1,-1};
     static const int8_t left[7]= { TOP_DC_PRED8x8,-1, 2,-1,DC_128_PRED8x8};
@@ -125,7 +125,7 @@ int ff_h264_check_intra_pred_mode(H264Context *h, int mode){
 
     if((h->left_samples_available&0x8080) != 0x8080){
         mode= left[ mode ];
-        if(h->left_samples_available&0x8080){ //mad cow disease mode, aka MBAFF + constrained_intra_pred
+        if(is_chroma && (h->left_samples_available&0x8080)){ //mad cow disease mode, aka MBAFF + constrained_intra_pred
             mode= ALZHEIMER_DC_L0T_PRED8x8 + (!(h->left_samples_available&0x8000)) + 2*(mode == DC_128_PRED8x8);
         }
         if(mode<0){

libavcodec/h264.h

@@ -657,7 +657,7 @@ int ff_h264_check_intra4x4_pred_mode(H264Context *h);
 /**
  * Check if the top & left blocks are available if needed & change the dc mode so it only uses the available blocks.
  */
-int ff_h264_check_intra_pred_mode(H264Context *h, int mode);
+int ff_h264_check_intra_pred_mode(H264Context *h, int mode, int is_chroma);
 
 void ff_h264_hl_decode_mb(H264Context *h);
 int ff_h264_frame_start(H264Context *h);

libavcodec/h264_cabac.c

@@ -2040,14 +2040,14 @@ decode_intra_mb:
             write_back_intra_pred_mode(h);
             if( ff_h264_check_intra4x4_pred_mode(h) < 0 ) return -1;
         } else {
-            h->intra16x16_pred_mode= ff_h264_check_intra_pred_mode( h, h->intra16x16_pred_mode );
+            h->intra16x16_pred_mode= ff_h264_check_intra_pred_mode( h, h->intra16x16_pred_mode, 0 );
             if( h->intra16x16_pred_mode < 0 ) return -1;
         }
         if(decode_chroma){
             h->chroma_pred_mode_table[mb_xy] =
             pred_mode                        = decode_cabac_mb_chroma_pre_mode( h );
 
-            pred_mode= ff_h264_check_intra_pred_mode( h, pred_mode );
+            pred_mode= ff_h264_check_intra_pred_mode( h, pred_mode, 1 );
             if( pred_mode < 0 ) return -1;
             h->chroma_pred_mode= pred_mode;
         } else {

libavcodec/h264_cavlc.c

@@ -822,12 +822,12 @@ decode_intra_mb:
             if( ff_h264_check_intra4x4_pred_mode(h) < 0)
                 return -1;
         }else{
-            h->intra16x16_pred_mode= ff_h264_check_intra_pred_mode(h, h->intra16x16_pred_mode);
+            h->intra16x16_pred_mode= ff_h264_check_intra_pred_mode(h, h->intra16x16_pred_mode, 0);
             if(h->intra16x16_pred_mode < 0)
                 return -1;
         }
         if(decode_chroma){
-            pred_mode= ff_h264_check_intra_pred_mode(h, get_ue_golomb_31(&s->gb));
+            pred_mode= ff_h264_check_intra_pred_mode(h, get_ue_golomb_31(&s->gb), 1);
             if(pred_mode < 0)
                 return -1;
             h->chroma_pred_mode= pred_mode;

libavcodec/svq3.c

@@ -612,7 +612,7 @@ static int svq3_decode_mb(SVQ3Context *svq3, unsigned int mb_type)
         dir = i_mb_type_info[mb_type - 8].pred_mode;
         dir = (dir >> 1) ^ 3*(dir & 1) ^ 1;
 
-        if ((h->intra16x16_pred_mode = ff_h264_check_intra_pred_mode(h, dir)) == -1){
+        if ((h->intra16x16_pred_mode = ff_h264_check_intra_pred_mode(h, dir, 0)) == -1){
             av_log(h->s.avctx, AV_LOG_ERROR, "check_intra_pred_mode = -1\n");
             return -1;
         }
@@ -711,7 +711,7 @@ static int svq3_decode_mb(SVQ3Context *svq3, unsigned int mb_type)
     s->current_picture.f.mb_type[mb_xy] = mb_type;
 
     if (IS_INTRA(mb_type)) {
-        h->chroma_pred_mode = ff_h264_check_intra_pred_mode(h, DC_PRED8x8);
+        h->chroma_pred_mode = ff_h264_check_intra_pred_mode(h, DC_PRED8x8, 1);
     }
 
     return 0;