default_get_buffer() cleanup

fixes probably exploitable heap overflow
heap overflow found by (Simon Kilvington <s D kilvington A eris D qinetiq D com>)

Originally committed as revision 4714 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/utils.c

@@ -292,49 +292,50 @@ int avcodec_default_get_buffer(AVCodecContext *s, AVFrame *pic){
         buf->last_pic_num= *picture_number;
     }else{
         int h_chroma_shift, v_chroma_shift;
-        int pixel_size;
+        int pixel_size, size[3];
+        AVPicture picture;
 
         avcodec_get_chroma_sub_sample(s->pix_fmt, &h_chroma_shift, &v_chroma_shift);
 
-        switch(s->pix_fmt){
-        case PIX_FMT_RGB555:
-        case PIX_FMT_RGB565:
-        case PIX_FMT_YUV422:
-        case PIX_FMT_UYVY422:
-            pixel_size=2;
-            break;
-        case PIX_FMT_RGB24:
-        case PIX_FMT_BGR24:
-            pixel_size=3;
-            break;
-        case PIX_FMT_RGBA32:
-            pixel_size=4;
-            break;
-        default:
-            pixel_size=1;
-        }
-
         avcodec_align_dimensions(s, &w, &h);
             
         if(!(s->flags&CODEC_FLAG_EMU_EDGE)){
             w+= EDGE_WIDTH*2;
             h+= EDGE_WIDTH*2;
         }
+        avpicture_fill(&picture, NULL, s->pix_fmt, w, h);
+        pixel_size= picture.linesize[0]*8 / w;
+//av_log(NULL, AV_LOG_ERROR, "%d %d %d %d\n", (int)picture.data[1], w, h, s->pix_fmt);
+        assert(pixel_size>=1);
+            //FIXME next ensures that linesize= 2^x uvlinesize, thats needed because some MC code assumes it
+        if(pixel_size == 3*8)
+            w= ALIGN(w, STRIDE_ALIGN<<h_chroma_shift);
+        else
+            w= ALIGN(pixel_size*w, STRIDE_ALIGN<<(h_chroma_shift+3)) / pixel_size;
+        size[1] = avpicture_fill(&picture, NULL, s->pix_fmt, w, h);
+        size[0] = picture.linesize[0] * h;
+        size[1] -= size[0];
+        if(picture.data[2])
+            size[1]= size[2]= size[1]/2;
+        else
+            size[2]= 0;
 
         buf->last_pic_num= -256*256*256*64;
+        memset(buf->base, 0, sizeof(buf->base));
+        memset(buf->data, 0, sizeof(buf->data));
 
-        for(i=0; i<3; i++){
+        for(i=0; i<3 && size[i]; i++){
             const int h_shift= i==0 ? 0 : h_chroma_shift;
             const int v_shift= i==0 ? 0 : v_chroma_shift;
 
-            //FIXME next ensures that linesize= 2^x uvlinesize, thats needed because some MC code assumes it
-            buf->linesize[i]= ALIGN(pixel_size*w>>h_shift, STRIDE_ALIGN<<(h_chroma_shift-h_shift)); 
+            buf->linesize[i]= picture.linesize[i];
 
-            buf->base[i]= av_malloc((buf->linesize[i]*h>>v_shift)+16); //FIXME 16
+            buf->base[i]= av_malloc(size[i]+16); //FIXME 16
             if(buf->base[i]==NULL) return -1;
-            memset(buf->base[i], 128, buf->linesize[i]*h>>v_shift);
+            memset(buf->base[i], 128, size[i]);
 
-            if(s->flags&CODEC_FLAG_EMU_EDGE)
+            // no edge if EDEG EMU or not planar YUV, we check for PAL8 redundantly to protect against a exploitable bug regression ...
+            if((s->flags&CODEC_FLAG_EMU_EDGE) || (s->pix_fmt == PIX_FMT_PAL8) || !size[2]) 
                 buf->data[i] = buf->base[i];
             else
                 buf->data[i] = buf->base[i] + ALIGN((buf->linesize[i]*EDGE_WIDTH>>v_shift) + (EDGE_WIDTH>>h_shift), STRIDE_ALIGN);