Don't use ff_cropTbl[] for IDCT.

Results of IDCT can by far outreach the range of ff_cropTbl[], leading
to overreads and potentially crashes.

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

1 files changed, 6 insertions, 9 deletions

diff --git a/libavcodec/rv34dsp.c b/libavcodec/rv34dsp.c
index ce5be93894..4145c4dd85 100644
--- a/libavcodec/rv34dsp.c
+++ b/libavcodec/rv34dsp.c
@@ -55,7 +55,6 @@ static av_always_inline void rv34_row_transform(int temp[16], DCTELEM *block)
  */
 static void rv34_idct_add_c(uint8_t *dst, ptrdiff_t stride, DCTELEM *block){
     int      temp[16];
-    uint8_t *cm = ff_cropTbl + MAX_NEG_CROP;
     int      i;
 
     rv34_row_transform(temp, block);
@@ -67,10 +66,10 @@ static void rv34_idct_add_c(uint8_t *dst, ptrdiff_t stride, DCTELEM *block){
         const int z2 =  7* temp[4*1+i] - 17*temp[4*3+i];
         const int z3 = 17* temp[4*1+i] +  7*temp[4*3+i];
 
-        dst[0] = cm[ dst[0] + ( (z0 + z3) >> 10 ) ];
-        dst[1] = cm[ dst[1] + ( (z1 + z2) >> 10 ) ];
-        dst[2] = cm[ dst[2] + ( (z1 - z2) >> 10 ) ];
-        dst[3] = cm[ dst[3] + ( (z0 - z3) >> 10 ) ];
+        dst[0] = av_clip_uint8( dst[0] + ( (z0 + z3) >> 10 ) );
+        dst[1] = av_clip_uint8( dst[1] + ( (z1 + z2) >> 10 ) );
+        dst[2] = av_clip_uint8( dst[2] + ( (z1 - z2) >> 10 ) );
+        dst[3] = av_clip_uint8( dst[3] + ( (z0 - z3) >> 10 ) );
 
         dst  += stride;
     }
@@ -103,15 +102,13 @@ static void rv34_inv_transform_noround_c(DCTELEM *block){
 
 static void rv34_idct_dc_add_c(uint8_t *dst, ptrdiff_t stride, int dc)
 {
-    const uint8_t *cm = ff_cropTbl + MAX_NEG_CROP;
     int i, j;
 
-    cm += (13*13*dc + 0x200) >> 10;
-
+    dc = (13*13*dc + 0x200) >> 10;
     for (i = 0; i < 4; i++)
     {
         for (j = 0; j < 4; j++)
-            dst[j] = cm[ dst[j] ];
+            dst[j] = av_clip_uint8( dst[j] + dc );
 
         dst += stride;
     }