avcodec/vp3: Fix undefined pointer arithmetic

When decoding a keyframe, last_frame and golden_frame are
not used at all and (at least when starting decoding)
are not set at all. But due to code sharing pointer arithmetic
on the NULL data-pointers of these frames has nevertheless
been performed. This is undefined behaviour and causes e.g.
"runtime error: applying non-zero offset 173440 to null pointer"
from UBSan in the vp31, vp4, theora-coeff-level64 and theora-offset
FATE-tests.

Fix this by reusing the current frame for unavailable frames.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

1 files changed, 10 insertions, 2 deletions

diff --git a/libavcodec/vp3.c b/libavcodec/vp3.c
index 6303068459..81dd6535c0 100644
--- a/libavcodec/vp3.c
+++ b/libavcodec/vp3.c
@@ -2056,6 +2056,14 @@ static void render_slice(Vp3DecodeContext *s, int slice)
 {
     int16_t *block = s->block;
     int motion_x = 0xdeadbeef, motion_y = 0xdeadbeef;
+    /* When decoding keyframes, the earlier frames may not be available,
+     * so to avoid using undefined pointer arithmetic on them we just
+     * use the current frame instead. Nothing is ever read from these
+     * frames in case of a keyframe. */
+    const AVFrame *last_frame   = s->last_frame.f->data[0]   ?
+                                      s->last_frame.f   : s->current_frame.f;
+    const AVFrame *golden_frame = s->golden_frame.f->data[0] ?
+                                      s->golden_frame.f : s->current_frame.f;
     int motion_halfpel_index;
     int first_pixel;
 
@@ -2065,9 +2073,9 @@ static void render_slice(Vp3DecodeContext *s, int slice)
     for (int plane = 0; plane < 3; plane++) {
         uint8_t *output_plane = s->current_frame.f->data[plane] +
                                 s->data_offset[plane];
-        const uint8_t *last_plane = s->last_frame.f->data[plane] +
+        const uint8_t *last_plane = last_frame->data[plane] +
                               s->data_offset[plane];
-        const uint8_t *golden_plane = s->golden_frame.f->data[plane] +
+        const uint8_t *golden_plane = golden_frame->data[plane] +
                                 s->data_offset[plane];
         ptrdiff_t stride = s->current_frame.f->linesize[plane];
         int plane_width  = s->width  >> (plane && s->chroma_x_shift);