avcodec/cbs: Fix potential overflow

The number of bits in a PutBitContext must fit into an int, yet nothing
guaranteed the size argument cbs_write_unit_data() uses in init_put_bits()
to be in the range 0..INT_MAX / 8. This has been changed.

Furthermore, the check 8 * data_size > data_bit_start that there is
data beyond the initial padding when writing mpeg2 or H.264/5 slices
could also overflow, so divide it by 8 to get an equivalent check
without this problem.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/cbs.c

@@ -309,7 +309,9 @@ static int cbs_write_unit_data(CodedBitstreamContext *ctx,
     if (ret < 0) {
         if (ret == AVERROR(ENOSPC)) {
             // Overflow.
-            ctx->write_buffer_size *= 2;
+            if (ctx->write_buffer_size == INT_MAX / 8)
+                return AVERROR(ENOMEM);
+            ctx->write_buffer_size = FFMIN(2 * ctx->write_buffer_size, INT_MAX / 8);
             goto reallocate_and_try_again;
         }
         // Write failed for some other reason.

libavcodec/cbs_h2645.c

@@ -1101,7 +1101,7 @@ static int cbs_h2645_write_slice_data(CodedBitstreamContext *ctx,
     const uint8_t *pos = data + data_bit_start / 8;
 
     av_assert0(data_bit_start >= 0 &&
-               8 * data_size > data_bit_start);
+               data_size > data_bit_start / 8);
 
     if (data_size * 8 + 8 > put_bits_left(pbc))
         return AVERROR(ENOSPC);

libavcodec/cbs_mpeg2.c

@@ -337,7 +337,7 @@ static int cbs_mpeg2_write_slice(CodedBitstreamContext *ctx,
         uint8_t *pos = slice->data + slice->data_bit_start / 8;
 
         av_assert0(slice->data_bit_start >= 0 &&
-                   8 * slice->data_size > slice->data_bit_start);
+                   slice->data_size > slice->data_bit_start / 8);
 
         if (slice->data_size * 8 + 8 > put_bits_left(pbc))
             return AVERROR(ENOSPC);