alsdec: ensure channel reordering is reversible

If the same idx is used for more than one i, at least one entry in
sconf->chan_pos remains uninitialized.

This can cause segmentation faults.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

1 files changed, 5 insertions, 1 deletions

diff --git a/libavcodec/alsdec.c b/libavcodec/alsdec.c
index 65aa7d2c90..f85f1e8ccf 100644
--- a/libavcodec/alsdec.c
+++ b/libavcodec/alsdec.c
@@ -358,10 +358,14 @@ static av_cold int read_specific_config(ALSDecContext *ctx)
         ctx->cs_switch = 1;
 
         for (i = 0; i < avctx->channels; i++) {
+            sconf->chan_pos[i] = -1;
+        }
+
+        for (i = 0; i < avctx->channels; i++) {
             int idx;
 
             idx = get_bits(&gb, chan_pos_bits);
-            if (idx >= avctx->channels) {
+            if (idx >= avctx->channels || sconf->chan_pos[idx] != -1) {
                 av_log(avctx, AV_LOG_WARNING, "Invalid channel reordering.\n");
                 ctx->cs_switch = 0;
                 break;