| Message ID | 20220930142009.5862-1-aidanmacdonald.0x0@gmail.com |
|---|---|
| State | Accepted |
| Commit | a4405cc0cc16413fe5222c888b3d4bb04cedc83f |
| Headers | show |
| Series | [FFmpeg-devel] avcodec/dvdsub_parser: Fix length check for short packets | expand |
Quoting Aidan MacDonald (2022-09-30 16:20:09) > The DVD subtitle parser handles two types of packets: "normal" > packets with a 16-bit length, and HD-DVD packets that set the > 16-bit length to 0 and encode a 32-bit length in the next four > bytes. This implies that HD-DVD packets are at least six bytes > long, but the code didn't actually verify this. > > The faulty length check results in an out of bounds read for > zero-length "normal" packets that occur in the input, which are > only 2 bytes long, but get misinterpreted as an HD-DVD packet. > When this happens the parser reads packet_len from beyond the > end of the input buffer. The subtitle stream is not correctly > decoded after this point due to the garbage packet_len. > > Fixing this is pretty simple: fix the length check so packets > less than 6 bytes long will not be mistakenly parsed as HD-DVD > packets. > > Signed-off-by: Aidan MacDonald <aidanmacdonald.0x0@gmail.com> > --- > libavcodec/dvdsub_parser.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/libavcodec/dvdsub_parser.c b/libavcodec/dvdsub_parser.c > index 44738a73d6..8871b6a383 100644 > --- a/libavcodec/dvdsub_parser.c > +++ b/libavcodec/dvdsub_parser.c > @@ -43,7 +43,7 @@ static int dvdsub_parse(AVCodecParserContext *s, > *poutbuf_size = buf_size; > > if (pc->packet_index == 0) { > - if (buf_size < 2 || AV_RB16(buf) && buf_size < 6) { > + if (buf_size < 2 || (AV_RB16(buf) == 0 && buf_size < 6)) { Looks good, will push.
diff --git a/libavcodec/dvdsub_parser.c b/libavcodec/dvdsub_parser.c index 44738a73d6..8871b6a383 100644 --- a/libavcodec/dvdsub_parser.c +++ b/libavcodec/dvdsub_parser.c @@ -43,7 +43,7 @@ static int dvdsub_parse(AVCodecParserContext *s, *poutbuf_size = buf_size; if (pc->packet_index == 0) { - if (buf_size < 2 || AV_RB16(buf) && buf_size < 6) { + if (buf_size < 2 || (AV_RB16(buf) == 0 && buf_size < 6)) { if (buf_size) av_log(avctx, AV_LOG_DEBUG, "Parser input %d too small\n", buf_size); return buf_size;
The DVD subtitle parser handles two types of packets: "normal" packets with a 16-bit length, and HD-DVD packets that set the 16-bit length to 0 and encode a 32-bit length in the next four bytes. This implies that HD-DVD packets are at least six bytes long, but the code didn't actually verify this. The faulty length check results in an out of bounds read for zero-length "normal" packets that occur in the input, which are only 2 bytes long, but get misinterpreted as an HD-DVD packet. When this happens the parser reads packet_len from beyond the end of the input buffer. The subtitle stream is not correctly decoded after this point due to the garbage packet_len. Fixing this is pretty simple: fix the length check so packets less than 6 bytes long will not be mistakenly parsed as HD-DVD packets. Signed-off-by: Aidan MacDonald <aidanmacdonald.0x0@gmail.com> --- libavcodec/dvdsub_parser.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)