In decode_seq(), the extension components loop iterates through bmp2_len
bits. However, it evaluates whether the component index exceeds the known
upper bound (`i >= f->ub`) and whether it contains a STOP attribute before
checking if the field is actually present in `bmp2`:

    if (i >= f->ub) {
        ...
        len = get_len(bs);
        ...
    }
    if (son->attr & STOP)
        return H323_ERROR_STOP;

    if (!((0x80000000 >> opt) & bmp2))
        continue;

If an unknown extension is marked absent in the bitmap (`bit == 0`), the
parser erroneously attempts to read a non-existent open type length from
the bitstream and advances `bs->cur` by garbage bytes. Furthermore, if a
known field has the STOP attribute but is absent from the transmission,
decoding aborts prematurely.

Move the presence bitmap check to the beginning of the loop body so absent
extensions are skipped immediately before attempting to decode or inspect
attributes. Also append a 'U' suffix to 0x80000000 to prevent signed shift
overflow.

Fixes: f587de0e2feb ("[NETFILTER]: nf_conntrack/nf_nat: add H.323 helper port")
Cc: stable@vger.kernel.org
Signed-off-by: Subham Pal <subhampal789@gmail.com>
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[Note: Raw patch attached to avoid webmail tab/whitespace corruption]