netfilter: nf_conntrack_h323: fix OOB read in decode_choice()

netfilter: nf_conntrack_h323: fix OOB read in decode_choice()

In decode_choice(), the boundary check before get_len() uses the
variable `len`, which is still 0 from its initialization at the top of
the function:

    unsigned int type, ext, len = 0;
    ...
    if (ext || (son->attr & OPEN)) {
        BYTE_ALIGN(bs);
        if (nf_h323_error_boundary(bs, len, 0))  /* len is 0 here */
            return H323_ERROR_BOUND;
        len = get_len(bs);                        /* OOB read */

When the bitstream is exactly consumed (bs->cur == bs->end), the check
nf_h323_error_boundary(bs, 0, 0) evaluates to (bs->cur + 0 > bs->end),
which is false.  The subsequent get_len() call then dereferences
*bs->cur++, reading 1 byte past the end of the buffer.  If that byte
has bit 7 set, get_len() reads a second byte as well.

This can be triggered remotely by sending a crafted Q.931 SETUP message
with a User-User Information Element containing exactly 2 bytes of
PER-encoded data ({0x08, 0x00}) to port 1720 through a firewall with
the nf_conntrack_h323 helper active.  The decoder fully consumes the
PER buffer before reaching this code path, resulting in a 1-2 byte
heap-buffer-overflow read confirmed by AddressSanitizer.

Fix this by checking for 2 bytes (the maximum that get_len() may read)
instead of the uninitialized `len`.  This matches the pattern used at
every other get_len() call site in the same file, where the caller
checks for 2 bytes of available data before calling get_len().

Fixes: ec8a8f3c31dd ("netfilter: nf_ct_h323: Extend nf_h323_error_boundary to work on bits as well")
Signed-off-by: Vahagn Vardanian <vahagn@redrays.io>
Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://patch.msgid.link/20260225130619.1248-2-fw@strlen.de
Signed-off-by: Paolo Abeni <pabeni@redhat.com>