drm/dp/mst: fix OOB reads on 2-byte fields in sideband reply parsers

drm/dp/mst: fix OOB reads on 2-byte fields in sideband reply parsers

Three sideband reply parsers read 16-bit fields as:

  val = (raw->msg[idx] << 8) | (raw->msg[idx+1]);

and check bounds only after the fact. When idx == raw->curlen,
raw->msg[idx+1] reads one byte past the received message data into
the following struct fields (curchunk_len, curchunk_idx, curlen).

Affected functions:
 - drm_dp_sideband_parse_enum_path_resources_ack()
   full_payload_bw_number and avail_payload_bw_number fields
 - drm_dp_sideband_parse_allocate_payload_ack()
   allocated_pbn field
 - drm_dp_sideband_parse_query_payload_ack()
   allocated_pbn field

Fix by using a single combined check (idx + 2 > curlen) before each
2-byte read. Since the check is strictly tighter than idx > curlen,
no separate step is needed.

Fixes: ad7f8a1f9ced ("drm/helper: add Displayport multi-stream helper (v0.6)")
Cc: <stable@vger.kernel.org> # v3.17+
Signed-off-by: Ashutosh Desai <ashutoshdesai993@gmail.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
[added fixes tag]
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patch.msgid.link/20260510203128.2884846-1-ashutoshdesai993@gmail.com