[PATCH net v2] sctp: fix uninit-value in __sctp_rcv_asconf_lookup()

[Michael Bommarito <michael.bommarito@gmail.com>]

__sctp_rcv_asconf_lookup() in net/sctp/input.c only checks that the ASCONF
chunk can hold the ADDIP header and a parameter header, then calls
af->from_addr_param(), which reads the full address (16 bytes for IPv6)
trusting the parameter's declared length.

An unauthenticated peer can send a truncated trailing ASCONF chunk that
declares an IPv6 address parameter but stops after the 4-byte parameter
header; reached from the no-association lookup path, from_addr_param() then
reads uninitialized bytes past the parameter.

Impact: an unauthenticated SCTP peer makes the receive path read up to 16
bytes of uninitialized memory past a truncated ASCONF address parameter.

The sibling __sctp_rcv_init_lookup() bounds parameters with
sctp_walk_params(); this path open-codes the fetch and omits the bound.
Verify the whole address parameter lies within the chunk before
from_addr_param() reads it, the same class of fix as commit 51e5ad549c43
("net: sctp: fix KMSAN uninit-value in sctp_inq_pop").

Fixes: df2185771439 ("[SCTP]: Update association lookup to look at ASCONF chunks as well")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
---
v2:
- Regenerate from net/main so the patch has index lines and applies
  cleanly (Xin Long).
- Use unsigned int for the decoded length and compare it against the
  remaining parameter space after the ADDIP header (David Laight).
v1: https://lore.kernel.org/all/20260604175803.2142975-1-michael.bommarito@gmail.com/

 net/sctp/input.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/net/sctp/input.c b/net/sctp/input.c
index e119e460ccde0..c63d42500aa28 100644
--- a/net/sctp/input.c
+++ b/net/sctp/input.c
@@ -1197,13 +1197,26 @@ static struct sctp_association *__sctp_rcv_asconf_lookup(
 	struct sctp_af *af;
 	union sctp_addr_param *param;
 	union sctp_addr paddr;
+	unsigned int param_space;
+	unsigned int plen;
 
 	if (ntohs(ch->length) < sizeof(*asconf) + sizeof(struct sctp_paramhdr))
 		return NULL;
 
+	param_space = ntohs(ch->length) - sizeof(*asconf);
+
 	/* Skip over the ADDIP header and find the Address parameter */
 	param = (union sctp_addr_param *)(asconf + 1);
 
+	/* The whole address parameter must lie within the chunk before
+	 * af->from_addr_param() reads the variable-length address; otherwise a
+	 * truncated trailing ASCONF chunk lets it read uninitialized bytes past
+	 * the parameter.
+	 */
+	plen = ntohs(param->p.length);
+	if (plen < sizeof(struct sctp_paramhdr) || plen > param_space)
+		return NULL;
+
 	af = sctp_get_af_specific(param_type2af(param->p.type));
 	if (unlikely(!af))
 		return NULL;
-- 
2.53.0