From: Charles Vosburgh SCTP chunks always include a four-byte generic header, but sctp_inq_pop() currently accepts shorter declared lengths. A zero-length chunk leaves chunk_end at the current header. When ASCONF is covered by the association's SCTP-AUTH policy, sctp_assoc_bh_rcv() can continue before the state machine performs its normal chunk-length check. sctp_inq_pop() then returns the same malformed chunk repeatedly and the receive softirq can lock up. A remote SCTP peer can trigger this after establishing an association on a kernel built with CONFIG_IP_SCTP and configured with net.sctp.addip_enable=1 and net.sctp.auth_enable=1. The reproducer did not require application credentials, a shared SCTP AUTH key, or net.sctp.addip_noauth_enable=1. On commit f967455fb2a5 ("seg6: reset IP6CB after IPv6 decapsulation"), one zero-length ASCONF caused repeated watchdog soft-lockup reports in a two-vCPU KVM guest. All 3 pre-trigger health probes succeeded, while 36 of 37 post-trigger probes failed. With this change, all 37 post-trigger probes succeeded and no equivalent soft-lockup signature appeared. Reject chunks shorter than the generic SCTP header at the shared inqueue parser boundary. Mark the packet for discard before either caller can continue processing it, while preserving the four-byte generic minimum. Declared-length 1 through 4 controls and kernel-generated ASCONF traffic remained healthy. The patched sctp_hello selftest passed for IPv4 and IPv6. The complete private reproducer and validation evidence are available directly to maintainers on request. Fixes: bbd0d59809f9 ("[SCTP]: Implement the receive and verification of AUTH chunk") Cc: stable@vger.kernel.org Assisted-by: ChatGPT:GPT-5.6-Sol Assisted-by: Vantix:claude-opus-5 Assisted-by: Codex:GPT-5 Signed-off-by: Charles Vosburgh --- Changes in v2: - Fold the minimum-header check into the existing chunk classifier. - Keep pr_debug() on the malformed path and leave chunk_end unchanged. - Retest lengths 0--4, legitimate ASCONF, and IPv4/IPv6 SCTP traffic. - Link to v1: https://patch.msgid.link/20260826-sctp-zero-chunk-inqueue-v1-1-86769dcc7f8c@gmail.com To: Marcelo Ricardo Leitner To: Xin Long To: "David S. Miller" To: Eric Dumazet To: Jakub Kicinski To: Paolo Abeni To: Simon Horman To: Vlad Yasevich Cc: linux-sctp@vger.kernel.org Cc: netdev@vger.kernel.org Cc: linux-kernel@vger.kernel.org --- net/sctp/inqueue.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/net/sctp/inqueue.c b/net/sctp/inqueue.c index 5f988b3a8814f..d666cec6b194e 100644 --- a/net/sctp/inqueue.c +++ b/net/sctp/inqueue.c @@ -212,8 +212,10 @@ struct sctp_chunk *sctp_inq_pop(struct sctp_inq *queue) chunk->chunk_end = ((__u8 *)ch) + SCTP_PAD4(ntohs(ch->length)); skb_pull(chunk->skb, sizeof(*ch)); chunk->subh.v = NULL; /* Subheader is no longer valid. */ - - if (chunk->chunk_end + sizeof(*ch) <= skb_tail_pointer(chunk->skb)) { + if (unlikely(ntohs(ch->length) < sizeof(*ch))) { + chunk->pdiscard = 1; + } else if (chunk->chunk_end + sizeof(*ch) <= + skb_tail_pointer(chunk->skb)) { /* This is not a singleton */ chunk->singleton = 0; } else if (chunk->chunk_end > skb_tail_pointer(chunk->skb)) { --- base-commit: f967455fb2a5a2079b9eb5823e9ccf359174bf9f change-id: 20260826-sctp-zero-chunk-inqueue-b478dca552c6 Best regards, -- Charles Vosburgh