sctp: fix soft lockup from unpadded ASCONF-ACK parameter iteration

In the Linux kernel, the following vulnerability has been resolved:

sctp: fix soft lockup from unpadded ASCONF-ACK parameter iteration

sctp_verify_asconf() walks ASCONF-ACK parameters with
sctp_walk_params(), which advances by SCTP_PAD4(length), while the
consumer sctp_get_asconf_response() iterates the same parameters
advancing by the raw length, without padding. A single odd-length
parameter desynchronises the two walks and makes the consumer
interpret attacker-controlled bytes at a misaligned offset.

When those bytes yield a length of zero, the while loop over
asconf_ack_len makes no progress, spinning forever in softirq
context, and the watchdog reports a soft lockup. All reads stay
within the received skb, so the lockup is a pure remote denial of
service. A remote peer can trigger it with a crafted ASCONF-ACK on
an ADD-IP enabled association with an outstanding ASCONF (RFC 5061
section 4.1.2 requires the chunk to be authenticated, but the
predefined empty key id 0 allows the peer to compute the same
association HMAC from publicly exchanged parameters, so the gate
does not help).

The SCTP_PARAM_ERR_CAUSE case of sctp_verify_asconf() also performs
no length check, letting a parameter without a complete error
header reach the consumer, which reads errhdr.cause past the end of
the parameter, an out-of-bounds read.

Reject SCTP_PARAM_ERR_CAUSE parameters shorter than
sizeof(struct sctp_addip_param) + sizeof(struct sctp_errhdr) at the
verifier, and advance the consumer iterator with the same padding
rule as the verifier to keep the two walks in lockstep. The verifier
change guarantees a complete error header in every ERR_CAUSE
parameter the consumer can see, so the consumer's asconf_ack_len
check is dropped and it returns err_param->cause directly. The
consumer padding fix is still required because odd lengths remain
valid for SCTP_PARAM_ERR_CAUSE per RFC 5061.

The issue was found by ZeroHive, a vulnerability hunting agent at
Tencent Yunding Lab.