On Fri, Jul 3, 2026 at 11:35 PM Weiming Shi [off-list ref] wrote:

When an ERROR chunk with a STALE_COOKIE cause is received in the
COOKIE_ECHOED state, sctp_sf_do_5_2_6_stale() reads the 4-byte Measure
of Staleness that follows the cause header:

        err   = (struct sctp_errhdr *)(chunk->skb->data);
        stale = ntohl(*(__be32 *)((u8 *)err + sizeof(*err)));

err is the first cause in the chunk, not the STALE_COOKIE cause that
caused the dispatch, and nothing guarantees the staleness field is
present. sctp_walk_errors() only requires a cause to be as long as the
4-byte header, so for a STALE_COOKIE cause of length 4 the read runs
past the cause, and for a minimal ERROR chunk past skb->tail. The value
is echoed to the peer in the Cookie Preservative of the reply INIT,
leaking uninitialized memory.

sctp_sf_cookie_echoed_err() already walks to the STALE_COOKIE cause, so
check its length there and pass it to sctp_sf_do_5_2_6_stale(), which
reads that cause instead of the first one. A STALE_COOKIE cause too
short to hold the staleness field is discarded.

The read is reachable by any peer that can drive an association into
COOKIE_ECHOED, including an unprivileged process using a raw SCTP socket
in a user and network namespace.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: Xiang Mei <redacted>
Assisted-by: Claude:claude-opus-4-8
Cc: stable@vger.kernel.org
Signed-off-by: Weiming Shi <redacted>

Acked-by: Xin Long <lucien.xin@gmail.com>