CVE-2026-52924

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

sctp: purge outqueue on stale COOKIE-ECHO handling

sctpstreamupdate() is only invoked when the association is moved into

COOKIE_WAIT during association setup/reconfiguration. In this path, the

outbound stream scheduler state (stream->out_curr) is expected to be

clean, since no user data should have been transmitted yet unless the

state machine has already partially progressed.

However, a corner case exists in sctpsfdo526stale(): when a

Stale Cookie ERROR is received, the association is rolled back from

COOKIEECHOED to COOKIEWAIT. In this scenario, user data may already

have been queued and even bundled with the COOKIE-ECHO chunk.

During the rollback, sctpstreamupdate() frees the old stream table

and installs a new one, but it does not invalidate stream->out_curr.

As a result, outcurr may still point to a freed sctpstream_out

entry from the previous stream state.

Later, SCTP scheduler dequeue paths (FCFS, RR, PRIO, etc.) rely on

stream->out_curr->ext, which can lead to use-after-free once the old

stream state has been released via sctpstreamfree().

This results in crashes such as (reported by Yuqi):

  BUG: KASAN: slab-use-after-free in sctpschedfcfs_dequeue+0x13a/0x140

  Read of size 8 at addr ff1100004d4d3208 by task mini_poc/9312

  CPU: 1 UID: 1001 PID: 9312 Comm: mini_poc Not tainted

     7.1.0-rc1-00305-gbd3a4795d574 #5 PREEMPT(full)

   sctpschedfcfs_dequeue+0x13a/0x140

   sctpoutqflush+0x1603/0x33e0

   sctpdosm+0x31c9/0x5d30

   sctpassocbh_rcv+0x392/0x6f0

   sctpinqpush+0x1db/0x270

   sctp_rcv+0x138d/0x3c10

Fix this by fully purging the association outqueue when handling the

Stale Cookie case. This ensures all pending transmit and retransmit

state is dropped, and any scheduler cached pointers are invalidated,

making it safe to rebuild stream state during COOKIE_WAIT restart.

Updating only stream->out_curr would be insufficient, since queued

and retransmittable data would still reference the old stream state and

trigger later use-after-free in dequeue paths.