[PATCH net] sctp: revalidate list cursor after sctp_sendmsg_to_asoc() in SCTP_SENDALL

[joycathacker@gmail.com]

From: Ben Morris <bmorris@anthropic.com>

The SCTP_SENDALL path in sctp_sendmsg() iterates ep->asocs with
list_for_each_entry_safe(), which caches the next entry in @tmp before
the loop body runs.  The body calls sctp_sendmsg_to_asoc(), which may
drop the socket lock inside sctp_wait_for_sndbuf().

While the lock is dropped, another thread can SCTP_SOCKOPT_PEELOFF the
association cached in @tmp, migrating it to a new endpoint via
sctp_sock_migrate() (list_del_init() + list_add_tail() to
newep->asocs), and optionally close the new socket which frees the
association via kfree_rcu().  The cached @tmp can also be freed by a
network ABORT for that association, processed in softirq while the
lock is dropped.

sctp_wait_for_sndbuf() revalidates @asoc (the current entry) on re-lock
via the "sk != asoc->base.sk" and "asoc->base.dead" checks, but nothing
revalidates @tmp.  After a successful return, the iterator advances to
the stale @tmp, yielding either a use-after-free (if the peeled socket
was closed) or a list-walk onto the new endpoint's list head (type
confusion of &newep->asocs as a struct sctp_association *).

Both are reachable from CapEff=0; the type-confusion path gives
controlled indirect call via the outqueue.sched->init_sid pointer.

Fix by re-deriving @tmp from @asoc after sctp_sendmsg_to_asoc()
returns.  @asoc is known to still be on ep->asocs at that point: the
only callers that list_del an association from ep->asocs are
sctp_association_free() (which sets asoc->base.dead) and
sctp_assoc_migrate() (which changes asoc->base.sk), and
sctp_wait_for_sndbuf() checks both under the lock before any
successful return; a tripped check propagates as err < 0 and the loop
bails before the re-derive.

The SCTP_ABORT path in sctp_sendmsg_check_sflags() returns 0 and the
loop hits 'continue' before sctp_sendmsg_to_asoc() is ever called, so
the @tmp cached by list_for_each_entry_safe() still covers the
lock-held free that ba59fb027307 ("sctp: walk the list of asoc
safely") was added for.

Fixes: 4910280503f3 ("sctp: add support for snd flag SCTP_SENDALL process in sendmsg")
Cc: stable@vger.kernel.org
Assisted-by: claude:mythos
Signed-off-by: Ben Morris <bmorris@anthropic.com>
---
 net/sctp/socket.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/net/sctp/socket.c b/net/sctp/socket.c
index 58d0d9747f0b..1d2568bb6bc2 100644
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -1986,6 +1986,15 @@ static int sctp_sendmsg(struct sock *sk, struct msghdr *msg, size_t msg_len)
 				goto out_unlock;

 			iov_iter_revert(&msg->msg_iter, err);
+
+			/* sctp_sendmsg_to_asoc() may have released the socket
+			 * lock (sctp_wait_for_sndbuf), during which other
+			 * associations on ep->asocs could have been peeled
+			 * off or freed.  @asoc itself is revalidated by the
+			 * base.dead and base.sk checks in sctp_wait_for_sndbuf,
+			 * so re-derive the cached cursor from it.
+			 */
+			tmp = list_next_entry(asoc, asocs);
 		}

 		goto out_unlock;
--
2.43.0