On Wed, Apr 2, 2025 at 6:26 AM Ricardo Cañuelo Navarro <rcn@xxxxxxxxxx> wrote: > > sctp_sendmsg() re-uses associations and transports when possible by > doing a lookup based on the socket endpoint and the message destination > address, and then sctp_sendmsg_to_asoc() sets the selected transport in > all the message chunks to be sent. > > There's a possible race condition if another thread triggers the removal > of that selected transport, for instance, by explicitly unbinding an > address with setsockopt(SCTP_SOCKOPT_BINDX_REM), after the chunks have > been set up and before the message is sent. This causes the access to > the transport data in sctp_outq_select_transport(), when the association > outqueue is flushed, to do a use-after-free read. > The data send path: sctp_endpoint_lookup_assoc() -> sctp_sendmsg_to_asoc() And the transport removal path: sctp_sf_do_asconf() -> sctp_process_asconf() -> sctp_assoc_rm_peer() are both protected by the same socket lock. Additionally, when a path is removed, sctp_assoc_rm_peer() updates the transport of all existing chunks in the send queues (peer->transmitted and asoc->outqueue.out_chunk_list) to NULL. It will be great if you can reproduce the issue locally and help check how the potential race occurs. > This patch addresses this scenario by checking if the transport still > exists right after the chunks to be sent are set up to use it and before > proceeding to sending them. If the transport was freed since it was > found, the send is aborted. The reason to add the check here is that > once the transport is assigned to the chunks, deleting that transport > is safe, since it will also set chunk->transport to NULL in the affected > chunks. This scenario is correctly handled already, see Fixes below. > > The bug was found by a private syzbot instance (see the error report [1] > and the C reproducer that triggers it [2]). > > Link: https://people.igalia.com/rcn/kernel_logs/20250402__KASAN_slab-use-after-free_Read_in_sctp_outq_select_transport.txt [1] > Link: https://people.igalia.com/rcn/kernel_logs/20250402__KASAN_slab-use-after-free_Read_in_sctp_outq_select_transport__repro.c [2] > Cc: stable@xxxxxxxxxxxxxxx > Fixes: df132eff4638 ("sctp: clear the transport of some out_chunk_list chunks in sctp_assoc_rm_peer") > Signed-off-by: Ricardo Cañuelo Navarro <rcn@xxxxxxxxxx> > --- > net/sctp/socket.c | 16 ++++++++++++++++ > 1 file changed, 16 insertions(+) > > diff --git a/net/sctp/socket.c b/net/sctp/socket.c > index 36ee34f483d703ffcfe5ca9e6cc554fba24c75ef..9c5ff44fa73cae6a6a04790800cc33dfa08a8da9 100644 > --- a/net/sctp/socket.c > +++ b/net/sctp/socket.c > @@ -1787,17 +1787,24 @@ static int sctp_sendmsg_check_sflags(struct sctp_association *asoc, > return 1; > } > > +static union sctp_addr *sctp_sendmsg_get_daddr(struct sock *sk, > + const struct msghdr *msg, > + struct sctp_cmsgs *cmsgs); > + > static int sctp_sendmsg_to_asoc(struct sctp_association *asoc, > struct msghdr *msg, size_t msg_len, > struct sctp_transport *transport, > struct sctp_sndrcvinfo *sinfo) > { > + struct sctp_transport *aux_transport = NULL; > struct sock *sk = asoc->base.sk; > + struct sctp_endpoint *ep = sctp_sk(sk)->ep; > struct sctp_sock *sp = sctp_sk(sk); > struct net *net = sock_net(sk); > struct sctp_datamsg *datamsg; > bool wait_connect = false; > struct sctp_chunk *chunk; > + union sctp_addr *daddr; > long timeo; > int err; > > @@ -1869,6 +1876,15 @@ static int sctp_sendmsg_to_asoc(struct sctp_association *asoc, > sctp_set_owner_w(chunk); > chunk->transport = transport; > } > + /* Fail if transport was deleted after lookup in sctp_sendmsg() */ > + daddr = sctp_sendmsg_get_daddr(sk, msg, NULL); > + if (daddr) { > + sctp_endpoint_lookup_assoc(ep, daddr, &aux_transport); > + if (!aux_transport || aux_transport != transport) { > + sctp_datamsg_free(datamsg); > + goto err; > + } > + } > We should avoid an extra hashtable lookup on this hot TX path, as it would negatively impact performance. Thanks. > err = sctp_primitive_SEND(net, asoc, datamsg); > if (err) { > > --- > base-commit: 38fec10eb60d687e30c8c6b5420d86e8149f7557 > change-id: 20250402-kasan_slab-use-after-free_read_in_sctp_outq_select_transport-46c9c30bcb7d >
