net: sctp: test if association is dead in sctp_wake_up_waiters
In function sctp_wake_up_waiters(), we need to involve a test if the association is declared dead. If so, we don't have any reference to a possible sibling association anymore and need to invoke sctp_write_space() instead, and normally walk the socket's associations and notify them of new wmem space. The reason for special casing is that otherwise, we could run into the following issue when a sctp_primitive_SEND() call from sctp_sendmsg() fails, and tries to flush an association's outq, i.e. in the following way: sctp_association_free() `-> list_del(&asoc->asocs) <-- poisons list pointer asoc->base.dead = true sctp_outq_free(&asoc->outqueue) `-> __sctp_outq_teardown() `-> sctp_chunk_free() `-> consume_skb() `-> sctp_wfree() `-> sctp_wake_up_waiters() <-- dereferences poisoned pointers if asoc->ep->sndbuf_policy=0 Therefore, only walk the list in an 'optimized' way if we find that the current association is still active. We could also use list_del_init() in addition when we call sctp_association_free(), but as Vlad suggests, we want to trap such bugs and thus leave it poisoned as is. Why is it safe to resolve the issue by testing for asoc->base.dead? Parallel calls to sctp_sendmsg() are protected under socket lock, that is lock_sock()/release_sock(). Only within that path under lock held, we're setting skb/chunk owner via sctp_set_owner_w(). Eventually, chunks are freed directly by an association still under that lock. So when traversing association list on destruction time from sctp_wake_up_waiters() via sctp_wfree(), a different CPU can't be running sctp_wfree() while another one calls sctp_association_free() as both happens under the same lock. Therefore, this can also not race with setting/testing against asoc->base.dead as we are guaranteed for this to happen in order, under lock. Further, Vlad says: the times we check asoc->base.dead is when we've cached an association pointer for later processing. In between cache and processing, the association may have been freed and is simply still around due to reference counts. We check asoc->base.dead under a lock, so it should always be safe to check and not race against sctp_association_free(). Stress-testing seems fine now, too. Fixes: cd253f9f357d ("net: sctp: wake up all assocs if sndbuf policy is per socket") Signed-off-by: Daniel Borkmann <dborkman@redhat.com> Cc: Vlad Yasevich <vyasevic@redhat.com> Acked-by: Neil Horman <nhorman@tuxdriver.com> Acked-by: Vlad Yasevich <vyasevic@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Please register or sign in to comment