[PATCH net v2 2/2] net/smc: do not dereference an unset send buffer on the SMC-D teardown path

From: Bryam Vargas <[email protected]>

smc_close_stream_wait() calls smc_tx_prepared_sends() from inside its
sk_wait_event() condition, and sk_wait_event() evaluates that condition
once with the socket lock released. smcd_buf_detach() clears
conn->sndbuf_desc from smc_conn_kill() under lock_sock(), so a link group
terminating while a socket waits there leaves the helper dereferencing
NULL, faulting out of close(). SIOCOUTQ reads the field by hand, and
smc_close_cancel_work() drops the lock across two cancel_*_sync() calls.

Sample the pointer once in the helper, report nothing prepared while it is
unset, and bound the ioctl the same way. The receive tasklet dereferences
the field directly in smc_cdc_msg_recv_action(), not through this helper;
1/2 is what keeps it from running that late.

Fixes: ae2be35cbed2 ("net/smc: {at|de}tach sndbuf to peer DMB if supported")
Cc: [email protected]
Signed-off-by: Bryam Vargas <[email protected]>
---
 net/smc/af_smc.c | 3 ++-
 net/smc/smc_tx.h | 6 +++++-
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c
index 00403175b740..cff910cedbfc 100644
--- a/net/smc/af_smc.c
+++ b/net/smc/af_smc.c
@@ -3233,7 +3233,8 @@ int smc_ioctl(struct socket *sock, unsigned int cmd,
 			return -EINVAL;
 		}
 		if (smc->sk.sk_state == SMC_INIT ||
-		    smc->sk.sk_state == SMC_CLOSED)
+		    smc->sk.sk_state == SMC_CLOSED ||
+		    !READ_ONCE(smc->conn.sndbuf_desc))
 			answ = 0;
 		else
 			answ = smc->conn.sndbuf_desc->len -
diff --git a/net/smc/smc_tx.h b/net/smc/smc_tx.h
index a59f370b8b43..610a945aefd6 100644
--- a/net/smc/smc_tx.h
+++ b/net/smc/smc_tx.h
@@ -20,11 +20,15 @@
 
 static inline int smc_tx_prepared_sends(struct smc_connection *conn)
 {
+	struct smc_buf_desc *sndbuf_desc = READ_ONCE(conn->sndbuf_desc);
 	union smc_host_cursor sent, prep;
 
+	if (!sndbuf_desc)
+		return 0;
+
 	smc_curs_copy(&sent, &conn->tx_curs_sent, conn);
 	smc_curs_copy(&prep, &conn->tx_curs_prep, conn);
-	return smc_curs_diff(conn->sndbuf_desc->len, &sent, &prep);
+	return smc_curs_diff(sndbuf_desc->len, &sent, &prep);
 }
 
 void smc_tx_pending(struct smc_connection *conn);

-- 
2.55.0