[PATCH net v2 2/2] net/smc: fix use-after-free in smc_rx_pipe_buf_release()

smc_rx_splice() hands RMB pages to a pipe and takes a socket reference
per entry so the smc_sock stays alive until the reader finishes. The
connection does not: a concurrent close runs smc_conn_free(), which
releases the receive buffer back to the link group pool.

smc_rx_pipe_buf_release() tests sk_state before taking the socket lock.
The state can change between the test and the lock, and
smc_rx_update_cons() then dereferences conn->rmb_desc and walks
conn->lgr, which smc_conn_free() has already released. On the
is_reg_err path smcr_buf_unuse() frees the descriptor outright, so
this is a use-after-free.

Take the socket lock first and test conn->freed instead.
smc_conn_free() sets that flag before releasing anything, and every
caller holds the socket lock. The two paths exclude each other: either
the pipe release runs first with everything valid, or it sees the flag
and skips the update.

Fixes: 9014db202cb7 ("smc: add support for splice()")
Cc: [email protected]
Reviewed-by: Mahanta Jambigi <[email protected]>
Signed-off-by: Hidayath Khan <[email protected]>
---
v2: Replace conn->rmb_desc = NULL with a conn->freed check under the
    socket lock. The NULL store exposed two paths without a NULL check:
    smc_cdc_msg_recv_action() in softirq and the sock_diag dump without
    the socket lock.

 net/smc/smc_rx.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/net/smc/smc_rx.c b/net/smc/smc_rx.c
index 5c9e4d8b57de..197fddc6271f 100644
--- a/net/smc/smc_rx.c
+++ b/net/smc/smc_rx.c
@@ -115,16 +115,15 @@ static void smc_rx_pipe_buf_release(struct pipe_inode_info *pipe,
 				    struct pipe_buffer *buf)
 {
 	struct smc_spd_priv *priv = (struct smc_spd_priv *)buf->private;
+	struct smc_connection *conn = &priv->smc->conn;
 	struct smc_sock *smc = priv->smc;
-	struct smc_connection *conn;
 	struct sock *sk = &smc->sk;
 
-	if (sk->sk_state == SMC_CLOSED ||
-	    sk->sk_state == SMC_PEERFINCLOSEWAIT ||
-	    sk->sk_state == SMC_APPFINCLOSEWAIT)
-		goto out;
-	conn = &smc->conn;
 	lock_sock(sk);
+	if (conn->freed) {
+		release_sock(sk);
+		goto out;
+	}
 	smc_rx_update_cons(smc, priv->len);
 	release_sock(sk);
 	if (atomic_sub_and_test(priv->len, &conn->splice_pending))
-- 
2.52.0