[PATCH net v3 1/2] kcm: reject attaching sockets with a ULP, and TLS on KCM sockets

[Rishikesh Jethwani <rjethwani@purestorage.com>]

KCM and kTLS RX do not work together, in either order.

KCM's psock_data_ready() is the lower socket's data_ready callback. It
can run in softirq context under read_lock_bh(&sk->sk_callback_lock),
and calls strp_data_ready(), which invokes sock->ops->read_sock()
inline. With kTLS RX, that is tls_sw_read_sock(), which can sleep: it
allocates with sk->sk_allocation, waits for async crypto, flushes the
socket backlog, and may wait in tls_rx_reader_acquire().

Found by code review. On an unmodified kernel with
CONFIG_DEBUG_ATOMIC_SLEEP, sending TLS records over loopback to a socket
with software kTLS RX attached to a KCM mux reports:

  BUG: sleeping function called from invalid context at include/linux/sched/mm.h:321
  in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 189, name: ksoftirqd/34
  Call Trace:
   __might_resched
   kmem_cache_alloc_node_noprof
   __alloc_skb
   alloc_skb_with_frags
   tls_rx_one_record
   tls_sw_read_sock
   strp_read_sock
   strp_data_ready
   psock_data_ready [kcm]

Before commit 662fbcec32f4 ("net/tls: implement ->read_sock()"), TLS
inherited tcp_read_sock(), so KCM parsed the ciphertext in the TCP
receive queue as messages. HW TLS RX uses the same proto_ops as SW
RX, so it is affected the same way.

The two also stack socket callbacks and neither restores the other's:
with TLS RX configured after the attach, kcm_unattach() overwrites
tls_data_ready(), and releasing TLS RX later reinstalls
psock_data_ready() on a socket no longer attached to KCM. On TX,
kcm_unattach() restores its saved sk_write_space, dropping
tls_write_space().

There is no known user of this combination, so reject it, as was done
for sockmap in commit 460e6486617c ("tls: reject the combination of TLS
and sockmap"):

 - kcm_attach() refuses sockets that already have any ULP, not only
   TLS, as sk_psock_init() does for sockmap, with -EOPNOTSUPP like its
   other refusals;
 - tls_init() refuses sockets attached to KCM with -EINVAL, like
   __tcp_set_ulp() and the sockmap check, tracked by a new SOCK_KCM
   flag set on attach and cleared on unattach.

sk_user_data cannot serve as the marker: sunrpc and nvme-tcp set it on
sockets that tlshd later upgrades to TLS, so it is already set when
tls_init() runs. Unlike sockmap, both sides run under the lower
socket's lock (tcp_set_ulp() asserts this), so the check is made at ULP
install rather than at key configuration.

This is a user-visible change that will also reach stable kernels:
attaching a socket with a ULP now fails with -EOPNOTSUPP, and installing
TLS on an attached socket fails with -EINVAL. Every refused setup was
already broken as described above, except one where only kTLS TX is
configured. Such a socket still passes data, because read_sock() stays
tcp_read_sock() until kTLS RX is configured, and KCM transmits with
sock_sendmsg(), so records reach tls_sw_sendmsg(). It is still wrong: if
TLS was armed before the attach, psock_write_space() shadows
tls_write_space() and does not chain to it, so a record deferred on a
full send buffer waits for the next send; if TLS was armed after the
attach, the chain works until kcm_unattach() drops tls_write_space(), as
above. The direction is not known at ULP install, so refusing it keeps
one rule for both orders and matches sk_psock_init().

Tested with CONFIG_DEBUG_ATOMIC_SLEEP: both orders are now rejected,
plain KCM still delivers messages, and the TLS ULP can be installed
after SIOCKCMUNATTACH.

Fixes: 662fbcec32f4 ("net/tls: implement ->read_sock()")
Cc: stable@vger.kernel.org
Suggested-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Link: https://lore.kernel.org/netdev/3108e934-ec00-458b-8720-450554b35b55@linux.dev/
Signed-off-by: Rishikesh Jethwani <rjethwani@purestorage.com>
---
 include/net/sock.h |  1 +
 net/kcm/kcmsock.c  | 12 ++++++++++++
 net/tls/tls_main.c |  6 ++++++
 3 files changed, 19 insertions(+)

diff --git a/include/net/sock.h b/include/net/sock.h
index 173b37b2b8bb..e579a4a96c8e 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -1024,6 +1024,7 @@ enum sock_flags {
 	SOCK_RCVMARK, /* Receive SO_MARK  ancillary data with packet */
 	SOCK_RCVPRIORITY, /* Receive SO_PRIORITY ancillary data with packet */
 	SOCK_TIMESTAMPING_ANY, /* Copy of sk_tsflags & TSFLAGS_ANY */
+	SOCK_KCM, /* Attached to a KCM mux as a lower socket */
 };
 
 #define SK_FLAGS_TIMESTAMP ((1UL << SOCK_TIMESTAMP) | (1UL << SOCK_TIMESTAMPING_RX_SOFTWARE))
diff --git a/net/kcm/kcmsock.c b/net/kcm/kcmsock.c
index a7f94d42036f..546d5b563382 100644
--- a/net/kcm/kcmsock.c
+++ b/net/kcm/kcmsock.c
@@ -27,6 +27,7 @@
 #include <linux/sched/signal.h>
 #include <linux/uio.h>
 
+#include <net/inet_connection_sock.h>
 #include <net/kcm.h>
 #include <net/netns/generic.h>
 #include <net/sock.h>
@@ -1272,6 +1273,15 @@ static int kcm_attach(struct socket *sock, struct socket *csock,
 		goto out;
 	}
 
+	/* KCM and ULPs are mutually exclusive. A ULP such as kTLS replaces
+	 * read_sock() and stacks its own socket callbacks with ours. tls_init()
+	 * rejects sockets already attached to KCM, see SOCK_KCM.
+	 */
+	if (inet_csk_has_ulp(csk)) {
+		err = -EOPNOTSUPP;
+		goto out;
+	}
+
 	psock = kmem_cache_zalloc(kcm_psockp, GFP_KERNEL);
 	if (!psock) {
 		err = -ENOMEM;
@@ -1305,6 +1315,7 @@ static int kcm_attach(struct socket *sock, struct socket *csock,
 	psock->save_write_space = csk->sk_write_space;
 	psock->save_state_change = csk->sk_state_change;
 	csk->sk_user_data = psock;
+	sock_set_flag(csk, SOCK_KCM);
 	WRITE_ONCE(csk->sk_data_ready, psock_data_ready);
 	WRITE_ONCE(csk->sk_write_space, psock_write_space);
 	csk->sk_state_change = psock_state_change;
@@ -1382,6 +1393,7 @@ static void kcm_unattach(struct kcm_psock *psock)
 	 */
 	write_lock_bh(&csk->sk_callback_lock);
 	csk->sk_user_data = NULL;
+	sock_reset_flag(csk, SOCK_KCM);
 	WRITE_ONCE(csk->sk_data_ready, psock->save_data_ready);
 	WRITE_ONCE(csk->sk_write_space, psock->save_write_space);
 	csk->sk_state_change = psock->save_state_change;
diff --git a/net/tls/tls_main.c b/net/tls/tls_main.c
index b8d5a843f761..e45062841950 100644
--- a/net/tls/tls_main.c
+++ b/net/tls/tls_main.c
@@ -1067,6 +1067,12 @@ static int tls_init(struct sock *sk)
 	if (sk->sk_state != TCP_ESTABLISHED)
 		return -ENOTCONN;
 
+	/* TLS and KCM are mutually exclusive. kcm_attach() rejects sockets
+	 * with a ULP; both run under the socket lock, so this cannot race.
+	 */
+	if (sock_flag(sk, SOCK_KCM))
+		return -EINVAL;
+
 	/* allocate tls context */
 	write_lock_bh(&sk->sk_callback_lock);
 	ctx = tls_ctx_create(sk);
-- 
2.50.1