[PATCH 6.1 73/79] can: isotp: fix timer drain order, wakeup handling and tx_gen ordering

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

6.1-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Oliver Hartkopp <socketcan@hartkopp.net>

commit 050f010f920da17c1044a4f174766ad553e770b6 upstream.

This patch is a follow-up to commit cf070fe33bfb ("can: isotp: serialize
TX state transitions under so->rx_lock") which addresses following
sashiko-bot findings:

- isotp_sendmsg(): drain so->txfrtimer first so a stale callback can't
  re-arm echotimer after the claim

- isotp_release(): wake so->wait after forcing ISOTP_SHUTDOWN so a
  sleeping sendmsg() claim isn't stranded

- isotp_sendmsg(): have both wait_event_interruptible() calls in
  isotp_sendmsg() also wake on ISOTP_SHUTDOWN and do not return claim to
  IDLE to avoid corrupting a concurrent isotp_release() process.

- isotp_sendmsg(): handle potential claim of a new transfer when
  the wait_event_interruptible() call returns in CAN_ISOTP_WAIT_TX_DONE
  mode. Don't touch timers and states of the new transfer if a new thread
  incremented so->tx_gen before getting the lock at err_event_drop.

- isotp_sendmsg(): handle a stuck can_send() and omit timer and state
  changes if a new transfer was claimed. wait_tx_done() returns the error
  recorded in so->tx_result[], tagged with the caller's own generation.

- isotp_tx_timeout(): on a claimed timeout, record the ECOMM error for
  the timed-out transfer's own generation in so->tx_result[]; sk->sk_err
  is raised unconditionally, same as every other error path here.

- isotp_tx_gen_done()/isotp_tx_timeout(): always read tx.state (acquire)
  before tx_gen - the reverse order let a weakly ordered CPU pair a fresh
  tx.state with a stale tx_gen/tx_result slot.

- isotp_sendmsg(): wait_tx_done: drain sk_err via sock_error() once we
  have read the result from so->tx_result[], so an already-reported error
  doesn't stay latched for a later poll()/SO_ERROR.

Also align the remaining lock-free so->tx.state/rx.state/cfecho accesses
and use skb->hash as unique loopback echo frame indicator.

Fixes: cf070fe33bfb ("can: isotp: serialize TX state transitions under so->rx_lock")
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Link: https://patch.msgid.link/20260724181525.43556-1-socketcan@hartkopp.net
Cc: stable@kernel.org
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 net/can/isotp.c |  317 ++++++++++++++++++++++++++++++++++++++++----------------
 1 file changed, 230 insertions(+), 87 deletions(-)

--- a/net/can/isotp.c
+++ b/net/can/isotp.c
@@ -114,6 +114,15 @@ MODULE_ALIAS("can-proto-6");
 #define ISOTP_FC_TIMEOUT 1	/* 1 sec */
 #define ISOTP_ECHO_TIMEOUT 2	/* 2 secs */
 
+/* so->tx_result[so->tx_gen % ISOTP_TX_RESULT_SLOTS] holds the packed value
+ * (err << ISOTP_TX_RESULT_GEN_BITS | gen) for each tx generation slot, so it
+ * can be handled with a single READ_ONCE()/WRITE_ONCE() access.
+ */
+#define ISOTP_TX_RESULT_SLOTS 4
+#define ISOTP_TX_RESULT_GEN_BITS 24
+#define ISOTP_TX_RESULT_GEN_MASK ((1U << ISOTP_TX_RESULT_GEN_BITS) - 1)
+#define ISOTP_TX_RESULT_ERR_MASK 0xFF
+
 enum {
 	ISOTP_IDLE = 0,
 	ISOTP_WAIT_FIRST_FC,
@@ -150,7 +159,8 @@ struct isotp_sock {
 	u32 force_tx_stmin;
 	u32 force_rx_stmin;
 	u32 cfecho; /* consecutive frame echo tag */
-	u32 tx_gen; /* generation, bumped per new tx transfer */
+	u32 tx_gen; /* transfer generation, increased per new tx transfer */
+	u32 tx_result[ISOTP_TX_RESULT_SLOTS]; /* per-generation result slots */
 	struct tpcon rx, tx;
 	struct list_head notifier;
 	wait_queue_head_t wait;
@@ -161,6 +171,65 @@ static LIST_HEAD(isotp_notifier_list);
 static DEFINE_SPINLOCK(isotp_notifier_lock);
 static struct isotp_sock *isotp_busy_notifier;
 
+/* increase (24 bit) tx generation value */
+static u32 isotp_inc_tx_gen(u32 gen)
+{
+	return (gen + 1) & ISOTP_TX_RESULT_GEN_MASK;
+}
+
+/* store 8 bit error and 24 bit tx generation values in packed u32 element */
+static u32 isotp_pack_tx_result(u32 gen, int err)
+{
+	return gen | ((u32)err << ISOTP_TX_RESULT_GEN_BITS);
+}
+
+/* get the 24 bit tx generation value from the tx result */
+static u32 isotp_get_tx_gen(u32 gen_err)
+{
+	return gen_err & ISOTP_TX_RESULT_GEN_MASK;
+}
+
+/* get the 8 bit error value from the tx result */
+static u32 isotp_get_tx_err(u32 gen_err)
+{
+	return (gen_err >> ISOTP_TX_RESULT_GEN_BITS) & ISOTP_TX_RESULT_ERR_MASK;
+}
+
+/* store transfer result in per-generation%4 so->tx_result[] slot */
+static void isotp_set_tx_result(struct isotp_sock *so, u32 gen, int err)
+{
+	WRITE_ONCE(so->tx_result[gen % ISOTP_TX_RESULT_SLOTS],
+		   isotp_pack_tx_result(gen, err));
+}
+
+/* fetch the result recorded for 'gen', as a (negative) errno (0 for success) */
+static int isotp_get_tx_result(struct isotp_sock *so, u32 gen)
+{
+	u32 result = READ_ONCE(so->tx_result[gen % ISOTP_TX_RESULT_SLOTS]);
+
+	if (isotp_get_tx_gen(result) != gen) {
+		pr_notice_once("can-isotp: tx_result[] slot reused before read\n");
+
+		/* report failure rather than risk a false success */
+		return -ECOMM;
+	}
+
+	return -(isotp_get_tx_err(result));
+}
+
+/* true if done, shut down or superseded ('gen' is no longer the active
+ * transfer). Reads tx.state first (acquire) so tx_gen/tx_result reads
+ * below see at least what that state write published (common sequence).
+ */
+static bool isotp_tx_gen_done(struct isotp_sock *so, u32 gen)
+{
+	/* read tx.state first for the common sequence */
+	u32 state = smp_load_acquire(&so->tx.state);
+
+	return state == ISOTP_IDLE || state == ISOTP_SHUTDOWN ||
+	       READ_ONCE(so->tx_gen) != gen;
+}
+
 static inline struct isotp_sock *isotp_sk(const struct sock *sk)
 {
 	return (struct isotp_sock *)sk;
@@ -183,7 +252,7 @@ static enum hrtimer_restart isotp_rx_tim
 					     rxtimer);
 	struct sock *sk = &so->sk;
 
-	if (so->rx.state == ISOTP_WAIT_DATA) {
+	if (READ_ONCE(so->rx.state) == ISOTP_WAIT_DATA) {
 		/* we did not get new data frames in time */
 
 		/* report 'connection timed out' */
@@ -192,7 +261,7 @@ static enum hrtimer_restart isotp_rx_tim
 			sk_error_report(sk);
 
 		/* reset rx state */
-		so->rx.state = ISOTP_IDLE;
+		WRITE_ONCE(so->rx.state, ISOTP_IDLE);
 	}
 
 	return HRTIMER_NORESTART;
@@ -349,20 +418,19 @@ static void isotp_send_cframe(struct iso
 static int isotp_rcv_fc(struct isotp_sock *so, struct canfd_frame *cf, int ae)
 {
 	struct sock *sk = &so->sk;
+	int tx_err = EBADMSG; /* default for unknown FC status */
 
-	if (so->tx.state != ISOTP_WAIT_FC &&
-	    so->tx.state != ISOTP_WAIT_FIRST_FC)
+	if (READ_ONCE(so->tx.state) != ISOTP_WAIT_FC &&
+	    READ_ONCE(so->tx.state) != ISOTP_WAIT_FIRST_FC)
 		return 0;
 
 	hrtimer_cancel(&so->txtimer);
 
 	/* isotp_tx_timeout() may have given up on this job while
-	 * hrtimer_cancel() above waited for it to finish; so->rx_lock
-	 * (held by our caller isotp_rcv()) rules out a concurrent claim,
-	 * so a plain recheck is enough here.
+	 * hrtimer_cancel() above waited for it to finish => recheck
 	 */
-	if (so->tx.state != ISOTP_WAIT_FC &&
-	    so->tx.state != ISOTP_WAIT_FIRST_FC)
+	if (READ_ONCE(so->tx.state) != ISOTP_WAIT_FC &&
+	    READ_ONCE(so->tx.state) != ISOTP_WAIT_FIRST_FC)
 		return 1;
 
 	if ((cf->len < ae + FC_CONTENT_SZ) ||
@@ -373,13 +441,15 @@ static int isotp_rcv_fc(struct isotp_soc
 		if (!sock_flag(sk, SOCK_DEAD))
 			sk_error_report(sk);
 
-		so->tx.state = ISOTP_IDLE;
+		isotp_set_tx_result(so, so->tx_gen, EBADMSG);
+		/* set to IDLE after publishing tx_result */
+		smp_store_release(&so->tx.state, ISOTP_IDLE);
 		wake_up_interruptible(&so->wait);
 		return 1;
 	}
 
 	/* get communication parameters only from the first FC frame */
-	if (so->tx.state == ISOTP_WAIT_FIRST_FC) {
+	if (READ_ONCE(so->tx.state) == ISOTP_WAIT_FIRST_FC) {
 		so->txfc.bs = cf->data[ae + 1];
 		so->txfc.stmin = cf->data[ae + 2];
 
@@ -402,13 +472,13 @@ static int isotp_rcv_fc(struct isotp_soc
 			so->tx_gap = ktime_add_ns(so->tx_gap,
 						  (so->txfc.stmin - 0xF0)
 						  * 100000);
-		so->tx.state = ISOTP_WAIT_FC;
+		WRITE_ONCE(so->tx.state, ISOTP_WAIT_FC);
 	}
 
 	switch (cf->data[ae] & 0x0F) {
 	case ISOTP_FC_CTS:
 		so->tx.bs = 0;
-		so->tx.state = ISOTP_SENDING;
+		WRITE_ONCE(so->tx.state, ISOTP_SENDING);
 		/* send CF frame and enable echo timeout handling */
 		hrtimer_start(&so->echotimer, ktime_set(ISOTP_ECHO_TIMEOUT, 0),
 			      HRTIMER_MODE_REL_SOFT);
@@ -423,14 +493,19 @@ static int isotp_rcv_fc(struct isotp_soc
 
 	case ISOTP_FC_OVFLW:
 		/* overflow on receiver side - report 'message too long' */
-		sk->sk_err = EMSGSIZE;
-		if (!sock_flag(sk, SOCK_DEAD))
-			sk_error_report(sk);
+		tx_err = EMSGSIZE;
 		fallthrough;
 
 	default:
-		/* stop this tx job */
-		so->tx.state = ISOTP_IDLE;
+		/* reserved/unknown flow status (tx_err defaults to EBADMSG) */
+
+		sk->sk_err = tx_err;
+		if (!sock_flag(sk, SOCK_DEAD))
+			sk_error_report(sk);
+
+		isotp_set_tx_result(so, so->tx_gen, tx_err);
+		/* set to IDLE after publishing tx_result */
+		smp_store_release(&so->tx.state, ISOTP_IDLE);
 		wake_up_interruptible(&so->wait);
 	}
 	return 0;
@@ -443,7 +518,7 @@ static int isotp_rcv_sf(struct sock *sk,
 	struct sk_buff *nskb;
 
 	hrtimer_cancel(&so->rxtimer);
-	so->rx.state = ISOTP_IDLE;
+	WRITE_ONCE(so->rx.state, ISOTP_IDLE);
 
 	if (!len || len > cf->len - pcilen)
 		return 1;
@@ -477,7 +552,7 @@ static int isotp_rcv_ff(struct sock *sk,
 	int ff_pci_sz;
 
 	hrtimer_cancel(&so->rxtimer);
-	so->rx.state = ISOTP_IDLE;
+	WRITE_ONCE(so->rx.state, ISOTP_IDLE);
 
 	/* get the used sender LL_DL from the (first) CAN frame data length */
 	so->rx.ll_dl = padlen(cf->len);
@@ -521,7 +596,7 @@ static int isotp_rcv_ff(struct sock *sk,
 
 	/* initial setup for this pdu reception */
 	so->rx.sn = 1;
-	so->rx.state = ISOTP_WAIT_DATA;
+	WRITE_ONCE(so->rx.state, ISOTP_WAIT_DATA);
 
 	/* no creation of flow control frames */
 	if (so->opt.flags & CAN_ISOTP_LISTEN_MODE)
@@ -539,7 +614,7 @@ static int isotp_rcv_cf(struct sock *sk,
 	struct sk_buff *nskb;
 	int i;
 
-	if (so->rx.state != ISOTP_WAIT_DATA)
+	if (READ_ONCE(so->rx.state) != ISOTP_WAIT_DATA)
 		return 0;
 
 	/* drop if timestamp gap is less than force_rx_stmin nano secs */
@@ -554,11 +629,9 @@ static int isotp_rcv_cf(struct sock *sk,
 	hrtimer_cancel(&so->rxtimer);
 
 	/* isotp_rx_timer_handler() may have raced us for so->rx.state
-	 * while hrtimer_cancel() above waited for it to finish, already
-	 * reporting ETIMEDOUT and resetting the reception; don't process
-	 * this CF into a reassembly that has already been given up on.
+	 * while hrtimer_cancel() above waited for it to finish => recheck
 	 */
-	if (so->rx.state != ISOTP_WAIT_DATA)
+	if (READ_ONCE(so->rx.state) != ISOTP_WAIT_DATA)
 		return 1;
 
 	/* CFs are never longer than the FF */
@@ -579,7 +652,7 @@ static int isotp_rcv_cf(struct sock *sk,
 			sk_error_report(sk);
 
 		/* reset rx state */
-		so->rx.state = ISOTP_IDLE;
+		WRITE_ONCE(so->rx.state, ISOTP_IDLE);
 		return 1;
 	}
 	so->rx.sn++;
@@ -593,7 +666,7 @@ static int isotp_rcv_cf(struct sock *sk,
 
 	if (so->rx.idx >= so->rx.len) {
 		/* we are done */
-		so->rx.state = ISOTP_IDLE;
+		WRITE_ONCE(so->rx.state, ISOTP_IDLE);
 
 		if ((so->opt.flags & ISOTP_CHECK_PADDING) &&
 		    check_pad(so, cf, i + 1, so->opt.rxpad_content)) {
@@ -664,8 +737,10 @@ static void isotp_rcv(struct sk_buff *sk
 
 	if (so->opt.flags & CAN_ISOTP_HALF_DUPLEX) {
 		/* check rx/tx path half duplex expectations */
-		if ((so->tx.state != ISOTP_IDLE && n_pci_type != N_PCI_FC) ||
-		    (so->rx.state != ISOTP_IDLE && n_pci_type == N_PCI_FC))
+		if ((READ_ONCE(so->tx.state) != ISOTP_IDLE &&
+		     n_pci_type != N_PCI_FC) ||
+		    (READ_ONCE(so->rx.state) != ISOTP_IDLE &&
+		     n_pci_type == N_PCI_FC))
 			goto out_unlock;
 	}
 
@@ -759,6 +834,7 @@ static void isotp_send_cframe(struct iso
 	struct canfd_frame *cf;
 	int can_send_ret;
 	int ae = (so->opt.flags & CAN_ISOTP_EXTEND_ADDR) ? 1 : 0;
+	u32 old_cfecho;
 
 	dev = dev_get_by_index(sock_net(sk), so->ifindex);
 	if (!dev)
@@ -773,6 +849,9 @@ static void isotp_send_cframe(struct iso
 	can_skb_reserve(skb);
 	can_skb_prv(skb)->ifindex = dev->ifindex;
 
+	/* set uid in tx skb to identify CF echo frames */
+	can_set_skb_uid(skb);
+
 	cf = (struct canfd_frame *)skb->data;
 	skb_put_zero(skb, so->ll.mtu);
 
@@ -789,12 +868,15 @@ static void isotp_send_cframe(struct iso
 	skb->dev = dev;
 	can_skb_set_owner(skb, sk);
 
-	/* cfecho should have been zero'ed by init/isotp_rcv_echo() */
-	if (so->cfecho)
-		pr_notice_once("can-isotp: cfecho is %08X != 0\n", so->cfecho);
+	/* zero'ed by init/isotp_rcv_echo(); reached lock-free via
+	 * isotp_txfr_timer_handler() too, so use READ_ONCE()/WRITE_ONCE()
+	 */
+	old_cfecho = READ_ONCE(so->cfecho);
+	if (old_cfecho)
+		pr_notice_once("can-isotp: cfecho is %08X != 0\n", old_cfecho);
 
 	/* set consecutive frame echo tag */
-	so->cfecho = *(u32 *)cf->data;
+	WRITE_ONCE(so->cfecho, skb->hash);
 
 	/* send frame with local echo enabled */
 	can_send_ret = can_send(skb, 1);
@@ -846,7 +928,6 @@ static void isotp_rcv_echo(struct sk_buf
 {
 	struct sock *sk = (struct sock *)data;
 	struct isotp_sock *so = isotp_sk(sk);
-	struct canfd_frame *cf = (struct canfd_frame *)skb->data;
 
 	/* only handle my own local echo CF/SF skb's (no FF!) */
 	if (skb->sk != sk)
@@ -858,32 +939,35 @@ static void isotp_rcv_echo(struct sk_buf
 	spin_lock(&so->rx_lock);
 
 	/* so->cfecho may since belong to a new transfer; recheck under lock */
-	if (so->cfecho != *(u32 *)cf->data)
+	if (READ_ONCE(so->cfecho) != skb->hash)
 		goto out_unlock;
 
 	/* cancel local echo timeout */
 	hrtimer_cancel(&so->echotimer);
 
 	/* local echo skb with consecutive frame has been consumed */
-	so->cfecho = 0;
+	WRITE_ONCE(so->cfecho, 0);
 
 	/* claiming a transfer also takes so->rx_lock, so a plain recheck
 	 * is enough: so->tx.state can't have flipped to ISOTP_SENDING for
 	 * a new claim while we're still in here
 	 */
-	if (so->tx.state != ISOTP_SENDING)
+	if (READ_ONCE(so->tx.state) != ISOTP_SENDING)
 		goto out_unlock;
 
 	if (so->tx.idx >= so->tx.len) {
 		/* we are done */
-		so->tx.state = ISOTP_IDLE;
+
+		isotp_set_tx_result(so, so->tx_gen, 0);
+		/* set to IDLE after publishing tx_result */
+		smp_store_release(&so->tx.state, ISOTP_IDLE);
 		wake_up_interruptible(&so->wait);
 		goto out_unlock;
 	}
 
 	if (so->txfc.bs && so->tx.bs >= so->txfc.bs) {
 		/* stop and wait for FC with timeout */
-		so->tx.state = ISOTP_WAIT_FC;
+		WRITE_ONCE(so->tx.state, ISOTP_WAIT_FC);
 		hrtimer_start(&so->txtimer, ktime_set(ISOTP_FC_TIMEOUT, 0),
 			      HRTIMER_MODE_REL_SOFT);
 		goto out_unlock;
@@ -905,16 +989,20 @@ out_unlock:
 	spin_unlock(&so->rx_lock);
 }
 
-/* shared by so->txtimer's and so->echotimer's callbacks. Both timers get
- * cancelled under so->rx_lock elsewhere, so this must stay lock-free to
- * avoid deadlocking with that; uses so->tx_gen instead to avoid tainting
- * a new transfer with an error from the one that just timed out.
+/* isotp_tx_timeout: we did not get any flow control or echo frame in time
+ *
+ * Shared by so->txtimer's and so->echotimer's callbacks. Both timers get
+ * cancelled under so->rx_lock elsewhere, so this must stay lock-free.
+ *
+ * tx.state is acquired before tx_gen. Common sequence in isotp_tx_gen_done().
+ * cmpxchg() only orders itself, not the two preceding loads.
  */
 static enum hrtimer_restart isotp_tx_timeout(struct isotp_sock *so)
 {
 	struct sock *sk = &so->sk;
+	/* read tx.state first for the common sequence */
+	u32 old_state = smp_load_acquire(&so->tx.state);
 	u32 gen = READ_ONCE(so->tx_gen);
-	u32 old_state = READ_ONCE(so->tx.state);
 
 	/* don't handle timeouts in IDLE or SHUTDOWN state */
 	if (old_state == ISOTP_IDLE || old_state == ISOTP_SHUTDOWN)
@@ -924,14 +1012,14 @@ static enum hrtimer_restart isotp_tx_tim
 	if (cmpxchg(&so->tx.state, old_state, ISOTP_IDLE) != old_state)
 		return HRTIMER_NORESTART;
 
-	/* we did not get any flow control or echo frame in time */
+	/* detected timeout: report 'communication error on send' */
 
-	if (READ_ONCE(so->tx_gen) == gen) {
-		/* report 'communication error on send' */
-		sk->sk_err = ECOMM;
-		if (!sock_flag(sk, SOCK_DEAD))
-			sk_error_report(sk);
-	}
+	/* a stale read of this slot by a waiter still falls back to ECOMM */
+	isotp_set_tx_result(so, gen, ECOMM);
+
+	sk->sk_err = ECOMM;
+	if (!sock_flag(sk, SOCK_DEAD))
+		sk_error_report(sk);
 
 	wake_up_interruptible(&so->wait);
 
@@ -966,7 +1054,7 @@ static enum hrtimer_restart isotp_txfr_t
 		      HRTIMER_MODE_REL_SOFT);
 
 	/* cfecho should be consumed by isotp_rcv_echo() here */
-	if (so->tx.state == ISOTP_SENDING && !so->cfecho)
+	if (READ_ONCE(so->tx.state) == ISOTP_SENDING && !READ_ONCE(so->cfecho))
 		isotp_send_cframe(so);
 
 	return HRTIMER_NORESTART;
@@ -984,10 +1072,12 @@ static int isotp_sendmsg(struct socket *
 	s64 hrtimer_sec = ISOTP_ECHO_TIMEOUT;
 	struct hrtimer *tx_hrt = &so->echotimer;
 	u32 new_state = ISOTP_SENDING;
+	u32 my_gen;
+	u32 old_cfecho;
 	int off;
 	int err;
 
-	if (!so->bound || so->tx.state == ISOTP_SHUTDOWN)
+	if (!so->bound || READ_ONCE(so->tx.state) == ISOTP_SHUTDOWN)
 		return -EADDRNOTAVAIL;
 
 	/* claim the socket under so->rx_lock: this serializes the claim
@@ -1004,29 +1094,33 @@ static int isotp_sendmsg(struct socket *
 		if (msg->msg_flags & MSG_DONTWAIT)
 			return -EAGAIN;
 
-		if (so->tx.state == ISOTP_SHUTDOWN)
+		if (READ_ONCE(so->tx.state) == ISOTP_SHUTDOWN)
 			return -EADDRNOTAVAIL;
 
 		/* wait for complete transmission of current pdu */
 		err = wait_event_interruptible(so->wait,
-					       so->tx.state == ISOTP_IDLE);
+					       READ_ONCE(so->tx.state) == ISOTP_IDLE ||
+					       READ_ONCE(so->tx.state) == ISOTP_SHUTDOWN);
 		if (err)
 			return err;
 	}
 
-	/* new transfer: bump so->tx_gen and drain the old one's timers,
-	 * still under the so->rx_lock we just claimed the socket with
-	 */
-	WRITE_ONCE(so->tx.state, ISOTP_SENDING);
-	WRITE_ONCE(so->tx_gen, READ_ONCE(so->tx_gen) + 1);
+	/* txfrtimer's callback re-arms echotimer lock-free: drain it first */
+	hrtimer_cancel(&so->txfrtimer);
 	hrtimer_cancel(&so->txtimer);
 	hrtimer_cancel(&so->echotimer);
-	hrtimer_cancel(&so->txfrtimer);
-	so->cfecho = 0;
+
+	/* new transfer: increment so->tx_gen and set tx.state after barrier */
+	my_gen = isotp_inc_tx_gen(READ_ONCE(so->tx_gen));
+	isotp_set_tx_result(so, my_gen, ECOMM); /* prevent stale slot matching */
+	WRITE_ONCE(so->tx_gen, my_gen);
+	smp_wmb(); /* see smp_load_acquire() in isotp_tx_[timeout|gen_done] */
+	WRITE_ONCE(so->tx.state, ISOTP_SENDING);
+	WRITE_ONCE(so->cfecho, 0);
 	spin_unlock_bh(&so->rx_lock);
 
 	/* so->bound is only checked once above - a wakeup may have
-	 * unbound/rebound the socket meanwhile, so re-validate it
+	 * unbound/rebound the socket meanwhile => recheck
 	 */
 	if (!so->bound) {
 		err = -EADDRNOTAVAIL;
@@ -1068,6 +1162,9 @@ static int isotp_sendmsg(struct socket *
 	can_skb_reserve(skb);
 	can_skb_prv(skb)->ifindex = dev->ifindex;
 
+	/* set uid in tx skb to identify CF echo frames */
+	can_set_skb_uid(skb);
+
 	so->tx.len = size;
 	so->tx.idx = 0;
 
@@ -1075,8 +1172,9 @@ static int isotp_sendmsg(struct socket *
 	skb_put_zero(skb, so->ll.mtu);
 
 	/* cfecho should have been zero'ed by init / former isotp_rcv_echo() */
-	if (so->cfecho)
-		pr_notice_once("can-isotp: uninit cfecho %08X\n", so->cfecho);
+	old_cfecho = READ_ONCE(so->cfecho);
+	if (old_cfecho)
+		pr_notice_once("can-isotp: uninit cfecho %08X\n", old_cfecho);
 
 	/* check for single frame transmission depending on TX_DL */
 	if (size <= so->tx.ll_dl - SF_PCI_SZ4 - ae - off) {
@@ -1104,7 +1202,7 @@ static int isotp_sendmsg(struct socket *
 			cf->data[ae] |= size;
 
 		/* set CF echo tag for isotp_rcv_echo() (SF-mode) */
-		so->cfecho = *(u32 *)cf->data;
+		WRITE_ONCE(so->cfecho, skb->hash);
 	} else {
 		/* send first frame */
 
@@ -1121,7 +1219,7 @@ static int isotp_sendmsg(struct socket *
 			so->txfc.bs = 0;
 
 			/* set CF echo tag for isotp_rcv_echo() (CF-mode) */
-			so->cfecho = *(u32 *)cf->data;
+			WRITE_ONCE(so->cfecho, skb->hash);
 		} else {
 			/* standard flow control check */
 			new_state = ISOTP_WAIT_FIRST_FC;
@@ -1131,12 +1229,12 @@ static int isotp_sendmsg(struct socket *
 			tx_hrt = &so->txtimer;
 
 			/* no CF echo tag for isotp_rcv_echo() (FF-mode) */
-			so->cfecho = 0;
+			WRITE_ONCE(so->cfecho, 0);
 		}
 	}
 
 	spin_lock_bh(&so->rx_lock);
-	if (so->tx.state == ISOTP_SHUTDOWN) {
+	if (READ_ONCE(so->tx.state) == ISOTP_SHUTDOWN) {
 		/* isotp_release() has since taken over and already drained
 		 * our timers - don't send into a socket that's going away
 		 */
@@ -1147,7 +1245,7 @@ static int isotp_sendmsg(struct socket *
 		return -EADDRNOTAVAIL;
 	}
 	/* WAIT_FIRST_FC for standard FF, else stays ISOTP_SENDING */
-	so->tx.state = new_state;
+	WRITE_ONCE(so->tx.state, new_state);
 	hrtimer_start(tx_hrt, ktime_set(hrtimer_sec, 0),
 		      HRTIMER_MODE_REL_SOFT);
 	spin_unlock_bh(&so->rx_lock);
@@ -1164,20 +1262,49 @@ static int isotp_sendmsg(struct socket *
 			       __func__, ERR_PTR(err));
 
 		spin_lock_bh(&so->rx_lock);
+
+		/* new transfer already claimed by a concurrent completion,
+		 * timeout or sendmsg() while we were stuck in can_send()?
+		 */
+		if (READ_ONCE(so->tx_gen) != my_gen) {
+			/* don't touch timers and state of the new transfer */
+			spin_unlock_bh(&so->rx_lock);
+			return err;
+		}
+
 		/* no transmission -> no timeout monitoring */
 		hrtimer_cancel(tx_hrt);
 		goto err_out_drop_locked;
 	}
 
 	if (wait_tx_done) {
-		/* wait for complete transmission of current pdu */
-		err = wait_event_interruptible(so->wait, so->tx.state == ISOTP_IDLE);
+		/* wake up for:
+		 * - concurrent sendmsg() claiming a new transfer
+		 * - complete transmission of current PDU
+		 * - shutdown state change in isotp_release()
+		 * isotp_tx_gen_done() uses common tx.state/tx_gen read sequence
+		 */
+		err = wait_event_interruptible(so->wait,
+					       isotp_tx_gen_done(so, my_gen));
 		if (err)
 			goto err_event_drop;
 
-		err = sock_error(sk);
-		if (err)
-			return err;
+		/* still our claim, but isotp_release() force-shut it down */
+		if (smp_load_acquire(&so->tx.state) == ISOTP_SHUTDOWN &&
+		    READ_ONCE(so->tx_gen) == my_gen) {
+			err = -EADDRNOTAVAIL;
+			goto err_event_drop;
+		}
+
+		/* own completion, or tx_gen moved on - either way this is
+		 * what isotp_get_tx_result() recorded for my_gen
+		 */
+		err = isotp_get_tx_result(so, my_gen);
+
+		/* drain to avoid stale error for a later poll()/SO_ERROR */
+		sock_error(sk);
+
+		return err ? err : size;
 	}
 
 	return size;
@@ -1187,15 +1314,26 @@ err_out_drop:
 	spin_lock_bh(&so->rx_lock);
 	goto err_out_drop_locked;
 err_event_drop:
-	/* interrupted waiting on our own transfer - drain its timers */
+	/* interrupted or shut down while waiting on our own transfer */
 	spin_lock_bh(&so->rx_lock);
+
+	/* new transfer already started by concurrent sendmsg()? */
+	if (READ_ONCE(so->tx_gen) != my_gen) {
+		/* don't touch timers and states of the new transfer */
+		spin_unlock_bh(&so->rx_lock);
+		return err;
+	}
+
 	hrtimer_cancel(&so->txfrtimer);
 	hrtimer_cancel(&so->txtimer);
 	hrtimer_cancel(&so->echotimer);
 err_out_drop_locked:
 	/* release the claim; so->rx_lock still held from above */
-	so->cfecho = 0;
-	so->tx.state = ISOTP_IDLE;
+	WRITE_ONCE(so->cfecho, 0);
+
+	/* only claim to IDLE if isotp_release() has not taken over */
+	if (READ_ONCE(so->tx.state) != ISOTP_SHUTDOWN)
+		WRITE_ONCE(so->tx.state, ISOTP_IDLE);
 	spin_unlock_bh(&so->rx_lock);
 	wake_up_interruptible(&so->wait);
 
@@ -1261,8 +1399,9 @@ static int isotp_release(struct socket *
 	/* best-effort: wait for a running pdu to finish, but don't block on
 	 * it forever - give up after the first signal
 	 */
-	while (so->tx.state != ISOTP_IDLE &&
-	       wait_event_interruptible(so->wait, so->tx.state == ISOTP_IDLE) == 0)
+	while (READ_ONCE(so->tx.state) != ISOTP_IDLE &&
+	       wait_event_interruptible(so->wait,
+					READ_ONCE(so->tx.state) == ISOTP_IDLE) == 0)
 		;
 
 	/* claim the socket under so->rx_lock like sendmsg() does, so its
@@ -1270,9 +1409,12 @@ static int isotp_release(struct socket *
 	 * unconditionally, even when a signal cut the wait above short
 	 */
 	spin_lock_bh(&so->rx_lock);
-	so->tx.state = ISOTP_SHUTDOWN;
+	WRITE_ONCE(so->tx.state, ISOTP_SHUTDOWN);
 	spin_unlock_bh(&so->rx_lock);
-	so->rx.state = ISOTP_IDLE;
+	WRITE_ONCE(so->rx.state, ISOTP_IDLE);
+
+	/* forced SHUTDOWN may have skipped IDLE (gave up on a signal) */
+	wake_up_interruptible(&so->wait);
 
 	spin_lock(&isotp_notifier_lock);
 	while (isotp_busy_notifier == so) {
@@ -1387,7 +1529,8 @@ static int isotp_bind(struct socket *soc
 	 * with so->bound in the same lock_sock() section above, so there is
 	 * no window in which a concurrent isotp_notify() could be missed.
 	 */
-	if (so->tx.state != ISOTP_IDLE || so->rx.state != ISOTP_IDLE) {
+	if (READ_ONCE(so->tx.state) != ISOTP_IDLE ||
+	    READ_ONCE(so->rx.state) != ISOTP_IDLE) {
 		err = -EAGAIN;
 		goto out;
 	}
@@ -1421,7 +1564,7 @@ static int isotp_bind(struct socket *soc
 				isotp_rcv, sk, "isotp", sk);
 
 	/* no consecutive frame echo skb in flight */
-	so->cfecho = 0;
+	WRITE_ONCE(so->cfecho, 0);
 
 	/* register for echo skb's */
 	can_rx_register(net, dev, tx_id, SINGLE_MASK(tx_id),
@@ -1764,7 +1907,7 @@ static __poll_t isotp_poll(struct file *
 	poll_wait(file, &so->wait, wait);
 
 	/* Check for false positives due to TX state */
-	if ((mask & EPOLLWRNORM) && (so->tx.state != ISOTP_IDLE))
+	if ((mask & EPOLLWRNORM) && (READ_ONCE(so->tx.state) != ISOTP_IDLE))
 		mask &= ~(EPOLLOUT | EPOLLWRNORM);
 
 	return mask;