net: mctp i2c: check length before marking flow active

[ Upstream commit 4ca07b9 ]

Currently, mctp_i2c_get_tx_flow_state() is called before the packet length
sanity check. This function marks a new flow as active in the MCTP core.

If the sanity check fails, mctp_i2c_xmit() returns early without calling
mctp_i2c_lock_nest(). This results in a mismatched locking state: the
flow is active, but the I2C bus lock was never acquired for it.

When the flow is later released, mctp_i2c_release_flow() will see the
active state and queue an unlock marker. The TX thread will then
decrement midev->i2c_lock_count from 0, causing it to underflow to -1.

This underflow permanently breaks the driver's locking logic, allowing
future transmissions to occur without holding the I2C bus lock, leading
to bus collisions and potential hardware hangs.

Move the mctp_i2c_get_tx_flow_state() call to after the length sanity
check to ensure we only transition the flow state if we are actually
going to proceed with the transmission and locking.

Fixes: f5b8abf ("mctp i2c: MCTP I2C binding driver")
Signed-off-by: William A. Kennington III <william@wkennington.com>
Acked-by: Jeremy Kerr <jk@codeconstruct.com.au>
Link: https://patch.msgid.link/20260423074741.201460-1-william@wkennington.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: wkennington

drivers/net/mctp/mctp-i2c.c

@@ -497,8 +497,6 @@ static void mctp_i2c_xmit(struct mctp_i2c_dev *midev, struct sk_buff *skb)
 	u8 *pecp;
 	int rc;
 
-	fs = mctp_i2c_get_tx_flow_state(midev, skb);
-
 	hdr = (void *)skb_mac_header(skb);
 	/* Sanity check that packet contents matches skb length,
 	 * and can't exceed MCTP_I2C_BUFSZ
@@ -510,6 +508,8 @@ static void mctp_i2c_xmit(struct mctp_i2c_dev *midev, struct sk_buff *skb)
 		return;
 	}
 
+	fs = mctp_i2c_get_tx_flow_state(midev, skb);
+
 	if (skb_tailroom(skb) >= 1) {
 		/* Linear case with space, we can just append the PEC */
 		skb_put(skb, 1);

net/sched/cls_flower.c

@@ -560,6 +560,7 @@ static int __fl_delete(struct tcf_proto *tp, struct cls_fl_filter *f,
 		       struct netlink_ext_ack *extack)
 {
 	struct cls_fl_head *head = fl_head_dereference(tp);
+	struct fl_flow_mask *mask;
 
 	*last = false;
 
@@ -576,11 +577,12 @@ static int __fl_delete(struct tcf_proto *tp, struct cls_fl_filter *f,
 	list_del_rcu(&f->list);
 	spin_unlock(&tp->lock);
 
-	*last = fl_mask_put(head, f->mask);
+	mask = f->mask;
 	if (!tc_skip_hw(f->flags))
 		fl_hw_destroy_filter(tp, f, rtnl_held, extack);
 	tcf_unbind_filter(tp, &f->res);
 	__fl_put(f);
+	*last = fl_mask_put(head, mask);
 
 	return 0;
 }