@@ -4,5 +4,5 @@ obj-$(CONFIG_ENIC) := enic.o
 enic-y := enic_main.o vnic_cq.o vnic_intr.o vnic_wq.o \
 	enic_res.o enic_dev.o enic_pp.o vnic_dev.o vnic_rq.o vnic_vic.o \
 	enic_ethtool.o enic_api.o enic_clsf.o enic_rq.o enic_wq.o \
-	enic_admin.o
+	enic_admin.o enic_mbox.o
 

@@ -297,6 +297,8 @@ struct enic {
 	 * left the resources freed.
 	 */
 	bool admin_chan_up;
+	/* set on send timeout; cleared on channel re-open */
+	bool mbox_send_disabled;
 	struct vnic_wq admin_wq;
 	struct vnic_rq admin_rq;
 	struct vnic_cq admin_cq[2];

@@ -309,6 +311,10 @@ struct enic {
 	unsigned int admin_msg_count;	/* current depth of admin_msg_list */
 	void (*admin_rq_handler)(struct enic *enic, void *buf,
 				 unsigned int len);
+
+	/* MBOX protocol state — mbox_lock serializes admin WQ sends */
+	struct mutex mbox_lock;
+	u64 mbox_msg_num;
 };
 
 static inline struct net_device *vnic_get_netdev(struct vnic_dev *vdev)

@@ -19,6 +19,7 @@
 #include "cq_enet_desc.h"
 #include "wq_enet_desc.h"
 #include "rq_enet_desc.h"
+#include "enic_mbox.h"
 
 /* Clean up any admin WQ buffers still held by hardware at close time.
  * Normally buffers are freed inline after send completion, but a timed-out

@@ -213,7 +214,26 @@ unsigned int enic_admin_rq_cq_service(struct enic *enic)
 			goto next_desc;
 		}
 
-		enic_admin_msg_enqueue(enic, buf->os_buf, bytes_written);
+		if (enic->admin_rq_handler) {
+			u16 sender_vlan;
+
+			/* Firmware sets the CQ VLAN field to identify the
+			 * sender: 0 = PF, 1-based = VF index.  Overwrite
+			 * the untrusted src_vnic_id in the MBOX header with
+			 * the hardware-verified value.
+			 */
+			sender_vlan = le16_to_cpu(rq_desc->vlan);
+			if (bytes_written >= sizeof(struct enic_mbox_hdr)) {
+				struct enic_mbox_hdr *hdr = buf->os_buf;
+
+				hdr->src_vnic_id = (sender_vlan == 0) ?
+					cpu_to_le16(ENIC_MBOX_DST_PF) :
+					cpu_to_le16(sender_vlan - 1);
+			}
+
+			enic_admin_msg_enqueue(enic, buf->os_buf,
+					       bytes_written);
+		}
 
 next_desc:
 		enic_admin_rq_buf_clean(rq, rq->to_clean);

@@ -456,8 +476,9 @@ static void enic_admin_init_resources(struct enic *enic)
 		     VNIC_CQ_MSG_DISABLE,
 		     intr_offset,
 		     0 /* cq_message_addr */);
+	/* coalescing_timer, coalescing_type, mask_on_assertion */
 	vnic_intr_init(&enic->admin_intr,
-		       0, 0, 1); /* coalescing_timer, coalescing_type, mask_on_assertion */
+		       0, 0, 1);
 }
 
 static void enic_admin_msg_drain(struct enic *enic)

@@ -522,6 +543,14 @@ int enic_admin_channel_open(struct enic *enic)
 
 	vnic_intr_unmask(&enic->admin_intr);
 
+	/* Only now that the admin WQ/RQ/CQ and interrupt are fully allocated,
+	 * programmed and enabled is it safe to allow MBOX sends.  Clearing this
+	 * earlier opened a window where a concurrent sender (e.g. link-notify
+	 * work scheduled by a post-reset link-up) could call enic_mbox_send_msg()
+	 * against a not-yet-allocated admin_wq and crash.
+	 */
+	WRITE_ONCE(enic->mbox_send_disabled, false);
+
 	netdev_dbg(enic->netdev,
 		   "admin channel open: intr=%u wq_avail=%u rq_avail=%u cq0_color=%u cq1_color=%u\n",
 		   enic->admin_intr_index,

@@ -563,6 +592,8 @@ void enic_admin_channel_close(struct enic *enic)
 	if (!enic->admin_chan_up)
 		return;
 
+	WRITE_ONCE(enic->mbox_send_disabled, true);
+
 	netdev_dbg(enic->netdev, "admin channel close\n");
 
 	vnic_intr_mask(&enic->admin_intr);

@@ -0,0 +1,170 @@
+// SPDX-License-Identifier: GPL-2.0-only
+// Copyright 2025 Cisco Systems, Inc.  All rights reserved.
+
+#include <linux/kernel.h>
+#include <linux/netdevice.h>
+#include <linux/dma-mapping.h>
+#include <linux/delay.h>
+
+#include "vnic_dev.h"
+#include "vnic_wq.h"
+#include "vnic_cq.h"
+#include "enic.h"
+#include "enic_admin.h"
+#include "enic_mbox.h"
+#include "wq_enet_desc.h"
+
+#define ENIC_MBOX_POLL_TIMEOUT_US	5000000
+#define ENIC_MBOX_POLL_INTERVAL_US	100
+
+static void enic_mbox_fill_hdr(struct enic *enic, struct enic_mbox_hdr *hdr,
+			       u8 msg_type, u16 dst_vnic_id, u16 msg_len)
+{
+	memset(hdr, 0, sizeof(*hdr));
+	hdr->dst_vnic_id = cpu_to_le16(dst_vnic_id);
+	hdr->msg_type = msg_type;
+	hdr->msg_len = cpu_to_le16(msg_len);
+	hdr->msg_num = cpu_to_le64(++enic->mbox_msg_num);
+}
+
+int enic_mbox_send_msg(struct enic *enic, u8 msg_type, u16 dst_vnic_id,
+		       void *payload, u16 payload_len)
+{
+	u16 total_len = sizeof(struct enic_mbox_hdr) + payload_len;
+	struct vnic_wq *wq = &enic->admin_wq;
+	struct wq_enet_desc *desc;
+	unsigned long timeout;
+	dma_addr_t dma_addr;
+	u16 vlan_tag;
+	void *buf;
+	int err;
+
+	/* Serialize MBOX sends. The admin channel is a low-frequency
+	 * control path; holding the mutex across the poll is acceptable.
+	 */
+	mutex_lock(&enic->mbox_lock);
+
+	if (!enic->has_admin_channel || READ_ONCE(enic->mbox_send_disabled)) {
+		err = -ENODEV;
+		goto unlock;
+	}
+
+	if (vnic_wq_desc_avail(wq) == 0) {
+		err = -ENOSPC;
+		goto unlock;
+	}
+
+	buf = kmalloc(total_len, GFP_KERNEL);
+	if (!buf) {
+		err = -ENOMEM;
+		goto unlock;
+	}
+
+	enic_mbox_fill_hdr(enic, buf, msg_type, dst_vnic_id, total_len);
+	if (payload_len) {
+		void *dst = buf + sizeof(struct enic_mbox_hdr);
+
+		memcpy(dst, payload, payload_len);
+	}
+
+	dma_addr = dma_map_single(&enic->pdev->dev, buf, total_len,
+				  DMA_TO_DEVICE);
+	if (dma_mapping_error(&enic->pdev->dev, dma_addr)) {
+		kfree(buf);
+		err = -ENOMEM;
+		goto unlock;
+	}
+
+	/* Firmware uses vlan field for routing: 0 = PF, 1-based = VF index */
+	if (dst_vnic_id == ENIC_MBOX_DST_PF)
+		vlan_tag = 0;
+	else
+		vlan_tag = dst_vnic_id + 1;
+
+	desc = vnic_wq_next_desc(wq);
+	wq_enet_desc_enc(desc, (u64)dma_addr | VNIC_PADDR_TARGET,
+			 total_len,
+			 0, 0, 0,       /* mss, hdr_len, offload_mode */
+			 1, 1,          /* eop, cq_entry */
+			 0,             /* fcoe_encap */
+			 1, vlan_tag,   /* vlan_tag_insert, vlan_tag */
+			 0);            /* loopback */
+	vnic_wq_post(wq, buf, dma_addr, total_len,
+		     1, 1,              /* sop, eop */
+		     1, 1,              /* desc_skip_cnt, cq_entry */
+		     0, 0);             /* compressed_send, wrid */
+	vnic_wq_doorbell(wq);
+
+	timeout = jiffies + usecs_to_jiffies(ENIC_MBOX_POLL_TIMEOUT_US);
+	err = -ETIMEDOUT;
+	while (time_before(jiffies, timeout)) {
+		if (enic_admin_wq_cq_service(enic)) {
+			err = 0;
+			break;
+		}
+		usleep_range(ENIC_MBOX_POLL_INTERVAL_US,
+			     ENIC_MBOX_POLL_INTERVAL_US + 50);
+	}
+	/* Final check in case completion arrived during the last sleep */
+	if (err && enic_admin_wq_cq_service(enic))
+		err = 0;
+
+	if (!err) {
+		wq->to_clean = wq->to_clean->next;
+		wq->ring.desc_avail++;
+		dma_unmap_single(&enic->pdev->dev, dma_addr, total_len,
+				 DMA_TO_DEVICE);
+		kfree(buf);
+	} else {
+		netdev_err(enic->netdev,
+			   "MBOX send timed out (type %u dst %u), disabling channel\n",
+			   msg_type, dst_vnic_id);
+		/*
+		 * The WQ descriptor is still live in hardware. Do not unmap
+		 * or free the buffer: the device may still DMA from dma_addr.
+		 * Mark the channel unusable so no further sends are attempted.
+		 */
+		WRITE_ONCE(enic->mbox_send_disabled, true);
+	}
+
+	netdev_dbg(enic->netdev,
+		   "MBOX send msg_type %u dst %u vlan %u err %d\n",
+		   msg_type, dst_vnic_id, vlan_tag, err);
+unlock:
+	mutex_unlock(&enic->mbox_lock);
+	return err;
+}
+
+static void enic_mbox_recv_handler(struct enic *enic, void *buf,
+				   unsigned int len)
+{
+	struct enic_mbox_hdr *hdr = buf;
+
+	if (len < sizeof(*hdr)) {
+		if (net_ratelimit())
+			netdev_warn(enic->netdev,
+				    "MBOX: truncated message (len %u < %zu)\n",
+				    len, sizeof(*hdr));
+		return;
+	}
+
+	if (hdr->msg_type >= ENIC_MBOX_MAX) {
+		if (net_ratelimit())
+			netdev_warn(enic->netdev,
+				    "MBOX: unknown msg type %u\n",
+				    hdr->msg_type);
+		return;
+	}
+
+	netdev_dbg(enic->netdev,
+		   "MBOX recv: type %u from vnic %u len %u\n",
+		   hdr->msg_type, le16_to_cpu(hdr->src_vnic_id),
+		   le16_to_cpu(hdr->msg_len));
+}
+
+void enic_mbox_init(struct enic *enic)
+{
+	enic->mbox_msg_num = 0;
+	mutex_init(&enic->mbox_lock);
+	enic->admin_rq_handler = enic_mbox_recv_handler;
+}

@@ -80,4 +80,12 @@ struct enic_mbox_pf_link_state_ack_msg {
 	struct enic_mbox_generic_reply ack;
 };
 
+#define ENIC_MBOX_DST_PF	0xFFFF
+
+struct enic;
+
+void enic_mbox_init(struct enic *enic);
+int enic_mbox_send_msg(struct enic *enic, u8 msg_type, u16 dst_vnic_id,
+		       void *payload, u16 payload_len);
+
 #endif /* _ENIC_MBOX_H_ */