diff --git a/drivers/platform/x86/amd/hsmp/acpi.c b/drivers/platform/x86/amd/hsmp/acpi.c
index 1fb09251d826..72f68cef1297 100644
--- a/drivers/platform/x86/amd/hsmp/acpi.c
+++ b/drivers/platform/x86/amd/hsmp/acpi.c
@@ -224,7 +224,6 @@ static int hsmp_parse_acpi_table(struct device *dev, u16 sock_ind)
 	int ret;
 
 	sock->sock_ind		= sock_ind;
-	sock->dev		= dev;
 	sock->amd_hsmp_rdwr	= amd_hsmp_acpi_rdwr;
 
 	sema_init(&sock->hsmp_sem, 1);
@@ -237,7 +236,22 @@ static int hsmp_parse_acpi_table(struct device *dev, u16 sock_ind)
 		return ret;
 
 	/* Read mailbox offsets from DSD table */
-	return hsmp_read_acpi_dsd(dev, sock);
+	ret = hsmp_read_acpi_dsd(dev, sock);
+	if (ret)
+		return ret;
+
+	/*
+	 * Publish sock->dev last.  hsmp_send_message() uses it (via
+	 * smp_load_acquire()) as the readiness gate for the lock-free data
+	 * plane, so it must become visible only after virt_base_addr, the
+	 * mailbox offsets and the semaphore are fully initialized.  On a
+	 * multi-socket system socket 0 exposes /dev/hsmp before later sockets
+	 * finish probing, so without this an ioctl aimed at a socket still in
+	 * bring-up could pass the gate and dereference a NULL virt_base_addr.
+	 */
+	smp_store_release(&sock->dev, dev);
+
+	return 0;
 }
 
 static ssize_t hsmp_metric_tbl_acpi_read(struct file *filp, struct kobject *kobj,
diff --git a/drivers/platform/x86/amd/hsmp/hsmp.c b/drivers/platform/x86/amd/hsmp/hsmp.c
index 6a26937fc2b5..1a87931136fd 100644
--- a/drivers/platform/x86/amd/hsmp/hsmp.c
+++ b/drivers/platform/x86/amd/hsmp/hsmp.c
@@ -223,6 +223,19 @@ int hsmp_send_message(struct hsmp_message *msg)
 	sock_ind = array_index_nospec(msg->sock_ind, hsmp_pdev.num_sockets);
 	sock = &hsmp_pdev.sock[sock_ind];
 
+	/*
+	 * A slot exists for every possible socket, but it is only usable once
+	 * that socket has actually been probed.  Reject messages aimed at a
+	 * socket that was never brought up or is still in bring-up, so we never
+	 * operate on a zero-initialized semaphore or an unmapped mailbox.  A
+	 * non-NULL dev also guarantees virt_base_addr, the mailbox offsets and
+	 * the semaphore are visible.
+	 *
+	 * Pairs with smp_store_release(&sock->dev) in hsmp_parse_acpi_table().
+	 */
+	if (!smp_load_acquire(&sock->dev))
+		return -ENODEV;
+
 	ret = down_interruptible(&sock->hsmp_sem);
 	if (ret < 0)
 		return ret;