xen/privcmd: restrict usage in unprivileged domU

The Xen privcmd driver allows to issue arbitrary hypercalls from
user space processes. This is normally no problem, as access is
usually limited to root and the hypervisor will deny any hypercalls
affecting other domains.

In case the guest is booted using secure boot, however, the privcmd
driver would be enabling a root user process to modify e.g. kernel
memory contents, thus breaking the secure boot feature.

The only known case where an unprivileged domU is really needing to
use the privcmd driver is the case when it is acting as the device
model for another guest. In this case all hypercalls issued via the
privcmd driver will target that other guest.

Fortunately the privcmd driver can already be locked down to allow
only hypercalls targeting a specific domain, but this mode can be
activated from user land only today.

The target domain can be obtained from Xenstore, so when not running
in dom0 restrict the privcmd driver to that target domain from the
beginning, resolving the potential problem of breaking secure boot.

This is XSA-482

Reported-by: Teddy Astie <[email protected]>
Fixes: 1c5de19 ("xen: add privcmd driver")
Signed-off-by: Juergen Gross <[email protected]>
---
V2:
- defer reading from Xenstore if Xenstore isn't ready yet (Jan Beulich)
- wait in open() if target domain isn't known yet
- issue message in case no target domain found (Jan Beulich)

Author: jgross1

drivers/xen/privcmd.c

@@ -12,6 +12,7 @@
 #include <linux/eventfd.h>
 #include <linux/file.h>
 #include <linux/kernel.h>
+#include <linux/kstrtox.h>
 #include <linux/module.h>
 #include <linux/mutex.h>
 #include <linux/poll.h>
@@ -30,7 +31,9 @@
 #include <linux/seq_file.h>
 #include <linux/miscdevice.h>
 #include <linux/moduleparam.h>
+#include <linux/notifier.h>
 #include <linux/virtio_mmio.h>
+#include <linux/wait.h>
 
 #include <asm/xen/hypervisor.h>
 #include <asm/xen/hypercall.h>
@@ -46,6 +49,7 @@
 #include <xen/page.h>
 #include <xen/xen-ops.h>
 #include <xen/balloon.h>
+#include <xen/xenbus.h>
 #ifdef CONFIG_XEN_ACPI
 #include <xen/acpi.h>
 #endif
@@ -72,6 +76,11 @@ struct privcmd_data {
 	domid_t domid;
 };
 
+/* DOMID_INVALID implies no restriction */
+static domid_t target_domain = DOMID_INVALID;
+static bool restrict_wait;
+static DECLARE_WAIT_QUEUE_HEAD(restrict_wait_wq);
+
 static int privcmd_vma_range_is_mapped(
                struct vm_area_struct *vma,
                unsigned long addr,
@@ -1563,13 +1572,16 @@ static long privcmd_ioctl(struct file *file,
 
 static int privcmd_open(struct inode *ino, struct file *file)
 {
-	struct privcmd_data *data = kzalloc_obj(*data);
+	struct privcmd_data *data;
+
+	if (wait_event_interruptible(restrict_wait_wq, !restrict_wait) < 0)
+		return -EINTR;
 
+	data = kzalloc_obj(*data);
 	if (!data)
 		return -ENOMEM;
 
-	/* DOMID_INVALID implies no restriction */
-	data->domid = DOMID_INVALID;
+	data->domid = target_domain;
 
 	file->private_data = data;
 	return 0;
@@ -1662,13 +1674,55 @@ static struct miscdevice privcmd_dev = {
 	.fops = &xen_privcmd_fops,
 };
 
+static int init_restrict(struct notifier_block *notifier,
+			 unsigned long event,
+			 void *data)
+{
+	char *target;
+	unsigned int domid;
+
+	/* Default to an guaranteed unused domain-id. */
+	target_domain = DOMID_IDLE;
+
+	target = xenbus_read(XBT_NIL, "target", "", NULL);
+	if (IS_ERR(target) || kstrtouint(target, 10, &domid)) {
+		pr_err("No target domain found, blocking all hypercalls\n");
+		goto out;
+	}
+
+	target_domain = domid;
+
+ out:
+	if (!IS_ERR(target))
+		kfree(target);
+
+	restrict_wait = false;
+	wake_up_all(&restrict_wait_wq);
+
+	return NOTIFY_DONE;
+}
+
+static struct notifier_block xenstore_notifier = {
+	.notifier_call = init_restrict,
+};
+
+static void __init restrict_driver(void)
+{
+	restrict_wait = true;
+
+	register_xenstore_notifier(&xenstore_notifier);
+}
+
 static int __init privcmd_init(void)
 {
 	int err;
 
 	if (!xen_domain())
 		return -ENODEV;
 
+	if (!xen_initial_domain())
+		restrict_driver();
+
 	err = misc_register(&privcmd_dev);
 	if (err != 0) {
 		pr_err("Could not register Xen privcmd device\n");