xen: XSA-482 security patches for v7.0

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Merge tag 'xsa482-7.0-tag' of git://git.kernel.org/pub/scm/linux/kernel/git/xen/tip

Pull xen fixes from Juergen Gross:
 "Restrict the xen privcmd driver in unprivileged domU to only allow
  hypercalls to target domain when using secure boot"

* tag 'xsa482-7.0-tag' of git://git.kernel.org/pub/scm/linux/kernel/git/xen/tip:
  xen/privcmd: add boot control for restricted usage in domU
  xen/privcmd: restrict usage in unprivileged domU

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 +31,10 @@
 #include <linux/seq_file.h>
 #include <linux/miscdevice.h>
 #include <linux/moduleparam.h>
+#include <linux/notifier.h>
+#include <linux/security.h>
 #include <linux/virtio_mmio.h>
+#include <linux/wait.h>
 
 #include <asm/xen/hypervisor.h>
 #include <asm/xen/hypercall.h>
@@ -46,6 +50,7 @@
 #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
@@ -68,10 +73,20 @@ module_param_named(dm_op_buf_max_size, privcmd_dm_op_buf_max_size, uint,
 MODULE_PARM_DESC(dm_op_buf_max_size,
 		 "Maximum size of a dm_op hypercall buffer");
 
+static bool unrestricted;
+module_param(unrestricted, bool, 0);
+MODULE_PARM_DESC(unrestricted,
+	"Don't restrict hypercalls to target domain if running in a domU");
+
 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 +1578,16 @@ 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;
@@ -1662,6 +1680,52 @@ 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)
+{
+	if (unrestricted) {
+		if (security_locked_down(LOCKDOWN_XEN_USER_ACTIONS))
+			pr_warn("Kernel is locked down, parameter \"unrestricted\" ignored\n");
+		else
+			return;
+	}
+
+	restrict_wait = true;
+
+	register_xenstore_notifier(&xenstore_notifier);
+}
+
 static int __init privcmd_init(void)
 {
 	int err;
@@ -1669,6 +1733,9 @@ static int __init privcmd_init(void)
 	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");

include/linux/security.h

@@ -145,6 +145,7 @@ enum lockdown_reason {
 	LOCKDOWN_BPF_WRITE_USER,
 	LOCKDOWN_DBG_WRITE_KERNEL,
 	LOCKDOWN_RTAS_ERROR_INJECTION,
+	LOCKDOWN_XEN_USER_ACTIONS,
 	LOCKDOWN_INTEGRITY_MAX,
 	LOCKDOWN_KCORE,
 	LOCKDOWN_KPROBES,

security/security.c

@@ -61,6 +61,7 @@ const char *const lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX + 1] = {
 	[LOCKDOWN_BPF_WRITE_USER] = "use of bpf to write user RAM",
 	[LOCKDOWN_DBG_WRITE_KERNEL] = "use of kgdb/kdb to write kernel RAM",
 	[LOCKDOWN_RTAS_ERROR_INJECTION] = "RTAS error injection",
+	[LOCKDOWN_XEN_USER_ACTIONS] = "Xen guest user action",
 	[LOCKDOWN_INTEGRITY_MAX] = "integrity",
 	[LOCKDOWN_KCORE] = "/proc/kcore access",
 	[LOCKDOWN_KPROBES] = "use of kprobes",