[RFC PATCH v3 3/3] devguard: added device guard for mknod in non-initial userns

[Michael Weiß]

devguard is a simple LSM to allow CAP_MKNOD in non-initial user
namespace in cooperation of an attached cgroup device program. We
just need to implement the security_inode_mknod() hook for this.
In the hook, we check if the current task is guarded by a device
cgroup using the lately introduced cgroup_bpf_current_enabled()
helper. If so, we strip out SB_I_NODEV from the super block.

Access decisions to those device nodes are then guarded by existing
device cgroups mechanism.

Signed-off-by: Michael Weiß <michael.weiss@xxxxxxxxxxxxxxxxxxx>
---
 security/Kconfig             | 11 +++++----
 security/Makefile            |  1 +
 security/devguard/Kconfig    | 12 ++++++++++
 security/devguard/Makefile   |  2 ++
 security/devguard/devguard.c | 44 ++++++++++++++++++++++++++++++++++++
 5 files changed, 65 insertions(+), 5 deletions(-)
 create mode 100644 security/devguard/Kconfig
 create mode 100644 security/devguard/Makefile
 create mode 100644 security/devguard/devguard.c

diff --git a/security/Kconfig b/security/Kconfig
index 52c9af08ad35..7ec4017745d4 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -194,6 +194,7 @@ source "security/yama/Kconfig"
 source "security/safesetid/Kconfig"
 source "security/lockdown/Kconfig"
 source "security/landlock/Kconfig"
+source "security/devguard/Kconfig"
 
 source "security/integrity/Kconfig"
 
@@ -233,11 +234,11 @@ endchoice
 
 config LSM
 	string "Ordered list of enabled LSMs"
-	default "landlock,lockdown,yama,loadpin,safesetid,smack,selinux,tomoyo,apparmor,bpf" if DEFAULT_SECURITY_SMACK
-	default "landlock,lockdown,yama,loadpin,safesetid,apparmor,selinux,smack,tomoyo,bpf" if DEFAULT_SECURITY_APPARMOR
-	default "landlock,lockdown,yama,loadpin,safesetid,tomoyo,bpf" if DEFAULT_SECURITY_TOMOYO
-	default "landlock,lockdown,yama,loadpin,safesetid,bpf" if DEFAULT_SECURITY_DAC
-	default "landlock,lockdown,yama,loadpin,safesetid,selinux,smack,tomoyo,apparmor,bpf"
+	default "landlock,lockdown,yama,loadpin,safesetid,smack,selinux,tomoyo,apparmor,bpf,devguard" if DEFAULT_SECURITY_SMACK
+	default "landlock,lockdown,yama,loadpin,safesetid,apparmor,selinux,smack,tomoyo,bpf,devguard" if DEFAULT_SECURITY_APPARMOR
+	default "landlock,lockdown,yama,loadpin,safesetid,tomoyo,bpf,devguard" if DEFAULT_SECURITY_TOMOYO
+	default "landlock,lockdown,yama,loadpin,safesetid,bpf,devguard" if DEFAULT_SECURITY_DAC
+	default "landlock,lockdown,yama,loadpin,safesetid,selinux,smack,tomoyo,apparmor,bpf,devguard"
 	help
 	  A comma-separated list of LSMs, in initialization order.
 	  Any LSMs left off this list, except for those with order
diff --git a/security/Makefile b/security/Makefile
index 18121f8f85cd..82a0d8cab3c3 100644
--- a/security/Makefile
+++ b/security/Makefile
@@ -24,6 +24,7 @@ obj-$(CONFIG_SECURITY_LOCKDOWN_LSM)	+= lockdown/
 obj-$(CONFIG_CGROUPS)			+= device_cgroup.o
 obj-$(CONFIG_BPF_LSM)			+= bpf/
 obj-$(CONFIG_SECURITY_LANDLOCK)		+= landlock/
+obj-$(CONFIG_SECURITY_DEVGUARD)		+= devguard/
 
 # Object integrity file lists
 obj-$(CONFIG_INTEGRITY)			+= integrity/
diff --git a/security/devguard/Kconfig b/security/devguard/Kconfig
new file mode 100644
index 000000000000..592684615a8f
--- /dev/null
+++ b/security/devguard/Kconfig
@@ -0,0 +1,12 @@
+config SECURITY_DEVGUARD
+	bool "Devguard for device node creation"
+	depends on SECURITY
+	depends on CGROUP_BPF
+	default n
+	help
+	  This enables devguard, an LSM that allows to guard device node
+	  creation in non-initial user namespace. It may allow mknod
+	  in cooperation of an attached cgroup device program.
+	  This security module stacks with other LSMs.
+
+	  If you are unsure how to answer this question, answer N.
diff --git a/security/devguard/Makefile b/security/devguard/Makefile
new file mode 100644
index 000000000000..fdaff8dc2fea
--- /dev/null
+++ b/security/devguard/Makefile
@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0-only
+obj-$(CONFIG_SECURITY_DEVGUARD) += devguard.o
diff --git a/security/devguard/devguard.c b/security/devguard/devguard.c
new file mode 100644
index 000000000000..3a0c9c27a691
--- /dev/null
+++ b/security/devguard/devguard.c
@@ -0,0 +1,44 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Device guard security module
+ *
+ * Simple in-kernel LSM to allow cap_mknod in non-initial
+ * user namespace if current task is guarded by device cgroup.
+ *
+ * Copyright (C) 2023 Fraunhofer AISEC. All rights reserved.
+ *
+ * Authors: Michael Weiß <michael.weiss@xxxxxxxxxxxxxxxxxxx>
+ */
+
+#include <linux/bpf-cgroup.h>
+#include <linux/lsm_hooks.h>
+
+static int devguard_inode_mknod(struct inode *dir, struct dentry *dentry,
+				umode_t mode, dev_t dev)
+{
+	if (dentry->d_sb->s_iflags & ~SB_I_NODEV)
+		return 0;
+
+	// strip SB_I_NODEV on super block if device cgroup is active
+	if (cgroup_bpf_current_enabled(CGROUP_DEVICE))
+		dentry->d_sb->s_iflags &= ~SB_I_NODEV;
+
+	return 0;
+}
+
+static struct security_hook_list devguard_hooks[] __ro_after_init = {
+	LSM_HOOK_INIT(inode_mknod, devguard_inode_mknod),
+};
+
+static int __init devguard_init(void)
+{
+	security_add_hooks(devguard_hooks, ARRAY_SIZE(devguard_hooks),
+			   "devguard");
+	pr_info("devguard: initialized\n");
+	return 0;
+}
+
+DEFINE_LSM(devguard) = {
+	.name = "devguard",
+	.init = devguard_init,
+};
-- 
2.30.2