[PATCH bpf-next v2 04/10] bpf: lsm: Add mutable hooks list for the BPF LSM

[KP Singh <kpsingh@chromium.org>]

From: KP Singh <kpsingh@google.com>

- The list of hooks registered by an LSM is currently immutable as they
  are declared with __lsm_ro_after_init and they are attached to a
  security_hook_heads struct.
- For the BPF LSM we need to de/register the hooks at runtime. Making
  the existing security_hook_heads mutable broadens an
  attack vector, so a separate security_hook_heads is added for only
  those that ~must~ be mutable.
- These mutable hooks are run only after all the static hooks have
  successfully executed.

This is based on the ideas discussed in:

  https://lore.kernel.org/lkml/20180408065916.GA2832@ircssh-2.c.rugged-nimbus-611.internal

Signed-off-by: KP Singh <kpsingh@google.com>
---
 MAINTAINERS             |  1 +
 include/linux/bpf_lsm.h | 71 +++++++++++++++++++++++++++++++++++++++++
 security/bpf/Kconfig    |  1 +
 security/bpf/Makefile   |  2 +-
 security/bpf/hooks.c    | 20 ++++++++++++
 security/bpf/lsm.c      |  9 +++++-
 security/security.c     | 24 +++++++-------
 7 files changed, 115 insertions(+), 13 deletions(-)
 create mode 100644 include/linux/bpf_lsm.h
 create mode 100644 security/bpf/hooks.c

diff --git a/MAINTAINERS b/MAINTAINERS
index 0941f478cfa5..02d7e05e9b75 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -3209,6 +3209,7 @@ L:	linux-security-module@vger.kernel.org
 L:	bpf@vger.kernel.org
 S:	Maintained
 F:	security/bpf/
+F:	include/linux/bpf_lsm.h
 
 BROADCOM B44 10/100 ETHERNET DRIVER
 M:	Michael Chan <michael.chan@broadcom.com>
diff --git a/include/linux/bpf_lsm.h b/include/linux/bpf_lsm.h
new file mode 100644
index 000000000000..9883cf25241c
--- /dev/null
+++ b/include/linux/bpf_lsm.h
@@ -0,0 +1,71 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+/*
+ * Copyright 2019 Google LLC.
+ */
+
+#ifndef _LINUX_BPF_LSM_H
+#define _LINUX_BPF_LSM_H
+
+#include <linux/bpf.h>
+#include <linux/lsm_hooks.h>
+
+#ifdef CONFIG_SECURITY_BPF
+
+/* Mutable hooks defined at runtime and executed after all the statically
+ * define LSM hooks.
+ */
+extern struct security_hook_heads bpf_lsm_hook_heads;
+
+int bpf_lsm_srcu_read_lock(void);
+void bpf_lsm_srcu_read_unlock(int idx);
+
+#define CALL_BPF_LSM_VOID_HOOKS(FUNC, ...)			\
+	do {							\
+		struct security_hook_list *P;			\
+		int _idx;					\
+								\
+		if (hlist_empty(&bpf_lsm_hook_heads.FUNC))	\
+			break;					\
+								\
+		_idx = bpf_lsm_srcu_read_lock();		\
+		hlist_for_each_entry(P, &bpf_lsm_hook_heads.FUNC, list) \
+			P->hook.FUNC(__VA_ARGS__);		\
+		bpf_lsm_srcu_read_unlock(_idx);			\
+	} while (0)
+
+#define CALL_BPF_LSM_INT_HOOKS(RC, FUNC, ...) ({		\
+	do {							\
+		struct security_hook_list *P;			\
+		int _idx;					\
+								\
+		if (hlist_empty(&bpf_lsm_hook_heads.FUNC))	\
+			break;					\
+								\
+		_idx = bpf_lsm_srcu_read_lock();		\
+								\
+		hlist_for_each_entry(P,				\
+			&bpf_lsm_hook_heads.FUNC, list) {	\
+			RC = P->hook.FUNC(__VA_ARGS__);		\
+			if (RC && IS_ENABLED(CONFIG_SECURITY_BPF_ENFORCE)) \
+				break;				\
+		}						\
+		bpf_lsm_srcu_read_unlock(_idx);			\
+	} while (0);						\
+	IS_ENABLED(CONFIG_SECURITY_BPF_ENFORCE) ? RC : 0;	\
+})
+
+#else /* !CONFIG_SECURITY_BPF */
+
+#define CALL_BPF_LSM_INT_HOOKS(RC, FUNC, ...) (RC)
+#define CALL_BPF_LSM_VOID_HOOKS(...)
+
+static inline int bpf_lsm_srcu_read_lock(void)
+{
+	return 0;
+}
+static inline void bpf_lsm_srcu_read_unlock(int idx) {}
+
+#endif /* CONFIG_SECURITY_BPF */
+
+#endif /* _LINUX_BPF_LSM_H */
diff --git a/security/bpf/Kconfig b/security/bpf/Kconfig
index a5f6c67ae526..595e4ad597ae 100644
--- a/security/bpf/Kconfig
+++ b/security/bpf/Kconfig
@@ -6,6 +6,7 @@ config SECURITY_BPF
 	bool "BPF-based MAC and audit policy"
 	depends on SECURITY
 	depends on BPF_SYSCALL
+	depends on SRCU
 	help
 	  This enables instrumentation of the security hooks with
 	  eBPF programs.
diff --git a/security/bpf/Makefile b/security/bpf/Makefile
index c78a8a056e7e..c526927c337d 100644
--- a/security/bpf/Makefile
+++ b/security/bpf/Makefile
@@ -2,4 +2,4 @@
 #
 # Copyright 2019 Google LLC.
 
-obj-$(CONFIG_SECURITY_BPF) := lsm.o ops.o
+obj-$(CONFIG_SECURITY_BPF) := lsm.o ops.o hooks.o
diff --git a/security/bpf/hooks.c b/security/bpf/hooks.c
new file mode 100644
index 000000000000..b123d9cb4cd4
--- /dev/null
+++ b/security/bpf/hooks.c
@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Copyright 2019 Google LLC.
+ */
+
+#include <linux/bpf_lsm.h>
+#include <linux/srcu.h>
+
+DEFINE_STATIC_SRCU(security_hook_srcu);
+
+int bpf_lsm_srcu_read_lock(void)
+{
+	return srcu_read_lock(&security_hook_srcu);
+}
+
+void bpf_lsm_srcu_read_unlock(int idx)
+{
+	return srcu_read_unlock(&security_hook_srcu, idx);
+}
diff --git a/security/bpf/lsm.c b/security/bpf/lsm.c
index 5c5c14f990ce..d4ea6aa9ddb8 100644
--- a/security/bpf/lsm.c
+++ b/security/bpf/lsm.c
@@ -4,14 +4,21 @@
  * Copyright 2019 Google LLC.
  */
 
+#include <linux/bpf_lsm.h>
 #include <linux/lsm_hooks.h>
 
 /* This is only for internal hooks, always statically shipped as part of the
- * BPF LSM. Statically defined hooks are appeneded to the security_hook_heads
+ * BPF LSM. Statically defined hooks are appended to the security_hook_heads
  * which is common for LSMs and R/O after init.
  */
 static struct security_hook_list lsm_hooks[] __lsm_ro_after_init = {};
 
+/* Security hooks registered dynamically by the BPF LSM and must be accessed
+ * by holding bpf_lsm_srcu_read_lock and bpf_lsm_srcu_read_unlock. The mutable
+ * hooks dynamically allocated by the BPF LSM are appeneded here.
+ */
+struct security_hook_heads bpf_lsm_hook_heads;
+
 static int __init lsm_init(void)
 {
 	security_add_hooks(lsm_hooks, ARRAY_SIZE(lsm_hooks), "bpf");
diff --git a/security/security.c b/security/security.c
index cd2d18d2d279..4a2eb4c089b2 100644
--- a/security/security.c
+++ b/security/security.c
@@ -27,6 +27,7 @@
 #include <linux/backing-dev.h>
 #include <linux/string.h>
 #include <linux/msg.h>
+#include <linux/bpf_lsm.h>
 #include <net/flow.h>
 
 #define MAX_LSM_EVM_XATTR	2
@@ -652,20 +653,21 @@ static void __init lsm_early_task(struct task_struct *task)
 								\
 		hlist_for_each_entry(P, &security_hook_heads.FUNC, list) \
 			P->hook.FUNC(__VA_ARGS__);		\
+		CALL_BPF_LSM_VOID_HOOKS(FUNC, __VA_ARGS__);	\
 	} while (0)
 
-#define call_int_hook(FUNC, IRC, ...) ({			\
-	int RC = IRC;						\
-	do {							\
-		struct security_hook_list *P;			\
-								\
+#define call_int_hook(FUNC, IRC, ...) ({				\
+	int RC = IRC;							\
+	do {								\
+		struct security_hook_list *P;				\
 		hlist_for_each_entry(P, &security_hook_heads.FUNC, list) { \
-			RC = P->hook.FUNC(__VA_ARGS__);		\
-			if (RC != 0)				\
-				break;				\
-		}						\
-	} while (0);						\
-	RC;							\
+			RC = P->hook.FUNC(__VA_ARGS__);			\
+			if (RC != 0)					\
+				break;					\
+		}							\
+		RC = CALL_BPF_LSM_INT_HOOKS(RC, FUNC, __VA_ARGS__);	\
+	} while (0);							\
+	RC;								\
 })
 
 /* Security operations */
-- 
2.20.1