[PATCH bpf-next v4 3/8] bpf: lsm: provide attachment points for BPF LSM programs

[KP Singh]

From: KP Singh <kpsingh@xxxxxxxxxx>

The BPF LSM programs are implemented as fexit trampolines to avoid the
overhead of retpolines. These programs cannot be attached to security_*
wrappers as there are quite a few security_* functions that do more than
just calling the LSM callbacks.

This was discussed on the lists in:

  https://lore.kernel.org/bpf/20200123152440.28956-1-kpsingh@xxxxxxxxxxxx/T/#m068becce588a0cdf01913f368a97aea4c62d8266

Adding a NOP callback after all the static LSM callbacks are called has
the following benefits:

- The BPF programs run at the right stage of the security_* wrappers.
- They run after all the static LSM hooks allowed the operation,
  therefore cannot allow an action that was already denied.

There are some hooks which do not call call_int_hooks or
call_void_hooks. It's not possible to call the bpf_lsm_* functions
without checking if there is BPF LSM program attached to these hooks.
This is added further in a subsequent patch. For now, these hooks are
marked as NO_BPF (i.e. attachment of BPF programs is not possible).

Signed-off-by: KP Singh <kpsingh@xxxxxxxxxx>
---
 include/linux/bpf_lsm.h | 34 ++++++++++++++++++++++++++++++++++
 kernel/bpf/bpf_lsm.c    | 16 ++++++++++++++++
 security/security.c     |  3 +++
 3 files changed, 53 insertions(+)
 create mode 100644 include/linux/bpf_lsm.h

diff --git a/include/linux/bpf_lsm.h b/include/linux/bpf_lsm.h
new file mode 100644
index 000000000000..f867f72f6aa9
--- /dev/null
+++ b/include/linux/bpf_lsm.h
@@ -0,0 +1,34 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+/*
+ * Copyright 2019 Google LLC.
+ */
+
+#ifndef _LINUX_BPF_LSM_H
+#define _LINUX_BPF_LSM_H
+
+#include <linux/bpf.h>
+
+#ifdef CONFIG_BPF_LSM
+
+#define LSM_HOOK(RET, NAME, ...) RET bpf_lsm_##NAME(__VA_ARGS__);
+#include <linux/lsm_hook_names.h>
+#undef LSM_HOOK
+
+#define RUN_BPF_LSM_VOID_PROGS(FUNC, ...) bpf_lsm_##FUNC(__VA_ARGS__)
+#define RUN_BPF_LSM_INT_PROGS(RC, FUNC, ...) ({				\
+	do {								\
+		if (RC == 0)						\
+			RC = bpf_lsm_##FUNC(__VA_ARGS__);		\
+	} while (0);							\
+	RC;								\
+})
+
+#else /* !CONFIG_BPF_LSM */
+
+#define RUN_BPF_LSM_INT_PROGS(RC, FUNC, ...) (RC)
+#define RUN_BPF_LSM_VOID_PROGS(FUNC, ...)
+
+#endif /* CONFIG_BPF_LSM */
+
+#endif /* _LINUX_BPF_LSM_H */
diff --git a/kernel/bpf/bpf_lsm.c b/kernel/bpf/bpf_lsm.c
index affb6941622e..abc847c9b9a1 100644
--- a/kernel/bpf/bpf_lsm.c
+++ b/kernel/bpf/bpf_lsm.c
@@ -7,6 +7,22 @@
 #include <linux/filter.h>
 #include <linux/bpf.h>
 #include <linux/btf.h>
+#include <linux/bpf_lsm.h>
+
+/* For every LSM hook  that allows attachment of BPF programs, declare a NOP
+ * function where a BPF program can be attached as an fexit trampoline.
+ */
+#define LSM_HOOK(RET, NAME, ...) LSM_HOOK_##RET(NAME, __VA_ARGS__)
+#define LSM_HOOK_int(NAME, ...) noinline int bpf_lsm_##NAME(__VA_ARGS__)  \
+{									  \
+	return 0;							  \
+}
+
+#define LSM_HOOK_void(NAME, ...) \
+	noinline void bpf_lsm_##NAME(__VA_ARGS__) {}
+
+#include <linux/lsm_hook_names.h>
+#undef LSM_HOOK
 
 const struct bpf_prog_ops lsm_prog_ops = {
 };
diff --git a/security/security.c b/security/security.c
index 565bc9b67276..aa111392a700 100644
--- a/security/security.c
+++ b/security/security.c
@@ -28,6 +28,7 @@
 #include <linux/string.h>
 #include <linux/msg.h>
 #include <net/flow.h>
+#include <linux/bpf_lsm.h>
 
 #define MAX_LSM_EVM_XATTR	2
 
@@ -684,6 +685,7 @@ 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__);		\
+		RUN_BPF_LSM_VOID_PROGS(FUNC, __VA_ARGS__);	\
 	} while (0)
 
 #define call_int_hook(FUNC, IRC, ...) ({			\
@@ -696,6 +698,7 @@ static void __init lsm_early_task(struct task_struct *task)
 			if (RC != 0)				\
 				break;				\
 		}						\
+		RC = RUN_BPF_LSM_INT_PROGS(RC, FUNC, __VA_ARGS__); \
 	} while (0);						\
 	RC;							\
 })
-- 
2.20.1