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

From: KP Singh
Date: Thu Feb 20 2020 - 12:53:31 EST


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