[PATCH bpf-next 1/7] bpf: Allow BPF LSM programs to attach to more hooks

[Anton Protopopov <a.s.protopopov@gmail.com>]

The BPF LSM programs are allowed to attach to LSM hooks, all of which
are defined in the <lsm_hook_defs.h> header file.  From BPF's point
of view the set of attachment points is defined in the bpf_lsm_hooks
BTF set. By analogy with existing code, add a new header file
<bpf_lsm_hook_defs.h> which will also be included in the bpf_lsm_hooks
BTF set.

This change allows attaching BPF LSM programs to more functions.
The actual hooks are added in subsequent commits.

Each BPF hook calls a [__weak] noinline function each time a hook is
reached. This may be too expensive for hot paths if a BPF program is
not attached. A future commit will optimize this by adding a per-hook
static key and inc/dec it on attach/detach. This way disabled hooks
will be bypassed efficiently.

Signed-off-by: Anton Protopopov <a.s.protopopov@gmail.com>
---
 MAINTAINERS                       |  1 +
 include/linux/bpf_lsm.h           | 12 ++++++++++++
 include/linux/bpf_lsm_hook_defs.h |  6 ++++++
 kernel/bpf/bpf_lsm.c              |  2 ++
 4 files changed, 21 insertions(+)
 create mode 100644 include/linux/bpf_lsm_hook_defs.h

diff --git a/MAINTAINERS b/MAINTAINERS
index 460cb7268845..d01dd1f096fc 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -5035,6 +5035,7 @@ L:	bpf@vger.kernel.org
 S:	Maintained
 F:	Documentation/bpf/prog_lsm.rst
 F:	include/linux/bpf_lsm.h
+F:	include/linux/bpf_lsm_hook_defs.h
 F:	kernel/bpf/bpf_lsm.c
 F:	kernel/bpf/bpf_lsm_proto.c
 F:	kernel/trace/bpf_trace.c
diff --git a/include/linux/bpf_lsm.h b/include/linux/bpf_lsm.h
index dda272d78f01..1e54c7cca27a 100644
--- a/include/linux/bpf_lsm.h
+++ b/include/linux/bpf_lsm.h
@@ -16,9 +16,19 @@
 
 extern bool bpf_lsm_initialized __ro_after_init;
 
+/*
+ * Technically, checking bpf_lsm_initialized is not necessary.
+ * But if it is off, then this means that all security_* calls
+ * do not call BPF, and it doesn't look reasonable to enable
+ * only "non-LSM" bpf hooks...
+ */
+#define bpf_lsm_hook(NAME, ...) \
+	(bpf_lsm_initialized ? bpf_lsm_##NAME(__VA_ARGS__) : 0)
+
 #define LSM_HOOK(RET, DEFAULT, NAME, ...) \
 	RET bpf_lsm_##NAME(__VA_ARGS__);
 #include <linux/lsm_hook_defs.h>
+#include <linux/bpf_lsm_hook_defs.h>
 #undef LSM_HOOK
 
 struct bpf_storage_blob {
@@ -114,6 +124,8 @@ static inline bool bpf_lsm_hook_returns_errno(u32 btf_id)
 {
 	return true;
 }
+
+#define bpf_lsm_hook(NAME, ...) 0
 #endif /* CONFIG_BPF_LSM */
 
 #endif /* _LINUX_BPF_LSM_H */
diff --git a/include/linux/bpf_lsm_hook_defs.h b/include/linux/bpf_lsm_hook_defs.h
new file mode 100644
index 000000000000..29bc0b514d16
--- /dev/null
+++ b/include/linux/bpf_lsm_hook_defs.h
@@ -0,0 +1,6 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+/*
+ * This is a set of BPF LSM hooks, which are _not_ fully implemented
+ * as LSM hooks. Thus, they only can be used by BPF LSM programs.
+ */
diff --git a/kernel/bpf/bpf_lsm.c b/kernel/bpf/bpf_lsm.c
index 82c5988417a0..add344ea2691 100644
--- a/kernel/bpf/bpf_lsm.c
+++ b/kernel/bpf/bpf_lsm.c
@@ -28,11 +28,13 @@ __weak noinline RET bpf_lsm_##NAME(__VA_ARGS__)	\
 }
 
 #include <linux/lsm_hook_defs.h>
+#include <linux/bpf_lsm_hook_defs.h>
 #undef LSM_HOOK
 
 #define LSM_HOOK(RET, DEFAULT, NAME, ...) BTF_ID(func, bpf_lsm_##NAME)
 BTF_SET_START(bpf_lsm_hooks)
 #include <linux/lsm_hook_defs.h>
+#include <linux/bpf_lsm_hook_defs.h>
 #undef LSM_HOOK
 BTF_SET_END(bpf_lsm_hooks)
 
-- 
2.43.0