[PATCH bpf-next v2 05/10] bpf: lsm: BTF API for LSM hooks

[KP Singh]

From: KP Singh <kpsingh at google.com>

The BTF API provides information required by the BPF verifier to
attach eBPF programs to the LSM hooks by using the BTF information of
two types:

- struct security_hook_heads: This type provides the offset which
  a new dynamically allocated security hook must be attached to.
- union security_list_options: This provides the information about the
  function prototype required by the hook.

When the program is loaded:

- The verifier receives the index of a member in struct
  security_hook_heads to which a program must be attached as
  prog->aux->lsm_hook_index. The index is one-based for better
  verification.
- bpf_lsm_type_by_index is used to determine the func_proto of
  the LSM hook and updates prog->aux->attach_func_proto
- bpf_lsm_head_by_index is used to determine the hlist_head to which
  the BPF program must be attached.

Signed-off-by: KP Singh <kpsingh at google.com>
---
 include/linux/bpf_lsm.h |  12 +++++
 security/bpf/Kconfig    |   1 +
 security/bpf/hooks.c    | 104 ++++++++++++++++++++++++++++++++++++++++
 3 files changed, 117 insertions(+)

diff --git a/include/linux/bpf_lsm.h b/include/linux/bpf_lsm.h
index 9883cf25241c..a9b4f7b41c65 100644
--- a/include/linux/bpf_lsm.h
+++ b/include/linux/bpf_lsm.h
@@ -19,6 +19,8 @@ extern struct security_hook_heads bpf_lsm_hook_heads;
 
 int bpf_lsm_srcu_read_lock(void);
 void bpf_lsm_srcu_read_unlock(int idx);
+const struct btf_type *bpf_lsm_type_by_index(struct btf *btf, u32 offset);
+const struct btf_member *bpf_lsm_head_by_index(struct btf *btf, u32 id);
 
 #define CALL_BPF_LSM_VOID_HOOKS(FUNC, ...)			\
 	do {							\
@@ -65,6 +67,16 @@ static inline int bpf_lsm_srcu_read_lock(void)
 	return 0;
 }
 static inline void bpf_lsm_srcu_read_unlock(int idx) {}
+static inline const struct btf_type *bpf_lsm_type_by_index(
+	struct btf *btf, u32 index)
+{
+	return ERR_PTR(-EOPNOTSUPP);
+}
+static inline const struct btf_member *bpf_lsm_head_by_index(
+	struct btf *btf, u32 id)
+{
+	return ERR_PTR(-EOPNOTSUPP);
+}
 
 #endif /* CONFIG_SECURITY_BPF */
 
diff --git a/security/bpf/Kconfig b/security/bpf/Kconfig
index 595e4ad597ae..9438d899b618 100644
--- a/security/bpf/Kconfig
+++ b/security/bpf/Kconfig
@@ -7,6 +7,7 @@ config SECURITY_BPF
 	depends on SECURITY
 	depends on BPF_SYSCALL
 	depends on SRCU
+	depends on DEBUG_INFO_BTF
 	help
 	  This enables instrumentation of the security hooks with
 	  eBPF programs.
diff --git a/security/bpf/hooks.c b/security/bpf/hooks.c
index b123d9cb4cd4..82725611693d 100644
--- a/security/bpf/hooks.c
+++ b/security/bpf/hooks.c
@@ -5,6 +5,8 @@
  */
 
 #include <linux/bpf_lsm.h>
+#include <linux/bpf.h>
+#include <linux/btf.h>
 #include <linux/srcu.h>
 
 DEFINE_STATIC_SRCU(security_hook_srcu);
@@ -18,3 +20,105 @@ void bpf_lsm_srcu_read_unlock(int idx)
 {
 	return srcu_read_unlock(&security_hook_srcu, idx);
 }
+
+static inline int validate_hlist_head(struct btf *btf, u32 type_id)
+{
+	s32 hlist_id;
+
+	hlist_id = btf_find_by_name_kind(btf, "hlist_head", BTF_KIND_STRUCT);
+	if (hlist_id < 0 || hlist_id != type_id)
+		return -EINVAL;
+
+	return 0;
+}
+
+/* Find the BTF representation of the security_hook_heads member for a member
+ * with a given index in struct security_hook_heads.
+ */
+const struct btf_member *bpf_lsm_head_by_index(struct btf *btf, u32 index)
+{
+	const struct btf_member *member;
+	const struct btf_type *t;
+	u32 off, i;
+	int ret;
+
+	t = btf_type_by_name_kind(btf, "security_hook_heads", BTF_KIND_STRUCT);
+	if (WARN_ON_ONCE(IS_ERR(t)))
+		return ERR_CAST(t);
+
+	for_each_member(i, t, member) {
+		/* We've found the id requested and need to check the
+		 * the following:
+		 *
+		 * - Is it at a valid alignment for struct hlist_head?
+		 *
+		 * - Is it a valid hlist_head struct?
+		 */
+		if (index == i) {
+			off = btf_member_bit_offset(t, member);
+			if (off % 8)
+				/* valid c code cannot generate such btf */
+				return ERR_PTR(-EINVAL);
+			off /= 8;
+
+			if (off % __alignof__(struct hlist_head))
+				return ERR_PTR(-EINVAL);
+
+			ret = validate_hlist_head(btf, member->type);
+			if (ret < 0)
+				return ERR_PTR(ret);
+
+			return member;
+		}
+	}
+
+	return ERR_PTR(-ENOENT);
+}
+
+/* Given an index of a member in security_hook_heads return the
+ * corresponding type for the LSM hook. The members of the union
+ * security_list_options have the same name as the security_hook_heads which
+ * is ensured by the LSM_HOOK_INIT macro defined in include/linux/lsm_hooks.h
+ */
+const struct btf_type *bpf_lsm_type_by_index(struct btf *btf, u32 index)
+{
+	const struct btf_member *member, *hook_head = NULL;
+	const struct btf_type *t, *hook_type = NULL;
+	u32 i;
+
+	hook_head = bpf_lsm_head_by_index(btf, index);
+	if (IS_ERR(hook_head))
+		return ERR_PTR(PTR_ERR(hook_head));
+
+	t = btf_type_by_name_kind(btf, "security_list_options", BTF_KIND_UNION);
+	if (WARN_ON_ONCE(IS_ERR(t)))
+		return ERR_CAST(t);
+
+	for_each_member(i, t, member) {
+		if (hook_head->name_off == member->name_off) {
+			/* There should be only one member with the same name
+			 * as the LSM hook. This should never really happen
+			 * and either indicates malformed BTF or someone trying
+			 * trick the LSM.
+			 */
+			if (WARN_ON(hook_type))
+				return ERR_PTR(-EINVAL);
+
+			hook_type = btf_type_by_id(btf, member->type);
+			if (unlikely(!hook_type))
+				return ERR_PTR(-EINVAL);
+
+			if (!btf_type_is_ptr(hook_type))
+				return ERR_PTR(-EINVAL);
+		}
+	}
+
+	if (!hook_type)
+		return ERR_PTR(-ENOENT);
+
+	t = btf_type_by_id(btf, hook_type->type);
+	if (unlikely(!t))
+		return ERR_PTR(-EINVAL);
+
+	return t;
+}
-- 
2.20.1