From: KP Singh <kpsingh@xxxxxxxxxx>
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@xxxxxxxxxx>
---
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
