bpf: lsm: Implement attach, detach and execution

JITed BPF programs are dynamically attached to the LSM hooks
using BPF trampolines. The trampoline prologue generates code to handle
conversion of the signature of the hook to the appropriate BPF context.

The allocated trampoline programs are attached to the nop functions
initialized as LSM hooks.

BPF_PROG_TYPE_LSM programs must have a GPL compatible license and
and need CAP_SYS_ADMIN (required for loading eBPF programs).

Upon attachment:

* A BPF fexit trampoline is used for LSM hooks with a void return type.
* A BPF fmod_ret trampoline is used for LSM hooks which return an
  int. The attached programs can override the return value of the
  bpf LSM hook to indicate a MAC Policy decision.

Signed-off-by: KP Singh <kpsingh@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Brendan Jackman <jackmanb@google.com>
Reviewed-by: Florent Revest <revest@google.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: James Morris <jamorris@linux.microsoft.com>
Link: https://lore.kernel.org/bpf/20200329004356.27286-5-kpsingh@chromium.org

kernel/bpf/btf.c

@@ -3710,7 +3710,21 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 	}
 
 	if (arg == nr_args) {
-		if (prog->expected_attach_type == BPF_TRACE_FEXIT) {
+		if (prog->expected_attach_type == BPF_TRACE_FEXIT ||
+		    prog->expected_attach_type == BPF_LSM_MAC) {
+			/* When LSM programs are attached to void LSM hooks
+			 * they use FEXIT trampolines and when attached to
+			 * int LSM hooks, they use MODIFY_RETURN trampolines.
+			 *
+			 * While the LSM programs are BPF_MODIFY_RETURN-like
+			 * the check:
+			 *
+			 *	if (ret_type != 'int')
+			 *		return -EINVAL;
+			 *
+			 * is _not_ done here. This is still safe as LSM hooks
+			 * have only void and int return types.
+			 */
 			if (!t)
 				return true;
 			t = btf_type_by_id(btf, t->type);