bpf: Remove unnecessary CAP_MAC_ADMIN check

While well intentioned, checking CAP_MAC_ADMIN for attaching
BPF_MODIFY_RETURN tracing programs to "security_" functions is not
necessary as tracing BPF programs already require CAP_SYS_ADMIN.

Fixes: 6ba43b76 ("bpf: Attachment verification for BPF_MODIFY_RETURN")
Signed-off-by: KP Singh <kpsingh@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200305204955.31123-1-kpsingh@chromium.org

kernel/bpf/verifier.c

@@ -9808,20 +9808,13 @@ static int check_attach_modify_return(struct bpf_verifier_env *env)
 	struct bpf_prog *prog = env->prog;
 	unsigned long addr = (unsigned long) prog->aux->trampoline->func.addr;
 
-	if (within_error_injection_list(addr))
-		return 0;
-
 	/* This is expected to be cleaned up in the future with the KRSI effort
 	 * introducing the LSM_HOOK macro for cleaning up lsm_hooks.h.
 	 */
-	if (!strncmp(SECURITY_PREFIX, prog->aux->attach_func_name,
-		     sizeof(SECURITY_PREFIX) - 1)) {
-
-		if (!capable(CAP_MAC_ADMIN))
-			return -EPERM;
-
+	if (within_error_injection_list(addr) ||
+	    !strncmp(SECURITY_PREFIX, prog->aux->attach_func_name,
+		     sizeof(SECURITY_PREFIX) - 1))
 		return 0;
-	}
 
 	verbose(env, "fmod_ret attach_btf_id %u (%s) is not modifiable\n",
 		prog->aux->attach_btf_id, prog->aux->attach_func_name);