bpf: Attachment verification for BPF_MODIFY_RETURN

- Allow BPF_MODIFY_RETURN attachment only to functions that are:

    * Whitelisted for error injection by checking
      within_error_injection_list. Similar discussions happened for the
      bpf_override_return helper.

    * security hooks, this is expected to be cleaned up with the LSM
      changes after the KRSI patches introduce the LSM_HOOK macro:

        https://lore.kernel.org/bpf/20200220175250.10795-1-kpsingh@chromium.org/

- The attachment is currently limited to functions that return an int.
  This can be extended later other types (e.g. PTR).

Signed-off-by: KP Singh <kpsingh@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200304191853.1529-5-kpsingh@chromium.org

1 files changed, 31 insertions, 0 deletions

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 2460c8e6b5be..ae32517d4ccd 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -19,6 +19,7 @@
 #include <linux/sort.h>
 #include <linux/perf_event.h>
 #include <linux/ctype.h>
+#include <linux/error-injection.h>
 
 #include "disasm.h"
 
@@ -9800,6 +9801,33 @@ static int check_struct_ops_btf_id(struct bpf_verifier_env *env)
 
 	return 0;
 }
+#define SECURITY_PREFIX "security_"
+
+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;
+
+		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);
+
+	return -EINVAL;
+}
 
 static int check_attach_btf_id(struct bpf_verifier_env *env)
 {
@@ -10000,6 +10028,9 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
 		}
 		tr->func.addr = (void *)addr;
 		prog->aux->trampoline = tr;
+
+		if (prog->expected_attach_type == BPF_MODIFY_RETURN)
+			ret = check_attach_modify_return(env);
 out:
 		mutex_unlock(&tr->mutex);
 		if (ret)