Commit 6ba43b76 authored by KP Singh's avatar KP Singh Committed by Alexei Starovoitov
Browse files

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: default avatarKP Singh <kpsingh@google.com>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Acked-by: default avatarAndrii Nakryiko <andriin@fb.com>
Acked-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200304191853.1529-5-kpsingh@chromium.org
parent ae240823

kernel/bpf/btf.c

+20 −8
Original line number Diff line number Diff line
@@ -3710,14 +3710,26 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type, 
		nr_args--;

	}



	if ((prog->expected_attach_type == BPF_TRACE_FEXIT ||

	     prog->expected_attach_type == BPF_MODIFY_RETURN) &&

	    arg == nr_args) {

	if (arg == nr_args) {

		if (prog->expected_attach_type == BPF_TRACE_FEXIT) {

			if (!t)

			/* Default prog with 5 args. 6th arg is retval. */

				return true;

		/* function return type */

			t = btf_type_by_id(btf, t->type);

		} else if (prog->expected_attach_type == BPF_MODIFY_RETURN) {

			/* For now the BPF_MODIFY_RETURN can only be attached to

			 * functions that return an int.

			 */

			if (!t)

				return false;



			t = btf_type_skip_modifiers(btf, t->type, NULL);

			if (!btf_type_is_int(t)) {

				bpf_log(log,

					"ret type %s not allowed for fmod_ret\n",

					btf_kind_str[BTF_INFO_KIND(t->info)]);

				return false;

			}

		}

	} else if (arg >= nr_args) {

		bpf_log(log, "func '%s' doesn't have %d-th argument\n",

			tname, arg + 1);

kernel/bpf/verifier.c

+31 −0
Original line number Diff line number Diff line
@@ -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)

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