diff mbox series

[bpf-next,v2,4/7] bpf: Attachment verification for BPF_MODIFY_RETURN

Message ID 20200304015528.29661-5-kpsingh@chromium.org (mailing list archive)
State New, archived
Headers show
Series Introduce BPF_MODIFY_RET tracing progs | expand

Commit Message

KP Singh March 4, 2020, 1:55 a.m. UTC
From: KP Singh <kpsingh@google.com>

- Allow BPF_MODIFY_RETURN attachment only to functions that are:

    * Whitelisted by for error injection i.e. by checking
      within_error_injection_list. Similar disucssions happened for the
      bpf_overrie_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>
---
 kernel/bpf/btf.c      | 28 ++++++++++++++++++++--------
 kernel/bpf/verifier.c | 31 +++++++++++++++++++++++++++++++
 2 files changed, 51 insertions(+), 8 deletions(-)

Comments

Andrii Nakryiko March 4, 2020, 5:12 a.m. UTC | #1
On Tue, Mar 3, 2020 at 5:56 PM KP Singh <kpsingh@chromium.org> wrote:
>
> From: KP Singh <kpsingh@google.com>
>
> - Allow BPF_MODIFY_RETURN attachment only to functions that are:
>
>     * Whitelisted by for error injection i.e. by checking
>       within_error_injection_list. Similar disucssions happened for the
>       bpf_overrie_return helper.

2 typos: discussions and bpf_override_return ;)

>
>     * 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>
> ---

Acked-by: Andrii Nakryiko <andriin@fb.com>

>  kernel/bpf/btf.c      | 28 ++++++++++++++++++++--------
>  kernel/bpf/verifier.c | 31 +++++++++++++++++++++++++++++++
>  2 files changed, 51 insertions(+), 8 deletions(-)
>

[...]
KP Singh March 4, 2020, 3:13 p.m. UTC | #2
On 03-Mär 21:12, Andrii Nakryiko wrote:
> On Tue, Mar 3, 2020 at 5:56 PM KP Singh <kpsingh@chromium.org> wrote:
> >
> > From: KP Singh <kpsingh@google.com>
> >
> > - Allow BPF_MODIFY_RETURN attachment only to functions that are:
> >
> >     * Whitelisted by for error injection i.e. by checking
> >       within_error_injection_list. Similar disucssions happened for the
> >       bpf_overrie_return helper.
> 
> 2 typos: discussions and bpf_override_return ;)

/me bows his head in shame ;) Fixed.

 -KP

> 
> >
> >     * 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>
> > ---
> 
> Acked-by: Andrii Nakryiko <andriin@fb.com>
> 
> >  kernel/bpf/btf.c      | 28 ++++++++++++++++++++--------
> >  kernel/bpf/verifier.c | 31 +++++++++++++++++++++++++++++++
> >  2 files changed, 51 insertions(+), 8 deletions(-)
> >
> 
> [...]
diff mbox series

Patch

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 30841fb8b3c0..50080add2ab9 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -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 (!t)
-			/* Default prog with 5 args. 6th arg is retval. */
-			return true;
-		/* function return type */
-		t = btf_type_by_id(btf, t->type);
+	if (arg == nr_args) {
+		if (prog->expected_attach_type == BPF_TRACE_FEXIT) {
+			if (!t)
+				return true;
+			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);
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)