diff mbox series

[bpf-next,v4,4/7] bpf: Attachment verification for BPF_MODIFY_RETURN

Message ID 20200304191853.1529-5-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, 7:18 p.m. UTC
From: KP Singh <kpsingh@google.com>

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

Comments

Stephen Smalley March 5, 2020, 1:43 p.m. UTC | #1
On Wed, Mar 4, 2020 at 2:20 PM KP Singh <kpsingh@chromium.org> wrote:
>
> From: KP Singh <kpsingh@google.com>
>
> - 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>
> Acked-by: Andrii Nakryiko <andriin@fb.com>
> ---
> 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
> @@ -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;

CAP_MAC_ADMIN was originally introduced for Smack and is not
all-powerful wrt SELinux, so this is not a sufficient check for
SELinux.
We would want an actual security hook called here so we can implement
a specific check over userspace
being able to attach BPF progs to LSM hooks.  CAP_MAC_ADMIN has other
connotations to SELinux (presently the
ability to set/get file security labels that are not known to the
currently loaded policy).
Alexei Starovoitov March 5, 2020, 5:21 p.m. UTC | #2
On Thu, Mar 05, 2020 at 08:43:11AM -0500, Stephen Smalley wrote:
> On Wed, Mar 4, 2020 at 2:20 PM KP Singh <kpsingh@chromium.org> wrote:
> >
> > From: KP Singh <kpsingh@google.com>
> >
> > - 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>
> > Acked-by: Andrii Nakryiko <andriin@fb.com>
> > ---
> > 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
> > @@ -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;
> 
> CAP_MAC_ADMIN was originally introduced for Smack and is not
> all-powerful wrt SELinux, so this is not a sufficient check for
> SELinux.

I think you're misunderstanding the intent here.
This facility is just a faster version of kprobe based fault injection.
It doesn't care about LSM. Security is not a focus here.
It can fault inject in a lot of places in the kernel: syscalls,
kmalloc, page_alloc, fs internals, etc
I think above capable() check created this confusion and
we should remove it.
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)