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

On Thu, Mar 05, 2020 at 08:43:11AM -0500, Stephen Smalley wrote:

On Wed, Mar 4, 2020 at 2:20 PM KP Singh [off-list ref] wrote:

quoted

From: KP Singh <redacted>

- 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/ (local)

- 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 <redacted>
Acked-by: Andrii Nakryiko <redacted>
---

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.