On Tue, Mar 24, 2020 at 12:25 PM Casey Schaufler [off-list ref] wrote:

On 3/24/2020 7:58 AM, Stephen Smalley wrote:

quoted

On Tue, Mar 24, 2020 at 10:50 AM KP Singh [off-list ref] wrote:

quoted

On 24-Mär 10:35, Stephen Smalley wrote:

quoted

On Mon, Mar 23, 2020 at 12:46 PM KP Singh [off-list ref] wrote:

quoted

From: KP Singh <redacted>

diff --git a/kernel/bpf/bpf_lsm.c b/kernel/bpf/bpf_lsm.c
index 530d137f7a84..2a8131b640b8 100644
--- a/kernel/bpf/bpf_lsm.c
+++ b/kernel/bpf/bpf_lsm.c

@@ -9,6 +9,9 @@
 #include <linux/btf.h>
 #include <linux/lsm_hooks.h>
 #include <linux/bpf_lsm.h>
+#include <linux/jump_label.h>
+#include <linux/kallsyms.h>
+#include <linux/bpf_verifier.h>

 /* For every LSM hook  that allows attachment of BPF programs, declare a NOP
  * function where a BPF program can be attached as an fexit trampoline.

@@ -27,6 +30,32 @@ noinline __weak void bpf_lsm_##NAME(__VA_ARGS__) {}
 #include <linux/lsm_hook_names.h>
 #undef LSM_HOOK

+#define BPF_LSM_SYM_PREFX  "bpf_lsm_"
+
+int bpf_lsm_verify_prog(struct bpf_verifier_log *vlog,
+                       const struct bpf_prog *prog)
+{
+       /* Only CAP_MAC_ADMIN users are allowed to make changes to LSM hooks
+        */
+       if (!capable(CAP_MAC_ADMIN))
+               return -EPERM;

I had asked before, and will ask again: please provide an explicit LSM
hook for mediating whether one can make changes to the LSM hooks.
Neither CAP_MAC_ADMIN nor CAP_SYS_ADMIN suffices to check this for SELinux.

What do you think about:

  int security_check_mutable_hooks(void)

Do you have any suggestions on the signature of this hook? Does this
hook need to be BPF specific?

I'd do something like int security_bpf_prog_attach_security(const
struct bpf_prog *prog) or similar.
Then the security module can do a check based on the current task
and/or the prog.  We already have some bpf-specific hooks.

I *strongly* disagree with Stephen on this. KRSI and SELinux are peers.
Just as Yama policy is independent of SELinux policy so KRSI policy should
be independent of SELinux policy. I understand the argument that BDF programs
ought to be constrained by SELinux, but I don't think it's right. Further,
we've got unholy layering when security modules call security_ functions.
I'm not saying there is no case where it would be appropriate, but this is not
one of them.

I explained this previously.  The difference is that the BPF programs
are loaded from a userspace
process, not a kernel-resident module.  They already recognize there
is a difference here or
they wouldn't have the CAP_MAC_ADMIN check above in their patch.  The
problem with that
check is just that CAP_MAC_ADMIN doesn't necessarily mean fully
privileged with respect to
SELinux, which is why I want an explicit hook.  This gets a NAK from
me until there is such a hook.