[RFC PATCH bpf-next seccomp 09/12] yama: (concept) restrict seccomp-eBPF with ptrace_scope

[YiFei Zhu <zhuyifei1999@gmail.com>]

From: YiFei Zhu <yifeifz2@illinois.edu>

LSM hook seccomp_extended is made to return -EPERM if the current
process may not ptrace its children, depending on the value of
ptrace_scope and CAP_SYS_PTRACE capability.

I'm not sure if this is the right way to do it, since ptrace_scope
is about ptrace and not seccomp. Is there a better policy that would
make more sense?

Signed-off-by: YiFei Zhu <yifeifz2@illinois.edu>
---
 security/yama/yama_lsm.c | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/security/yama/yama_lsm.c b/security/yama/yama_lsm.c
index 06e226166aab..3b7b408b47a3 100644
--- a/security/yama/yama_lsm.c
+++ b/security/yama/yama_lsm.c
@@ -421,9 +421,39 @@ static int yama_ptrace_traceme(struct task_struct *parent)
 	return rc;
 }
 
+#ifdef CONFIG_SECCOMP_FILTER_EXTENDED
+static int yama_seccomp_extended(void)
+{
+	int rc = 0;
+
+	/* seccomp filter attach can only affect itself and children */
+	switch (ptrace_scope) {
+	case YAMA_SCOPE_DISABLED:
+	case YAMA_SCOPE_RELATIONAL:
+		/* No additional restrictions. */
+		break;
+	case YAMA_SCOPE_CAPABILITY:
+		rcu_read_lock();
+		if (!ns_capable(current_user_ns(), CAP_SYS_PTRACE))
+			rc = -EPERM;
+		rcu_read_unlock();
+		break;
+	case YAMA_SCOPE_NO_ATTACH:
+	default:
+		rc = -EPERM;
+		break;
+	}
+
+	return rc;
+}
+#endif /* CONFIG_SECCOMP_FILTER_EXTENDED */
+
 static struct security_hook_list yama_hooks[] __lsm_ro_after_init = {
 	LSM_HOOK_INIT(ptrace_access_check, yama_ptrace_access_check),
 	LSM_HOOK_INIT(ptrace_traceme, yama_ptrace_traceme),
+#ifdef CONFIG_SECCOMP_FILTER_EXTENDED
+	LSM_HOOK_INIT(seccomp_extended, yama_seccomp_extended),
+#endif
 	LSM_HOOK_INIT(task_prctl, yama_task_prctl),
 	LSM_HOOK_INIT(task_free, yama_task_free),
 };
-- 
2.31.1