Re: [PATCH v14 00/26] ima: Namespace IMA with audit support in IMA-ns

[Stefan Berger]

On 9/16/22 06:54, Stefan Berger wrote:
> On 9/15/22 20:56, Casey Schaufler wrote:
> > On 9/15/2022 12:31 PM, Stefan Berger wrote:
> > > The goal of this series of patches is to start with the namespacing of
> > > IMA and support auditing within an IMA namespace (IMA-ns) as the first
> > > step.
> > >
> > > In this series the IMA namespace is piggybacking on the user namespace
> > > and therefore an IMA namespace is created when a user namespace is
> > > created, although this is done late when SecurityFS is mounted inside
> > > a user namespace. The advantage of piggybacking on the user namespace
> > > is that the user namespace can provide the keys infrastructure that IMA
> > > appraisal support will need later on.
> > >
> > > We chose the goal of supporting auditing within an IMA namespace 
> > > since it
> > > requires the least changes to IMA. Following this series, auditing 
> > > within
> > > an IMA namespace can be activated by a root running the following lines
> > > that rely on a statically linked busybox to be installed on the host for
> > > execution within the minimal container environment:
> > >
> > > As root (since audit rules may now only be set by root):
> >
> > How about calling out the required capabilities? You don't need
> > to be root, you need a specific set of capabilities. It would be
> > very useful for the purposes of understanding the security value
> > of the patch set to know this.
> CAP_AUDIT_WRITE?

Currently the capabilities that are required are CAP_SYS_ADMIN, which I 
could change to CAP_AUDIT_WRITE. This would result in the following 
change to 26/26:

 diff --git a/security/integrity/ima/ima_policy.c 
b/security/integrity/ima/ima_policy.c
index 760e79bb5a34..40cd19d38f23 100644
--- a/security/integrity/ima/ima_policy.c
+++ b/security/integrity/ima/ima_policy.c
@@ -1396,15 +1396,15 @@ static unsigned int 
ima_parse_appraise_algos(char *arg)
 }

 /*
- * Either host root with CAP_SYS_ADMIN in current user namespace or
- * root with CAP_SYS_ADMIN on the host entering a namespace may set
+ * Either host root with CAP_AUDIT_WRITE in current user namespace or
+ * root with CAP_AUDIT_WRITE on the host entering a namespace may set
  * audit rules inside a namespace.
  */
 static bool may_set_audit_rule_in_ns(kuid_t uid, struct user_namespace 
*user_ns)
 {
        return (uid_eq(uid, GLOBAL_ROOT_UID) &&
-               ns_capable(user_ns, CAP_SYS_ADMIN))
-               || capable(CAP_SYS_ADMIN);
+               ns_capable(user_ns, CAP_AUDIT_WRITE))
+               || capable(CAP_AUDIT_WRITE);
 }
 static int ima_parse_rule(struct user_namespace *user_ns,
                          char *rule, struct ima_rule_entry *entry)


What this check is to prevent is that non-root users spawn a user 
namespace and set IMA-audit rules which enables them to flood the audit 
log. From what I see non-root users have the full capability set in a 
user namespace they spawn. So it has to be filtered to root maybe with 
CAP_SYS_WRITE instead of CAP_SYS_ADMIN.