Re: [PATCH] ptrace: being capable wrt a process requires mapped uids/gids

[Serge E. Hallyn]

On Sat, Dec 26, 2015 at 09:55:50PM +0100, Jann Horn wrote:
> On Sat, Dec 26, 2015 at 02:23:45PM -0600, Serge E. Hallyn wrote:
> > On Sat, Dec 26, 2015 at 02:10:38AM +0100, Jann Horn wrote:
> > > On Sat, Dec 12, 2015 at 09:12:41PM +0100, Jann Horn wrote:
> > > > With this change, the entering process can first enter the
> > > > namespace and then safely inspect the namespace's
> > > > properties, e.g. through /proc/self/{uid_map,gid_map},
> > > > assuming that the namespace owner doesn't have access to
> > > > uid 0.
> > > 
> > > Actually, I think I missed something there. Well, at least it
> > > should not directly lead to a container escape.
> > > 
> > > 
> > > > -static int ptrace_has_cap(struct user_namespace *ns, unsigned int mode)
> > > > +static bool ptrace_has_cap(const struct cred *tcred, unsigned int mode)
> > > >  {
> > > > +	struct user_namespace *tns = tcred->user_ns;
> > > > +	struct user_namespace *curns = current_cred()->user_ns;
> > > > +
> > > > +	/* When a root-owned process enters a user namespace created by a
> > > > +	 * malicious user, the user shouldn't be able to execute code under
> > > > +	 * uid 0 by attaching to the root-owned process via ptrace.
> > > > +	 * Therefore, similar to the capable_wrt_inode_uidgid() check,
> > > > +	 * verify that all the uids and gids of the target process are
> > > > +	 * mapped into the current namespace.
> > > > +	 * No fsuid/fsgid check because __ptrace_may_access doesn't do it
> > > > +	 * either.
> > > > +	 */
> > > > +	if (!kuid_has_mapping(curns, tcred->euid) ||
> > > > +			!kuid_has_mapping(curns, tcred->suid) ||
> > > > +			!kuid_has_mapping(curns, tcred->uid)  ||
> > > > +			!kgid_has_mapping(curns, tcred->egid) ||
> > > > +			!kgid_has_mapping(curns, tcred->sgid) ||
> > > > +			!kgid_has_mapping(curns, tcred->gid))
> > > > +		return false;
> > > > +
> > > >  	if (mode & PTRACE_MODE_NOAUDIT)
> > > > -		return has_ns_capability_noaudit(current, ns, CAP_SYS_PTRACE);
> > > > +		return has_ns_capability_noaudit(current, tns, CAP_SYS_PTRACE);
> > > >  	else
> > > > -		return has_ns_capability(current, ns, CAP_SYS_PTRACE);
> > > > +		return has_ns_capability(current, tns, CAP_SYS_PTRACE);
> > > >  }
> > > 
> > > If the namespace owner can run code in the init namespace, the kuids are
> > > mapped into curns but he is still capable wrt the target namespace.
> > > 
> > > I think a proper fix should first determine the highest parent of
> > > tcred->user_ns in which the caller still has privs, then do the
> > > kxid_has_mapping() checks in there.
> > 
> > Hi,
> > 
> > I don't quite follow what you are concerned about.  Based on the new
> > patch you sent, I assume it's not the case where the tcred's kuid is
> > actually mapped into the container.  So is it the case where I
> > unshare a userns which unshares a userns, then setns from the grandparent
> > into the child?  And if so, the concern is that if the setns()ing task's
> > kuid is mappable all along into the grandhild, then container root should
> > be able to ptrace it?
> 
> Consider the following scenario:
> 
> init_user_ns has a child namespace (I'll call it child_ns).
> child_ns is owned by an attacker (child_ns->owner == attacker_kuid).
> The attacking process has current_cred()->euid == attacker_kuid and lives
> in init_user_ns (which means it's capable in child_ns).

Ah, right.  Special.

Thanks.

-serge
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