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

[Serge E. Hallyn]

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?

thanks,
-serge
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