On November 17, 2014 1:46:59 PM EST, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: >On Mon, Nov 17, 2014 at 10:31 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> >wrote: >> On Mon, Nov 17, 2014 at 10:06 AM, Casey Schaufler >> <casey@xxxxxxxxxxxxxxxx> wrote: >>> On 11/15/2014 1:01 AM, Josh Triplett wrote: >>>> Currently, unprivileged processes (without CAP_SETGID) cannot call >>>> setgroups at all. In particular, processes with a set of >supplementary >>>> groups cannot further drop permissions without obtaining elevated >>>> permissions first. >>> >>> Has anyone put any thought into how this will interact with >>> POSIX ACLs? I don't see that anywhere in the discussion. >> >> That means that user namespaces are a problem, too, and we need to >fix >> it. Or we should add some control to turn unprivileged user >namespace >> creation on and off and document that turning it on defeats POSIX >ACLs >> with a group entry that is more restrictive than the other entry. >> > >This is a significant enough issue that I posted it to oss-security: > >http://www.openwall.com/lists/oss-security/2014/11/17/19 > >It's not at all obvious to me how to fix it. We could disallow userns >creation of any supplementary groups don't match fsuid, or we could >keep negative-only groups around in the userns. > >It may be worth adding a sysctl to change the behavior, too. IMO it's >absurd to use groups to deny permissions that are otherwise available. There is an obvious user namespace fix. Don't allow dropping supplemental groups that are not mapped. That will require a little bit of fancy footwork if you want to play with supplemental groups in your unprivileged user namespace. I would like to get a grip on what hoops would be required before we add the additional restriction. Possibly something as simple as calling sg. I also want to look at what Tizen and any other concrete pieces of code I can find using this negative permission pattern are actually doing. Bugs definitely exist, but I have this erie feeling that the bugs may be in instances of userspace using this negative group permission pattern. I think we may have a hideous case of one setuid binary defeating a privilege check of another piece of code. This issue looks like it is worth a full scale investigation. Sigh. Eric -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
