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From: "Serge E. Hallyn" <serge@hallyn.com>
To: Casey Schaufler <casey@schaufler-ca.com>
Cc: "Serge E. Hallyn" <serge@hallyn.com>,
	Micah Morton <mortonm@chromium.org>,
	Kees Cook <keescook@chromium.org>,
	jmorris@namei.org, linux-security-module@vger.kernel.org
Subject: Re: [PATCH] LSM: add SafeSetID module that gates setid calls
Date: Thu, 1 Nov 2018 15:56:36 +0000
Message-ID: <20181101155636.GA11038@mail.hallyn.com> (raw)
In-Reply-To: <2d8b8ee8-0087-b9c2-f20d-90108e0b18bd@schaufler-ca.com>

Quoting Casey Schaufler (casey@schaufler-ca.com):
> On 10/31/2018 11:13 PM, Serge E. Hallyn wrote:
> > On Wed, Oct 31, 2018 at 06:12:46PM -0700, Micah Morton wrote:
> >> On Wed, Oct 31, 2018 at 3:37 PM Casey Schaufler <casey@schaufler-ca.com> wrote:
> >>> On 10/31/2018 2:57 PM, Kees Cook wrote:
> >>>> On Wed, Oct 31, 2018 at 2:02 PM, Serge E. Hallyn <serge@hallyn.com> wrote:
> >>>>> Just to be sure - your end-goal is to have a set of tasks which have
> >>>>> some privileges, including CAP_SETUID, but which cannot transition to
> >>>>> certain uids, perhaps including root?
> >> Correct, only whitelisted uids can be switched to. This only pertains
> >> to CAP_SETUID, other capabilities are not affected.
> >>
> >>>> AIUI, the issue is that CAP_SETUID is TOO permissive. Instead, run
> >>>> _without_ CAP_SETUID and still allow whitelisted uid transitions.
> >> Kees is right that this LSM only pertains to a single capability:
> >> CAP_SETUID (future work could tackle CAP_SETGID in the same fashion)
> >> -- although the idea here is to put in per-user limitations on what a
> >> process running as that user can do even when it _has_ CAP_SETUID. So
> >> it doesn't grant any extra privileges to processes that don't have
> >> CAP_SETUID, only restricts processes that _do_ have CAP_SETUID if the
> >> user they are running under is restricted.
> >>
> >>> I don't like that thought at all at all. You need CAP_SETUID for
> >>> some transitions but not all. I can call setreuid() and restore
> >>> the saved UID to the effective UID. If this LSM works correctly
> >>> (I haven't examined it carefully yet) it should prevent restoring
> >>> the effective UID if there isn't an appropriate whitelist entry.
> >> Yep, thats how it works. The idea here is that you still need
> >> CAP_SETUID for all transitions, regardless of whether whitelist
> >> policies exist or not.
> >>
> >>> It also violates the "additional restriction" model of LSMs.
> > Does it, or does the fact that CAP_SETUID is still required in order
> > to change uids address that?
> 
> Yes, it does. Reading Kees' response had me a little concerned.
> 
> >>> That has the potential to introduce a failure when a process tries
> >>> to give up privilege. If 0:1000 isn't on the whitelist but 1000:0
> >> As above, if a process drops CAP_SETUID it wouldn't be able to do any
> >> transitions (if this is what you mean by give up privilege). The
> >> whitelist is a one-way policy so if one wanted to restrict user 123
> >> but let it switch to 456 and back, 2 policies would need to be added:
> >> 123 -> 456 and 456 -> 123.
> >>
> >>> is Bad Things can happen. A SUID root program would be unable to
> >>> give up its privilege by going back to the real UID in this case.
> > Yes, this was the root cause of the "sendmail capabilities bug"
> 
> I'm very familiar with that particular bug, as Bob Mende's
> work to convert sendmail to using capabilities was done for
> a project I owned. The blowback against all things security
> was pretty intense.
> 
> >  - a
> > privileged daemon which could be made to run with slightly less
> > privilege in such a way that it failed to drop privilege, then continued
> > ot run with some privilege.
> >
> > But the key trigger there was that an unprivileged task could prevent
> > the more privileged task from dropping its privilege.
> >
> > Is that the case here?
> 
> I think it is reasonably safe to assume that there
> are many instances of programs that don't handle errors
> from setreuid() in the reset case. Without privilege
> setreuid() can be used to swap effective and real UIDs.
> 
> >   It might be...  If one of the uid-restricted
> > tasks running with CAP_SETUID runs a filter over some malicious data
> > which forces it to run a program which intends to change its uid and
> > fails to detect that that failed.  It's not quite as cut-and-dried
> > though, and if we simply do not allow uid 0 to be in the set of uids,
> > that may prevent any such cases.
> 
> Alas, UID 0 is not the only case we have to worry about.
> If I run a program owned by tss (Trousers) with the setuid
> bit set it will change the effective UID to tss. If this
> program expects to switch effective UID back to me and
> the SafeSetID whitelist prevents it, Bad Things may happen
> even though no capabilities or root privilege where ever
> involved.

Yes, but I don't think an unprivileged user can make that happen.
If you look at the patch, you require cap_sys_admin againt your
user namespace in order to limit the uid range.  So either you
were privileged to begin with, or you create a new user namespace.
If you create a new userns, you can only map uids which are delegated
to you - presumably not tss - into that namespace.

> It would be easy for an inexperienced or malicious admin to
> include cschaufler:tss in the whitelist but miss on adding
> tss:cschaufler.

Well, it's also pretty easy for an admin to add 0 or tss into
serge's delegated mappings in /etc/subuid, I suppose...

Now I hadn't noticed the one-way directional nature of these
whitelist entries.  I'd been asuming there was just a set of
ids it was allowed to transition to it.  Not sure which is
better, I can see pros/cons to both.

  reply index

Thread overview: 88+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-31 15:28 mortonm
2018-10-31 21:02 ` Serge E. Hallyn
2018-10-31 21:57   ` Kees Cook
2018-10-31 22:37     ` Casey Schaufler
2018-11-01  1:12       ` Micah Morton
2018-11-01  6:13         ` Serge E. Hallyn
2018-11-01 15:39           ` Casey Schaufler
2018-11-01 15:56             ` Serge E. Hallyn [this message]
2018-11-01 16:18             ` Micah Morton
2018-11-01  6:07   ` Serge E. Hallyn
2018-11-01 16:11     ` Micah Morton
2018-11-01 16:22       ` Micah Morton
2018-11-01 16:41       ` Micah Morton
2018-11-01 17:08       ` Casey Schaufler
2018-11-01 19:52         ` Micah Morton
2018-11-02 16:05           ` Casey Schaufler
2018-11-02 17:12             ` Micah Morton
2018-11-02 18:19               ` Casey Schaufler
2018-11-02 18:30                 ` Serge E. Hallyn
2018-11-02 19:02                   ` Casey Schaufler
2018-11-02 19:22                     ` Serge E. Hallyn
2018-11-08 20:53                       ` Micah Morton
2018-11-08 21:34                         ` Casey Schaufler
2018-11-09  0:30                           ` Micah Morton
2018-11-09 23:21                             ` [PATCH] LSM: generalize flag passing to security_capable mortonm
2018-11-21 16:54                             ` [PATCH] LSM: add SafeSetID module that gates setid calls mortonm
2018-12-06  0:08                               ` Kees Cook
2018-12-06 17:51                                 ` Micah Morton
2019-01-11 17:13                                 ` [PATCH v2] " mortonm
2019-01-15  0:38                                   ` Kees Cook
2019-01-15 18:04                                     ` [PATCH v3 1/2] LSM: mark all set*uid call sites in kernel/sys.c mortonm
2019-01-15 19:34                                       ` Kees Cook
2019-01-15 18:04                                     ` [PATCH v3 2/2] LSM: add SafeSetID module that gates setid calls mortonm
2019-01-15 19:44                                       ` Kees Cook
2019-01-15 21:50                                         ` [PATCH v4 " mortonm
2019-01-15 22:32                                           ` Kees Cook
2019-01-16 15:46                                             ` [PATCH v5 " mortonm
2019-01-16 16:10                                               ` Casey Schaufler
2019-01-22 20:40                                                 ` Micah Morton
2019-01-22 22:28                                                   ` James Morris
2019-01-22 22:40                                                     ` Micah Morton
2019-01-22 22:42                                                       ` [PATCH v3 1/2] " mortonm
2019-01-25 15:51                                                         ` Micah Morton
2019-01-25 20:15                                               ` [PATCH v5 2/2] " James Morris
2019-01-25 21:06                                                 ` Micah Morton
2019-01-28 19:47                                                   ` Micah Morton
2019-01-28 19:56                                                     ` Kees Cook
2019-01-28 20:09                                                       ` James Morris
2019-01-28 20:19                                                       ` Micah Morton
2019-01-28 20:30                                                         ` [PATCH] LSM: Add 'name' field for SafeSetID in DEFINE_LSM mortonm
2019-01-28 22:12                                                           ` James Morris
2019-01-28 22:33                                                         ` [PATCH v5 2/2] LSM: add SafeSetID module that gates setid calls Micah Morton
2019-01-29 17:25                                                           ` James Morris
2019-01-29 21:14                                                             ` Micah Morton
2019-01-30  7:15                                                               ` Kees Cook
2019-02-06 19:03                                                                 ` [PATCH] LSM: SafeSetID: add selftest mortonm
2019-02-06 19:26                                                                   ` Edwin Zimmerman
2019-02-07 21:54                                                                     ` Micah Morton
2019-02-12 19:01                                                                   ` James Morris
2019-01-15 21:58                                         ` [PATCH v3 2/2] LSM: add SafeSetID module that gates setid calls Micah Morton
2019-01-15 19:49                                     ` [PATCH v2] " Micah Morton
2019-01-15 19:53                                       ` Kees Cook
2019-01-15  4:07                                   ` James Morris
2019-01-15 19:42                                     ` Micah Morton
2018-11-02 19:28                 ` [PATCH] " Micah Morton
2018-11-06 19:09                 ` [PATCH v2] " mortonm
2018-11-06 20:59       ` [PATCH] " James Morris
2018-11-06 21:21         ` [PATCH v3] " mortonm
2018-11-02 18:07 ` [PATCH] " Stephen Smalley
2018-11-02 19:13   ` Micah Morton
2018-11-19 18:54   ` [PATCH] [PATCH] LSM: generalize flag passing to security_capable mortonm
2018-12-13 22:29     ` Micah Morton
2018-12-13 23:09       ` Casey Schaufler
2018-12-14  0:05         ` Micah Morton
2018-12-18 22:37         ` [PATCH v2] " mortonm
2019-01-07 17:55           ` Micah Morton
2019-01-07 18:16             ` Casey Schaufler
2019-01-07 18:36               ` Micah Morton
2019-01-07 18:46                 ` Casey Schaufler
2019-01-07 19:02                   ` Micah Morton
2019-01-07 22:57                     ` [PATCH v3] " mortonm
2019-01-07 23:13           ` [PATCH v2] " Kees Cook
2019-01-08  0:10             ` [PATCH v4] " mortonm
2019-01-08  0:20               ` Kees Cook
2019-01-09 18:39                 ` Micah Morton
2019-01-10 22:31               ` James Morris
2019-01-10 23:03                 ` Micah Morton
2019-01-08  0:10             ` [PATCH v2] " Micah Morton

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