From: "Serge E. Hallyn" <firstname.lastname@example.org> To: Casey Schaufler <email@example.com> Cc: "Serge E. Hallyn" <firstname.lastname@example.org>, Micah Morton <email@example.com>, Kees Cook <firstname.lastname@example.org>, email@example.com, firstname.lastname@example.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: <email@example.com> Quoting Casey Schaufler (firstname.lastname@example.org): > 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 <email@example.com> wrote: > >>> On 10/31/2018 2:57 PM, Kees Cook wrote: > >>>> On Wed, Oct 31, 2018 at 2:02 PM, Serge E. Hallyn <firstname.lastname@example.org> 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.
next prev parent 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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