From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andy Lutomirski Subject: Re: [PATCH v3 2/2] modules:capabilities: add a per-task modules autoload restriction Date: Fri, 21 Apr 2017 17:00:36 -0700 Message-ID: References: <1492640420-27345-1-git-send-email-tixxdz@gmail.com> <1492640420-27345-3-git-send-email-tixxdz@gmail.com> <5f07b121-62e1-b9a8-4faa-f6630b24ff48@schaufler-ca.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Return-path: In-Reply-To: <5f07b121-62e1-b9a8-4faa-f6630b24ff48@schaufler-ca.com> Sender: linux-kernel-owner@vger.kernel.org To: Casey Schaufler Cc: Kees Cook , Andy Lutomirski , Djalal Harouni , Linux Kernel Mailing List , Andrew Morton , "Serge E. Hallyn" , "kernel-hardening@lists.openwall.com" , LSM List , Linux API , Dongsu Park , James Morris , Paul Moore , Tetsuo Handa , Greg Kroah-Hartman , Jonathan Corbet , Jessica Yu , Rusty Russell , Arnaldo Carvalho de Melo List-Id: linux-api@vger.kernel.org On Fri, Apr 21, 2017 at 4:52 PM, Casey Schaufler wrote: > On 4/21/2017 4:28 PM, Andy Lutomirski wrote: >> On Fri, Apr 21, 2017 at 4:19 PM, Kees Cook wrote: >>> On Wed, Apr 19, 2017 at 7:41 PM, Andy Lutomirski wrote: >>>> On Wed, Apr 19, 2017 at 4:43 PM, Kees Cook wrote: >>>>> On Wed, Apr 19, 2017 at 4:15 PM, Andy Lutomirski wrote: >>>>>> On Wed, Apr 19, 2017 at 3:20 PM, Djalal Harouni wrote: >>>>>>> +/* Sets task's modules_autoload */ >>>>>>> +static inline int task_set_modules_autoload(struct task_struct *task, >>>>>>> + unsigned long value) >>>>>>> +{ >>>>>>> + if (value > MODULES_AUTOLOAD_DISABLED) >>>>>>> + return -EINVAL; >>>>>>> + else if (task->modules_autoload > value) >>>>>>> + return -EPERM; >>>>>>> + else if (task->modules_autoload < value) >>>>>>> + task->modules_autoload = value; >>>>>>> + >>>>>>> + return 0; >>>>>>> +} >>>>>> This needs to be more locked down. Otherwise someone could set this >>>>>> and then run a setuid program. Admittedly, it would be quite odd if >>>>>> this particular thing causes a problem, but the issue exists >>>>>> nonetheless. >>>>> Eeeh, I don't agree this needs to be changed. APIs provided by modules >>>>> are different than the existing privilege-manipulation syscalls this >>>>> concern stems from. Applications are already forced to deal with >>>>> things being missing like this in the face of it simply not being >>>>> built into the kernel. >>>>> >>>>> Having to hide this behind nnp seems like it'd reduce its utility... >>>>> >>>> I think that adding an inherited boolean to task_struct that can be >>>> set by unprivileged tasks and passed to privileged tasks is a terrible >>>> precedent. Ideally someone would try to find all the existing things >>>> like this and kill them off. >>> (Tristate, not boolean, but yeah.) >>> >>> I see two others besides seccomp and nnp: >>> >>> PR_MCE_KILL >> Well, that's interesting. That should presumably be reset on setuid >> exec or something. >> >>> PR_SET_THP_DISABLE >> Um. At least that's just a performance issue. >> >>> I really don't think this needs nnp protection. >>> >>>> I agree that I don't see how one would exploit this particular >>>> feature, but I still think I dislike the approach. This is a slippery >>>> slope to adding a boolean for perf_event_open(), unshare(), etc, and >>>> we should solve these for real rather than half-arsing them IMO. >>> I disagree (obviously); this would be protecting the entire module >>> autoload attack surface. That's hardly a specific control, and it's a >>> demonstrably needed flag. >>> >> The list is just going to get longer. We should probably have controls for: >> >> - Use of perf. Unclear how fine grained they should be. >> >> - Creation of new user namespaces. Possibly also use of things like >> iptables without global privilege. >> >> - Ability to look up tasks owned by different uids (or maybe other >> tasks *at all*) by pid/tid. Conceptually, this is easy. The API is >> the only hard part, I think. >> >> - Ability to bind ports, maybe? > > One of my longer term (i.e. after stacking) projects > is to create sensible access control on ports. Why shouldn't > they have owners and mode bits (or ACLs, if you prefer) > or real names. I kind of think we should be able to eliminate > the need for dbus without resorting to kdbus. My implicit_rights concept gives any type of access control you can use on inodes because they *are* inodes. So you get ACLs, etc. Brief summary for those who didn't read my old email: We add a new kind of filesystem object called a "right". It's a special kind of socket inode that can't be bound or connected but is instead created by a new syscall. It has a name, so "port:1234" might be a name of a right. To use an implicit right, you do whatever syscall you would do normally. The kernel looks for a right object at /dev/implicit_rights/. If that object exists, is a right of the correct type (i.e. the right's name matches ) and you have execute access, you win. Otherwise you lose. To avoid breaking existing distros, for things like modules_autoload, you would set a sysctl /proc/sys/kernel/required_implicit_rights/modules_autoload=1. With that set, to autoload a module without CAP_SYS_MODULE, you need the /dev/implicit_rights/modules_autoload. > > So I don't like the idea of treating that as a special case. > I'd rather see ports controlled properly. (Of course, the > SELinux crowd will point out they have this handled, but I > remain unconvinced of the overall solution) Agreed. But I think we should address all of these things together.