linux-kernel - Re: kernel BUG at kernel/cred.c:434!

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Date:   Wed, 17 Apr 2019 17:24:56 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Paul Moore <paul@...l-moore.com>, Oleg Nesterov <oleg@...hat.com>,
        john.johansen@...onical.com
Cc:     "chengjian (D)" <cj.chengjian@...wei.com>,
        Kees Cook <keescook@...omium.org>, NeilBrown <neilb@...e.com>,
        Anna Schumaker <Anna.Schumaker@...app.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        "Xiexiuqi (Xie XiuQi)" <xiexiuqi@...wei.com>,
        Li Bin <huawei.libin@...wei.com>,
        Jason Yan <yanaijie@...wei.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Linux Security Module list 
        <linux-security-module@...r.kernel.org>,
        SELinux <selinux@...r.kernel.org>,
        Yang Yingliang <yangyingliang@...wei.com>
Subject: Re: kernel BUG at kernel/cred.c:434!

On 4/17/2019 4:39 PM, Paul Moore wrote:
> On Wed, Apr 17, 2019 at 12:27 PM Oleg Nesterov <oleg@...hat.com> wrote:
>> On 04/17, Paul Moore wrote:
>>> On Wed, Apr 17, 2019 at 10:57 AM Oleg Nesterov <oleg@...hat.com> wrote:
>>>> On 04/17, Paul Moore wrote:
>>>>> I'm tempted to simply return an error in selinux_setprocattr() if
>>>>> the task's credentials are not the same as its real_cred;
>>>> What about other modules? I have no idea what smack_setprocattr() is,
>>>> but it too does prepare_creds/commit creds.
>>>>
>>>> it seems that the simplest workaround should simply add the additional
>>>> cred == real_cred into proc_pid_attr_write().
>>> Yes, that is simple, but I worry about what other LSMs might want to
>>> do.  While I believe failing if the effective creds are not the same
>>> as the real_creds is okay for SELinux (possibly Smack too), I worry
>>> about what other LSMs may want to do.  After all,
>>> proc_pid_attr_write() doesn't change the the creds itself, that is
>>> something the specific LSMs do.
>> Yes, but if proc_pid_attr_write() is called with cred != real_cred then
>> something is already wrong?
> True, or at least I would think so.
>
> Looking at the current tree there are three LSMs which implement
> setprocattr hooks: SELinux, Smack, and AppArmor.  I know Casey has
> already mentioned that he wasn't able to trigger the problem in Smack,
> but looking at smack_setprocattr() I see the similar commit_creds()
> usage so I would expect the same problem in Smack; what say you Casey?

I say that my test program runs without ill effect. I call acct()
with "/proc/self/attr/current", which succeeds and enables accounting
just like it is supposed to. I then have the program open
"/proc/self/attr/current" and read it, all of which goes swimmingly.
When Smack frees a cred it usually does not free any memory of its
own, so it is conceivable that I'm just getting lucky. Or, I may not
have sufficient debug enabled.

>   Looking at apparmor_setprocattr(), it appears that it too could end
> up calling commit_creds() via aa_set_current_hat().
>
> Since it looks like all three LSMs which implement the setprocattr
> hook are vulnerable I'm open to the idea that proc_pid_attr_write() is
> a better choice for the cred != read_cred check, but I would want to
> make sure John and Casey are okay with that.
>
> John?
>
> Casey?

I'm fine with the change going into proc_pid_attr_write().