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

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Date:   Wed, 17 Apr 2019 09:42:11 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Oleg Nesterov <oleg@...hat.com>, Paul Moore <paul@...l-moore.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>,
        casey@...aufler-ca.com
Subject: Re: kernel BUG at kernel/cred.c:434!

On 4/17/2019 9:27 AM, Oleg Nesterov 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?
>
> In fact, I think that something is already wrong if it is not called by
> user-space directly. Too late to ask, but why is this /proc/self/attr/
> magic not implemented via syscall(s) ?

Shell scripts, for one thing. It's a straightforward and appropriate
use of the /proc interface. System calls would require additional change
to existing programs, whereas using the /proc interface allows a good
deal to be done in the containing scripts.