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

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Message-ID: <20190417162723.GK32622@redhat.com>
Date:   Wed, 17 Apr 2019 18:27:24 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Paul Moore <paul@...l-moore.com>
Cc:     "chengjian (D)" <cj.chengjian@...wei.com>,
        Kees Cook <keescook@...omium.org>,
        Casey Schaufler <casey@...aufler-ca.com>,
        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 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) ?

But, Paul, this is up to you. I don't understand this all even remotely.

Oleg.