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

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Date:   Wed, 17 Apr 2019 19:39:19 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Oleg Nesterov <oleg@...hat.com>,
        Casey Schaufler <casey@...aufler-ca.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 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?
 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?

-- 
paul moore
www.paul-moore.com