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From:   Kees Cook <keescook@chromium.org>
Date:   Mon, 15 Apr 2019 22:40:52 -0500
Message-ID: <CAGXu5jKYmoaR6bxW82ogN4iOeKi8AtEwkvVvCXd_gn3CCwFU2Q@mail.gmail.com>
Subject: Re: kernel BUG at kernel/cred.c:434!
To:     Paul Moore <paul@paul-moore.com>
Cc:     Oleg Nesterov <oleg@redhat.com>,
        Casey Schaufler <casey@schaufler-ca.com>,
        "chengjian (D)" <cj.chengjian@huawei.com>,
        NeilBrown <neilb@suse.com>,
        Anna Schumaker <Anna.Schumaker@netapp.com>,
        Kees Cook <keescook@chromium.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        "Xiexiuqi (Xie XiuQi)" <xiexiuqi@huawei.com>,
        Li Bin <huawei.libin@huawei.com>,
        Jason Yan <yanaijie@huawei.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Linux Security Module list 
        <linux-security-module@vger.kernel.org>,
        SELinux <selinux@vger.kernel.org>
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On Mon, Apr 15, 2019 at 11:20 AM Paul Moore <paul@paul-moore.com> wrote:
>
> On Mon, Apr 15, 2019 at 11:05 AM Oleg Nesterov <oleg@redhat.com> wrote:
> > On 04/15, Paul Moore wrote:
> > >
> > > On Mon, Apr 15, 2019 at 9:43 AM Oleg Nesterov <oleg@redhat.com> wrote:
> > > > Well, acct("/proc/self/attr/current") doesn't look like a good idea, but I do
> > > > not know where should we put the additional check... And probably
> > > > "echo /proc/self/attr/current > /proc/sys/kernel/core_pattern" can hit the
> > > > same problem, do_coredump() does override_creds() too.
> > > >
> > > > May be just add
> > > >
> > > >         if (current->cred != current->real_cred)
> > > >                 return -EACCES;
> > > >
> > > > into proc_pid_attr_write(), I dunno.
> > >
> > > Is the problem that do_acct_process() is calling override_creds() and
> > > the returned/old credentials are being freed before do_acct_process()
> > > can reinstall the creds via revert_creds()?  Presumably because the
> > > process accounting is causing the credentials to be replaced?
> >
> > Afaics, the problem is that do_acct_process() does override_creds() and
> > then __kernel_write(). Which calls proc_pid_attr_write(), which in turn calls
> > selinux_setprocattr(), which does another prepare_creds() + commit_creds();
> > and commit_creds() hits
> >
> >         BUG_ON(task->cred != old);
>
> Gotcha.  In the process of looking at the backtrace I forgot about the
> BUG_ON() at the top of the oops message.
>
> I wonder what terrible things would happen if we changed the BUG_ON()
> in commit_creds to simple returning an error an error code to the
> caller.  There is a warning/requirement in commit_creds() function
> header comment that it should always return 0.

Would callers be expected to call abort_creds() on failure? There are
a number of places where it'd need fixing up. And would likely be best
with a __must_check marking.

It seems like avoiding the pathological case might be simpler?

-- 
Kees Cook