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

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Date:   Mon, 15 Apr 2019 12:20:08 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Casey Schaufler <casey@...aufler-ca.com>,
        "chengjian (D)" <cj.chengjian@...wei.com>, neilb@...e.com,
        Anna.Schumaker@...app.com, keescook@...omium.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        viro@...iv.linux.org.uk,
        "Xiexiuqi (Xie XiuQi)" <xiexiuqi@...wei.com>,
        Li Bin <huawei.libin@...wei.com>, yanaijie@...wei.com,
        peterz@...radead.org, mingo@...hat.com,
        Linux Security Module list 
        <linux-security-module@...r.kernel.org>, selinux@...r.kernel.org
Subject: Re: kernel BUG at kernel/cred.c:434!

On Mon, Apr 15, 2019 at 11:05 AM Oleg Nesterov <oleg@...hat.com> wrote:
> On 04/15, Paul Moore wrote:
> >
> > On Mon, Apr 15, 2019 at 9:43 AM Oleg Nesterov <oleg@...hat.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.

-- 
paul moore
www.paul-moore.com