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

From: Oleg Nesterov <oleg@redhat.com>
To: Paul Moore <paul@paul-moore.com>
Cc: Casey Schaufler <casey@schaufler-ca.com>,
	"chengjian (D)" <cj.chengjian@huawei.com>,
	neilb@suse.com, Anna.Schumaker@netapp.com, keescook@chromium.org,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	viro@zeniv.linux.org.uk,
	"Xiexiuqi (Xie XiuQi)" <xiexiuqi@huawei.com>,
	Li Bin <huawei.libin@huawei.com>,
	yanaijie@huawei.com, peterz@infradead.org, mingo@redhat.com,
	Linux Security Module list 
	<linux-security-module@vger.kernel.org>,
	selinux@vger.kernel.org
Subject: Re: kernel BUG at kernel/cred.c:434!
Date: Mon, 15 Apr 2019 17:05:21 +0200	[thread overview]
Message-ID: <20190415150520.GA13257@redhat.com> (raw)
In-Reply-To: <CAHC9VhQ7ihqU0K57ctwzoHq0t0QVJPKiQQZn5TXB1FUqsvOSEQ@mail.gmail.com>

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);

Oleg.