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

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Date:   Fri, 19 Apr 2019 12:13:29 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Yang Yingliang <yangyingliang@...wei.com>
Cc:     Casey Schaufler <casey@...aufler-ca.com>,
        Oleg Nesterov <oleg@...hat.com>, john.johansen@...onical.com,
        "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>
Subject: Re: kernel BUG at kernel/cred.c:434!

On Fri, Apr 19, 2019 at 10:34 AM Yang Yingliang
<yangyingliang@...wei.com> wrote:
> On 2019/4/19 21:24, Paul Moore wrote:
> > On Thu, Apr 18, 2019 at 10:42 PM Yang Yingliang
> > <yangyingliang@...wei.com> wrote:
> >> On 2019/4/19 10:04, Paul Moore wrote:
> >>> On Wed, Apr 17, 2019 at 10:50 PM Yang Yingliang
> >>> <yangyingliang@...wei.com> wrote:
> >>>> On 2019/4/18 8:24, Casey Schaufler wrote:
> >>>>> On 4/17/2019 4:39 PM, Paul Moore wrote:
> >>>>>> 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?
> >>>>> I'm fine with the change going into proc_pid_attr_write().
> >>>> The cred != real_cred checking is not enough.
> >>>>
> >>>> Consider this situation, when doing override, cred, real_cred and
> >>>> new_cred are all same:
> >>>>
> >>>> after override_creds()    cred == real_cred == new1_cred
> >>> I'm sorry, you've lost me.  After override_creds() returns
> >>> current->cred and current->real_cred are not going to be the same,
> >>> yes?
> >> It's possible the new  cred is equal to current->real_cred and
> >> current->cred,
> >> so after overrides_creds(), they have the same value.
> > Both task_struct.cred and task_struct.real_cred are pointer values,
> > assuming that one uses prepare_creds() to allocate/initialize a new
> > cred struct for use with override_creds() then the newly created cred
> > should never be equal to task_struct.real_cred.  Am I missing
> > something, or are you thinking of something else?
>
> In do_acct_process(), file->f_cred may equal to current->real_cred, I
> confirm
> it by adding some debug message in do_acct_process() like this:

I would expect that; real_cred is the task's objective DAC
credentials, so using it for f_cred makes sense.

What we are now talking about is the task's subjective credentials,
which can be overridden via override_creds(), and are what the LSMs
change via proc_pid_attr_write().

> --- a/kernel/acct.c
> +++ b/kernel/acct.c
> @@ -481,6 +481,7 @@ static void do_acct_process(struct bsd_acct_struct
> *acct)
>          flim = current->signal->rlim[RLIMIT_FSIZE].rlim_cur;
>          current->signal->rlim[RLIMIT_FSIZE].rlim_cur = RLIM_INFINITY;
>          /* Perform file operations on behalf of whoever enabled
> accounting */
> +       pr_info("task:%px new cred:%px real cred:%px cred:%px\n",
> current, file->f_cred, current->real_cred, current->cred);
>          orig_cred = override_creds(file->f_cred);

-- 
paul moore
www.paul-moore.com