linux-kernel - Re: [PATCH v3 seccomp 5/5] seccomp/cache: Report cache data through /proc/pid/seccomp

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Date:   Wed, 30 Sep 2020 16:12:24 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Jann Horn <jannh@...gle.com>
Cc:     YiFei Zhu <zhuyifei1999@...il.com>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        YiFei Zhu <yifeifz2@...inois.edu>, bpf <bpf@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andy Lutomirski <luto@...capital.net>,
        David Laight <David.Laight@...lab.com>,
        Dimitrios Skarlatos <dskarlat@...cmu.edu>,
        Giuseppe Scrivano <gscrivan@...hat.com>,
        Hubertus Franke <frankeh@...ibm.com>,
        Jack Chen <jianyan2@...inois.edu>,
        Josep Torrellas <torrella@...inois.edu>,
        Tianyin Xu <tyxu@...inois.edu>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Tycho Andersen <tycho@...ho.pizza>,
        Valentin Rothberg <vrothber@...hat.com>,
        Will Drewry <wad@...omium.org>
Subject: Re: [PATCH v3 seccomp 5/5] seccomp/cache: Report cache data through
 /proc/pid/seccomp_cache

On Thu, Oct 01, 2020 at 12:00:46AM +0200, Jann Horn wrote:
> On Wed, Sep 30, 2020 at 5:20 PM YiFei Zhu <zhuyifei1999@...il.com> wrote:
> [...]
> > diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> [...]
> > +int proc_pid_seccomp_cache(struct seq_file *m, struct pid_namespace *ns,
> > +                          struct pid *pid, struct task_struct *task)
> > +{
> > +       struct seccomp_filter *f;
> > +
> > +       /*
> > +        * We don't want some sandboxed process know what their seccomp
> > +        * filters consist of.
> > +        */
> > +       if (!file_ns_capable(m->file, &init_user_ns, CAP_SYS_ADMIN))
> > +               return -EACCES;
> > +
> > +       f = READ_ONCE(task->seccomp.filter);
> > +       if (!f)
> > +               return 0;
> 
> Hmm, this won't work, because the task could be exiting, and seccomp
> filters are detached in release_task() (using
> seccomp_filter_release()). And at the moment, seccomp_filter_release()
> just locklessly NULLs out the tsk->seccomp.filter pointer and drops
> the reference.

Oh nice catch. Yeah, this would only happen if it was the only filter
remaining on a process with no children, etc.

> 
> The locking here is kind of gross, but basically I think you can
> change this code to use lock_task_sighand() / unlock_task_sighand()
> (see the other examples in fs/proc/base.c), and bail out if
> lock_task_sighand() returns NULL. And in seccomp_filter_release(), add
> something like this:
> 
> /* We are effectively holding the siglock by not having any sighand. */
> WARN_ON(tsk->sighand != NULL);

Yeah, good idea.

-- 
Kees Cook