Re: [PATCH v3 1/8] exec: introduce cred_guard_light

From: Oleg Nesterov <oleg@redhat.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Jann Horn <jann@thejh.net>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Roland McGrath <roland@hack.frob.com>,
	John Johansen <john.johansen@canonical.com>,
	James Morris <james.l.morris@oracle.com>,
	"Serge E. Hallyn" <serge@hallyn.com>,
	Paul Moore <aul@paul-moore.com>,
	Stephen Smalley <sds@tycho.nsa.gov>,
	Eric Paris <eparis@parisplace.org>,
	Casey Schaufler <casey@schaufler-ca.com>,
	Kees Cook <keescook@chromium.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Janis Danisevskis <jdanis@google.com>,
	Seth Forshee <seth.forshee@canonical.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Benjamin LaHaise <bcrl@kvack.org>,
	Ben Hutchings <ben@decadent.org.uk>,
	Andy Lutomirski <luto@amacapital.net>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Krister Johansen <kjlx@templeofstupid.com>,
	linux-fsdevel@vger.kernel.org,
	linux-security-module@vger.kernel.org, security@kernel.org
Subject: Re: [PATCH v3 1/8] exec: introduce cred_guard_light
Date: Fri, 4 Nov 2016 19:45:05 +0100	[thread overview]
Message-ID: <20161104184505.GA21320@redhat.com> (raw)
In-Reply-To: <20161104180416.GA19221@redhat.com>

On 11/04, Oleg Nesterov wrote:
>
> On 11/04, Eric W. Biederman wrote:
> >
> > The following mostly correct patch modifies zap_other_threads in
> > the case of a de_thread to not wait for zombies to be reaped.  The only
> > case that cares is ptrace (as threads are self reaping).  So I don't
> > think this will cause any problems except removing the strace -f race.
>
> From my previous email:
>
> 	So the only plan I currently have is change de_thread() to wait until
> 	other threads pass exit_notify() or even exit_signals(), but I don't
> 	like this.
>
> And yes, I don't like this, but perhaps this is what we should do.
>
> The patch is incomplete and racy (afaics), and the SIGNAL_GROUP_EXIT
> checks doesn't look right, but off course technically this change should
> be simple enough.
>
> But not that simple. Just for example, the exiting sub-threads should
> not run with ->group_leader pointing to nowhere, in case it was reaped
> by de_thread.

Not to mention other potential problems outside of ptrace/exec. For example
userns_install() can fail after mt-exec even without ptrace, simply because
thread_group_empty() can be false. Sure, easy to fix, and probably _install()
should use signal->live anyway, but still.

And I didn't mention the fun with sighand unsharing. We simply can't do this
until all sub-threads go away. IOW, your patch breaks the usage of ->siglock.
The execing thread and the zombie threads will use different locks to, say,
remove the task from thread-group. Again, this is fixable, but not that
simple.

> And we have another problem with PTRACE_EVENT_EXIT which can lead to the
> same deadlock. Unfortunately, the semantics of PTRACE_EVENT_EXIT was never
> defined. But this change will add the user-visible change.
>
> And if we add the user-visible changes, then perhaps we could simply untrace
> the traced sub-threads on exec. This change is simple, we do not even need
> to touch exec/de_thread, we could just change exit_notify() to ignore ->ptrace
> if exec is in progress. But I'm afraid we can't do this.

Eric, I hope you see my emails, I got the "Undelivered Mail Returned to Sender"
	...
	This is the mail system at host mail.kernel.org.
	...
	<ebiederm@xmission.com> (expanded from <security@kernel.org>): host
	    mx.xmission.com[166.70.12.20] said: 550-XM-RJCT16: SPF Failure
	    (ip=198.145.29.136, frm=oleg@redhat.com, 550 result=fail) (in reply to RCPT
	    TO command)

right now I have no idea what does this mean.

Oleg.