linux-kernel - Re: [PATCHv4] exec: Fix a deadlock in ptrace

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Date:   Tue, 3 Mar 2020 15:20:47 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Bernd Edlinger <bernd.edlinger@...mail.de>
Cc:     Kees Cook <keescook@...omium.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Oleg Nesterov <oleg@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Andrei Vagin <avagin@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Yuyang Du <duyuyang@...il.com>,
        David Hildenbrand <david@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Anshuman Khandual <anshuman.khandual@....com>,
        David Howells <dhowells@...hat.com>,
        James Morris <jamorris@...ux.microsoft.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christian Kellner <christian@...lner.me>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Aleksa Sarai <cyphar@...har.com>,
        "Dmitry V. Levin" <ldv@...linux.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCHv4] exec: Fix a deadlock in ptrace

On Tue, Mar 03, 2020 at 11:23:31AM +0000, Bernd Edlinger wrote:
> On 3/3/20 11:34 AM, Bernd Edlinger wrote:
> > On 3/3/20 9:58 AM, Christian Brauner wrote:
> >> So one issue I see with having to reacquire the cred_guard_mutex might
> >> be that this would allow tasks holding the cred_guard_mutex to block a
> >> killed exec'ing task from exiting, right?
> >>
> > 
> > Yes maybe, but I think it will not be worse than it is now.
> > Since the second time the mutex is acquired it is done with
> > mutex_lock_killable, so at least kill -9 should get it terminated.
> > 
> 
> 
> 
> >  static void free_bprm(struct linux_binprm *bprm)
> >  {
> >  	free_arg_pages(bprm);
> >  	if (bprm->cred) {
> > +		if (!bprm->called_flush_old_exec)
> > +			mutex_lock(&current->signal->cred_guard_mutex);
> > +		current->signal->cred_locked_for_ptrace = false;
> >  		mutex_unlock(&current->signal->cred_guard_mutex);
> 
> 
> Hmm, cough...
> actually when the mutex_lock_killable fails, due to kill -9, in flush_old_exec
> free_bprm locks the same mutex, this time unkillable, but I should better do
> mutex_lock_killable here, and if that fails, I can leave cred_locked_for_ptrace,
> it shouldn't matter, since this is a fatal signal anyway, right?

I think so, yes.