linux-kernel - Re: [PATCHv2] exec: Fix a deadlock in ptrace

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Date:   Mon, 2 Mar 2020 13:28:29 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Bernd Edlinger <bernd.edlinger@...mail.de>
Cc:     Jann Horn <jannh@...gle.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Jonathan Corbet <corbet@....net>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        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>,
        Kees Cook <keescook@...omium.org>,
        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: [PATCHv2] exec: Fix a deadlock in ptrace

On 03/01, Bernd Edlinger wrote:
>
> This fixes a deadlock in the tracer when tracing a multi-threaded
> application that calls execve while more than one thread are running.

Heh. Yes, known problem. See my attempt to fix it:
https://lore.kernel.org/lkml/20170213141452.GA30203@redhat.com/

> @@ -1224,7 +1224,7 @@ struct mm_struct *mm_access(struct task_struct *task, unsigned int mode)
>  	struct mm_struct *mm;
>  	int err;
>  
> -	err =  mutex_lock_killable(&task->signal->cred_guard_mutex);
> +	err =  mutex_lock_killable(&task->signal->cred_change_mutex);

So if I understand correctly your patch doesn't fix other problems
with debugger waiting for cred_guard_mutex.

I too do not think this can justify the new mutex in signal_struct...

Oleg.