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

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Message-ID: <AM6PR03MB51706D69E3F0126237DDD110E4E70@AM6PR03MB5170.eurprd03.prod.outlook.com>
Date:   Mon, 2 Mar 2020 15:56:04 +0000
From:   Bernd Edlinger <bernd.edlinger@...mail.de>
To:     Oleg Nesterov <oleg@...hat.com>
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 3/2/20 1:28 PM, Oleg Nesterov wrote:
> 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.
> 

No, but I see this just as a first step.

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

I think for the vm_access the semantic of this mutex is clear, that it
prevents the credentials to change while it is held by vm_access,
and probably other places can take advantage of this mutex as well.

While on the other hand, the cred_guard_mutex is needed to avoid two
threads calling execve at the same time.  So that is needed as well.

What remains is probably making PTHREAD_ATTACH detect that the process
is currently in execve, and make that call fail in that situation.
I have not thought in depth about that problem, but it will probably
just need the right mutex to access current->in_execve.


That's at least how I see it.


Thanks
Bernd.