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

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Message-ID: <AM6PR03MB51707ABF20B6CBBECC34865FE4E70@AM6PR03MB5170.eurprd03.prod.outlook.com>
Date:   Mon, 2 Mar 2020 17:13:31 +0000
From:   Bernd Edlinger <bernd.edlinger@...mail.de>
To:     "Eric W. Biederman" <ebiederm@...ssion.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>,
        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>,
        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 5:17 PM, Eric W. Biederman wrote:
> Bernd Edlinger <bernd.edlinger@...mail.de> writes:
> 
>> On 3/2/20 4:57 PM, Eric W. Biederman wrote:
>>> Bernd Edlinger <bernd.edlinger@...mail.de> writes:
>>>
>>>>
>>>> I tried this with s/EACCESS/EACCES/.
>>>>
>>>> The test case in this patch is not fixed, but strace does not freeze,
>>>> at least with my setup where it did freeze repeatable.
>>>
>>> Thanks, That is what I was aiming at.
>>>
>>> So we have one method we can pursue to fix this in practice.
>>>
>>>> That is
>>>> obviously because it bypasses the cred_guard_mutex.  But all other
>>>> process that access this file still freeze, and cannot be
>>>> interrupted except with kill -9.
>>>>
>>>> However that smells like a denial of service, that this
>>>> simple test case which can be executed by guest, creates a /proc/$pid/mem
>>>> that freezes any process, even root, when it looks at it.
>>>> I mean: "ln -s README /proc/$pid/mem" would be a nice bomb.
>>>
>>> Yes.  Your the test case in your patch a variant of the original
>>> problem.
>>>
>>>
>>> I have been staring at this trying to understand the fundamentals of the
>>> original deeper problem.
>>>
>>> The current scope of cred_guard_mutex in exec is because being ptraced
>>> causes suid exec to act differently.  So we need to know early if we are
>>> ptraced.
>>>
>>
>> It has a second use, that it prevents two threads entering execve,
>> which would probably result in disaster.
> 
> Exec can fail with an error code up until de_thread.  de_thread causes
> exec to fail with the error code -EAGAIN for the second thread to get
> into de_thread.
> 
> So no.  The cred_guard_mutex is not needed for that case at all.
> 

Okay, but that will reset current->in_execve, right?

>>> If that case did not exist we could reduce the scope of the
>>> cred_guard_mutex in exec to where your patch puts the cred_change_mutex.
>>>
>>> I am starting to think reworking how we deal with ptrace and exec is the
>>> way to solve this problem.
> 
> 
> I am 99% convinced that the fix is to move cred_guard_mutex down.
> 
> Then right after we take cred_guard_mutex do:
> 	if (ptraced) {
> 		use_original_creds();
> 	}
> 
> And call it a day.
> 
> The details suck but I am 99% certain that would solve everyones
> problems, and not be too bad to audit either.
> 
> Eric
>