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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Bernd Edlinger <bernd.edlinger@hotmail.de>
Cc:     Jann Horn <jannh@google.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
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        Thomas Gleixner <tglx@linutronix.de>,
        Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Andrei Vagin <avagin@gmail.com>,
        Ingo Molnar <mingo@kernel.org>,
        "Peter Zijlstra \(Intel\)" <peterz@infradead.org>,
        Yuyang Du <duyuyang@gmail.com>,
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        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
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Date:   Mon, 02 Mar 2020 15:49:09 -0600
In-Reply-To: <AM6PR03MB51707ABF20B6CBBECC34865FE4E70@AM6PR03MB5170.eurprd03.prod.outlook.com>
        (Bernd Edlinger's message of "Mon, 2 Mar 2020 17:13:31 +0000")
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Subject: Re: [PATCHv2] exec: Fix a deadlock in ptrace
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Bernd Edlinger <bernd.edlinger@hotmail.de> writes:

> On 3/2/20 5:17 PM, Eric W. Biederman wrote:
>> Bernd Edlinger <bernd.edlinger@hotmail.de> writes:
>> 
>>> On 3/2/20 4:57 PM, Eric W. Biederman wrote:
>>>> Bernd Edlinger <bernd.edlinger@hotmail.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?

Absolutely.

The error handling kicks in and exec_binprm fails with a negative
return code.  Then __do_excve_file cleans up and clears
current->in_execve.

Eric