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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>,
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Date:   Mon, 02 Mar 2020 10:17:32 -0600
In-Reply-To: <AM6PR03MB51704206634C009500A8080DE4E70@AM6PR03MB5170.eurprd03.prod.outlook.com>
        (Bernd Edlinger's message of "Mon, 2 Mar 2020 16:02:46 +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 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.

>> 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