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

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Date:   Mon, 2 Mar 2020 17:01:15 +0000
From:   Bernd Edlinger <bernd.edlinger@...mail.de>
To:     Jann Horn <jannh@...gle.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        James Morris <jamorris@...ux.microsoft.com>
CC:     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>,
        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>,
        linux-security-module <linux-security-module@...r.kernel.org>
Subject: Re: [PATCHv2] exec: Fix a deadlock in ptrace

On 3/2/20 5:43 PM, Jann Horn wrote:
> On Mon, Mar 2, 2020 at 5:19 PM Eric W. Biederman <ebiederm@...ssion.com> 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.
>>
>>>> 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.
> 
> "move cred_guard_mutex down" as in "take it once we've already set up
> the new process, past the point of no return"?
> 
>> 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.
> 
> Ah, hmm, that sounds like it'll work fine at least when no LSMs are involved.
> 
> SELinux normally doesn't do the execution-degrading thing, it just
> blocks the execution completely - see their selinux_bprm_set_creds()
> hook. So I think they'd still need to set some state on the task that
> says "we're currently in the middle of an execution where the target
> task will run in context X", and then check against that in the
> ptrace_may_access hook. Or I suppose they could just kill the task
> near the end of execve, although that'd be kinda ugly.
> 

We have current->in_execve for that, right?
I think when the cred_guard_mutex is taken only in the critical section,
then PTRACE_ATTACH could take the guard_mutex, and look at current->in_execve,
and just return -EAGAIN in that case, right, everybody happy :)


Bernd.