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

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Date:   Mon, 2 Mar 2020 16:02:46 +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 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.

> 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.
> 
> Eric
>