linux-kernel - Re: [PATCH] pidfd: Stop taking cred_guard

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Date:   Tue, 10 Mar 2020 21:22:41 +0100
From:   Bernd Edlinger <bernd.edlinger@...mail.de>
To:     Jann Horn <jannh@...gle.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        Kees Cook <keescook@...omium.org>,
        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>,
        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-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Sargun Dhillon <sargun@...gun.me>
Subject: Re: [PATCH] pidfd: Stop taking cred_guard_mutex

On 3/10/20 9:10 PM, Jann Horn wrote:
> On Tue, Mar 10, 2020 at 9:00 PM Jann Horn <jannh@...gle.com> wrote:
>> On Tue, Mar 10, 2020 at 8:29 PM Eric W. Biederman <ebiederm@...ssion.com> wrote:
>>> Jann Horn <jannh@...gle.com> writes:
>>>> On Tue, Mar 10, 2020 at 7:54 PM Eric W. Biederman <ebiederm@...ssion.com> wrote:
>>>>> During exec some file descriptors are closed and the files struct is
>>>>> unshared.  But all of that can happen at other times and it has the
>>>>> same protections during exec as at ordinary times.  So stop taking the
>>>>> cred_guard_mutex as it is useless.
>>>>>
>>>>> Furthermore he cred_guard_mutex is a bad idea because it is deadlock
>>>>> prone, as it is held in serveral while waiting possibly indefinitely
>>>>> for userspace to do something.
> [...]
>>>> If you make this change, then if this races with execution of a setuid
>>>> program that afterwards e.g. opens a unix domain socket, an attacker
>>>> will be able to steal that socket and inject messages into
>>>> communication with things like DBus. procfs currently has the same
>>>> race, and that still needs to be fixed, but at least procfs doesn't
>>>> let you open things like sockets because they don't have a working
>>>> ->open handler, and it enforces the normal permission check for
>>>> opening files.
>>>
>>> It isn't only exec that can change credentials.  Do we need a lock for
>>> changing credentials?
> [...]
>>> If we need a lock around credential change let's design and build that.
>>> Having a mismatch between what a lock is designed to do, and what
>>> people use it for can only result in other bugs as people get confused.
>>
>> Hmm... what benefits do we get from making it a separate lock? I guess
>> it would allow us to make it a per-task lock instead of a
>> signal_struct-wide one? That might be helpful...
> 
> But actually, isn't the core purpose of the cred_guard_mutex to guard
> against concurrent credential changes anyway? That's what almost
> everyone uses it for, and it's in the name...
> 

The main reason d'etre of exec_update_mutex is to get a consitent
view of task->mm and task credentials.

The reason why you want the cred_guard_mutex, is that some action
is changing the resulting credentials that the execve is about
to install, and that is the data flow in the opposite direction.


Bernd.