linux-kernel - Re: [PATCH v2 5/5] exec: Add a exec_update_mutex to replace cred_guard

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Message-ID: <87a74nsz2f.fsf@x220.int.ebiederm.org>
Date:   Tue, 10 Mar 2020 16:02:48 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Kees Cook <keescook@...omium.org>
Cc:     Bernd Edlinger <bernd.edlinger@...mail.de>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Jann Horn <jannh@...gle.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>,
        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\@vger.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel\@vger.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-mm\@kvack.org" <linux-mm@...ck.org>,
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        "linux-api\@vger.kernel.org" <linux-api@...r.kernel.org>
Subject: Re: [PATCH v2 5/5] exec: Add a exec_update_mutex to replace cred_guard_mutex

Kees Cook <keescook@...omium.org> writes:

> On Mon, Mar 09, 2020 at 02:02:37PM -0500, Eric W. Biederman wrote:
>>     exec: Add exec_update_mutex to replace cred_guard_mutex
>>     
>>     The cred_guard_mutex is problematic as it is held over possibly
>>     indefinite waits for userspace.  The possilbe indefinite waits for
>>     userspace that I have identified are: The cred_guard_mutex is held in
>>     PTRACE_EVENT_EXIT waiting for the tracer.  The cred_guard_mutex is
>>     held over "put_user(0, tsk->clear_child_tid)" in exit_mm().  The
>>     cred_guard_mutex is held over "get_user(futex_offset, ...")  in
>>     exit_robust_list.  The cred_guard_mutex held over copy_strings.
>
> I suspect you're not trying to make a comprehensive list here, but do
> you want to mention seccomp too (since it's yet another weird case).

I was calling out all of the places I have found so far where
cred_guard_mutex is held over waiting for userspace to maybe do
something.  Those places are what cause our deadlocks.

>> [...]
>>     Holding a mutex over any of those possibly indefinite waits for
>>     userspace does not appear necessary.  Add exec_update_mutex that will
>>     just cover updating the process during exec where the permissions and
>>     the objects pointed to by the task struct may be out of sync.
>
> Should the specific resources be pointed out here? creds, mm, ... ?
>
> But otherwise, yup, looks sane:

Probably not.  The design is if exec changes it we will hold the
cred_guard_mutex over it, so things are semi-atomic.

> Reviewed-by: Kees Cook <keescook@...omium.org>

Eric