linux-kernel - Re: [PATCH 2/2] exec: Add a exec_update_mutex to replace cred_guard

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Date:   Fri, 06 Mar 2020 15:58:52 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Bernd Edlinger <bernd.edlinger@...mail.de>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        Kees Cook <keescook@...omium.org>,
        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>,
        "stable\@vger.kernel.org" <stable@...r.kernel.org>,
        "linux-api\@vger.kernel.org" <linux-api@...r.kernel.org>
Subject: Re: [PATCH 2/2] exec: Add a exec_update_mutex to replace cred_guard_mutex

Bernd Edlinger <bernd.edlinger@...mail.de> writes:

> On 3/6/20 6:17 AM, Eric W. Biederman wrote:
>> Bernd Edlinger <bernd.edlinger@...mail.de> writes:
>> 
>>> On 3/5/20 10:16 PM, Eric W. Biederman wrote:
>>>>
>>>> The cred_guard_mutex is problematic.  The cred_guard_mutex is held
>>>> over the userspace accesses as the arguments from userspace are read.
>>>> The cred_guard_mutex is held of PTRACE_EVENT_EXIT as the the other
>>>> threads are killed.  The cred_guard_mutex is held over
>>>> "put_user(0, tsk->clear_child_tid)" in exit_mm().
>>>>
>
> I am all for this patch, and the direction it is heading, Eric.
>
> I just wanted to add a note that I think it is
> possible that exec_mm_release can also invoke put_user(0, tsk->clear_child_tid),
> under the new exec_update_mutex, since vm_access increments the
> mm->mm_users, under the cred_update_mutex, but releases the mutex,
> and the caller can hold the reference for a while and then exec_mmap is not
> releasing the last reference.

Good catch.  I really appreciate your close look at the details.

I am wondering if process_vm_readv and process_vm_writev could be
safely changed to use mmgrab and mmdrop, instead of mmget and mmput.

That would resolve the potential issue you have pointed out.  I just
haven't figured out if it is safe.  The mm code has been seriously
refactored since I knew how it all worked.

Eric