linux-kernel - Re: [PATCH 3/4] proc: io_accounting: Use new infrastructure to fix deadlocks in execve

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Date:   Tue, 10 Mar 2020 16:25:09 -0500
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 3/4] proc: io_accounting: Use new infrastructure to fix deadlocks in execve

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

> On 3/10/20 8:06 PM, Eric W. Biederman wrote:
>> Bernd Edlinger <bernd.edlinger@...mail.de> writes:
>> 
>>> This changes do_io_accounting to use the new exec_update_mutex
>>> instead of cred_guard_mutex.
>>>
>>> This fixes possible deadlocks when the trace is accessing
>>> /proc/$pid/io for instance.
>>>
>>> This should be safe, as the credentials are only used for reading.
>> 
>> This is an improvement.
>> 
>> We probably want to do this just as an incremental step in making things
>> better but perhaps I am blind but I am not finding the reason for
>> guarding this with the cred_guard_mutex to be at all persuasive.
>> 
>> I think moving the ptrace_may_access check down to after the
>> unlock_task_sighand would be just as effective at addressing the
>> concerns raised in the original commit.  I think the task_lock provides
>> all of the barrier we need to make it safe to move the ptrace_may_access
>> checks safe.
>> 
>> The reason I say this is I don't see exec changing ->ioac.  Just
>> performing some I/O which would update the io accounting statistics.
>> 
>
> Maybe the suid executable is starting up and doing io or not,
> and what the program does immediately at startup is a secret,
> that we want to keep secret but evil eve want to find out.
> eve is using /proc/alice/io to do that.
>
> It is a bit constructed, but seems like a security concern.
> when we keep the exec_update_mutex while collecting the data, we
> cannot see any io of the new process when the new credentials
> don't allow that.

Jann Horn has convinced me we should just convert these to the
exec_change_mutex today.  Because while not 100% correct in theory, the
only really interesting case is exec.  So the code does something
interesting and worth while, and mostly correct.  The last thing I want
to do is to cause an unnecessary regression.

Eric