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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Bernd Edlinger <bernd.edlinger@hotmail.de>
Cc:     Christian Brauner <christian.brauner@ubuntu.com>,
        Kees Cook <keescook@chromium.org>,
        Jann Horn <jannh@google.com>, Jonathan Corbet <corbet@lwn.net>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Andrei Vagin <avagin@gmail.com>,
        Ingo Molnar <mingo@kernel.org>,
        "Peter Zijlstra \(Intel\)" <peterz@infradead.org>,
        Yuyang Du <duyuyang@gmail.com>,
        David Hildenbrand <david@redhat.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        David Howells <dhowells@redhat.com>,
        James Morris <jamorris@linux.microsoft.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Shakeel Butt <shakeelb@google.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Christian Kellner <christian@kellner.me>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Aleksa Sarai <cyphar@cyphar.com>,
        "Dmitry V. Levin" <ldv@altlinux.org>,
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Date:   Tue, 10 Mar 2020 16:25:09 -0500
In-Reply-To: <AM6PR03MB5170A29F8D885147F1A3981EE4FF0@AM6PR03MB5170.eurprd03.prod.outlook.com>
        (Bernd Edlinger's message of "Tue, 10 Mar 2020 21:19:05 +0100")
Message-ID: <878sk7q4wa.fsf@x220.int.ebiederm.org>
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Subject: Re: [PATCH 3/4] proc: io_accounting: Use new infrastructure to fix deadlocks in execve
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Bernd Edlinger <bernd.edlinger@hotmail.de> writes:

> On 3/10/20 8:06 PM, Eric W. Biederman wrote:
>> Bernd Edlinger <bernd.edlinger@hotmail.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