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

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Date:   Wed, 11 Mar 2020 20:48:31 +0100
From:   Bernd Edlinger <bernd.edlinger@...mail.de>
To:     Kees Cook <keescook@...omium.org>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        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@...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>
Subject: Re: [PATCH 3/4] proc: io_accounting: Use new infrastructure to fix
 deadlocks in execve

On 3/11/20 8:08 PM, Kees Cook wrote:
> On Tue, Mar 10, 2020 at 06:45:47PM +0100, Bernd Edlinger wrote:
>> 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.
> 
> I'd like to see the rationale described better here for why it should be
> safe. I'm still not seeing why this is safe here, as we might check
> ptrace_may_access() with one cred and then iterate io accounting with a
> different credential...
> 
> What am I missing?
> 

The same here, even if execve is already started, the credentials
are not actually changed until the execve acquired the exec_update_mutex.

The data flow is from the task->cred => do_io_accounting,
if the data flow would be from do_io_accounting => task's no new privs
you would see an entirely different patch.

I am open for suggestions how to improve the description, or even
add a comment from time to time :)

Thanks
Bernd.

> -Kees
> 
>>
>> Signed-off-by: Bernd Edlinger <bernd.edlinger@...mail.de>
>> ---
>>  fs/proc/base.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/proc/base.c b/fs/proc/base.c
>> index 4fdfe4f..529d0c6 100644
>> --- a/fs/proc/base.c
>> +++ b/fs/proc/base.c
>> @@ -2770,7 +2770,7 @@ static int do_io_accounting(struct task_struct *task, struct seq_file *m, int wh
>>  	unsigned long flags;
>>  	int result;
>>  
>> -	result = mutex_lock_killable(&task->signal->cred_guard_mutex);
>> +	result = mutex_lock_killable(&task->signal->exec_update_mutex);
>>  	if (result)
>>  		return result;
>>  
>> @@ -2806,7 +2806,7 @@ static int do_io_accounting(struct task_struct *task, struct seq_file *m, int wh
>>  	result = 0;
>>  
>>  out_unlock:
>> -	mutex_unlock(&task->signal->cred_guard_mutex);
>> +	mutex_unlock(&task->signal->exec_update_mutex);
>>  	return result;
>>  }
>>  
>> -- 
>> 1.9.1
>