linux-kernel - Re: [PATCH] Change task

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Message-ID: <20140307155415.GB16640@madcap2.tricolour.ca>
Date:	Fri, 7 Mar 2014 10:54:15 -0500
From:	Richard Guy Briggs <rgb@...hat.com>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:	peterz@...radead.org, paulmck@...ux.vnet.ibm.com,
	laijs@...fujitsu.com, akpm@...ux-foundation.org, joe@...ches.com,
	keescook@...omium.org, geert@...ux-m68k.org, jkosina@...e.cz,
	viro@...iv.linux.org.uk, davem@...emloft.net,
	linux-kernel@...r.kernel.org, mingo@...e.hu, rostedt@...dmis.org,
	tglx@...utronix.de, linux-security-module@...r.kernel.org
Subject: Re: [PATCH] Change task_struct->comm to use RCU.

On 14/03/07, Tetsuo Handa wrote:
> Peter Zijlstra wrote:
> >   https://lkml.org/lkml/2011/5/17/516

> Thank you for pointing that thread out. I found the following comment in that
> thread.
> 
> Linus Torvalds wrote:
> | What folks?
> | 
> | I don't think a new lock (or any lock) is at all appropriate.
> | 
> | There's just no point. Just guarantee that the last byte is always
> | zero, and you're done.
> | 
> | If you just guarantee that, THERE IS NO RACE. The last byte never
> | changes. You may get odd half-way strings, but you've trivially
> | guaranteed that they are C NUL-terminated, with no locking, no memory
> | ordering, no nothing.

> > >   Likewise, audit_log_untrustedstring(ab, current->comm) is racy.
> > >   If task->comm was "Hello Linux" until audit_string_contains_control() in
> > >   audit_log_n_untrustedstring() returns false, and becomes "Penguin" before
> > >   memcpy() in audit_log_n_string() is called, memcpy() will emit "Penguin\0nux"
> > >   into the audit log, which results in loss of information (e.g. SELinux
> > >   context) due to the unexpected '\0' byte.
> > 
> > I expect the audit people don't like this? Also, how do audit and the
> > LSM crap things interact? I thought they were both different piles of
> > ignorable goo?
> 
> I think the audit people do not like loss of information. Some of LSM modules
> are using audit subsystem for recording security related events. An example is
> shown later.

This is true, however since comm it untrusted because it can be modified
by the user audit doesn't trust it anyways, so who cares?

> > How about you do what you're supposed to do when you want a reliable
> > ->comm and use get_task_comm()?
> 
> I always want a reliable ->comm . But get_task_comm() is not for calling from
> vsnprintf(), for somebody might read task's commname from NMI context.
> I tried to use RCU for reading from vsnprintf() but Linus will not accept it.

- RGB

--
Richard Guy Briggs <rbriggs@...hat.com>
Senior Software Engineer, Kernel Security, AMER ENG Base Operating Systems, Red Hat
Remote, Ottawa, Canada
Voice: +1.647.777.2635, Internal: (81) 32635, Alt: +1.613.693.0684x3545
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