Re: [PATCH] sanitize task->comm to avoid leaking escape codes

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Jun 29, 2010 at 07:18:46PM +0200, Oleg Nesterov wrote:
> On 06/29, Paul E. McKenney wrote:
> >
> > On Tue, Jun 29, 2010 at 03:31:31PM +0200, Oleg Nesterov wrote:
> >
> > > So, afaics, set_task_comm()->wmb() buys nothing and should be removed.
> > > The last zero char in task_struct->comm[] is always here, at least this
> > > guarantees that strcpy(char *dest, tsk->comm) is always safe.
> > >
> > > (I cc'ed the expert, Paul can correct me)
> >
> > First, all of the implementations that I can see do task_lock(tsk), which
> > should prevent readers from seeing any changes.  So I am guessing that
> > you guys want to allow readers to get at ->comm without having to acquire
> > this lock.
> 
> Ah, sorry for confusion.
> 
> No, we are not trying to invent the lockless get_task_comm(). I'd say
> it is not needed, if we really care about the precise ->comm we can
> take task->alloc_lock.
> 
> The only problem is that I believe that set_task_comm() wrongly pretends
> wmb() can help the lockless reader, it does:
> 
> 	task_lock(tsk);
> 
> 	/*
> 	 * Threads may access current->comm without holding
> 	 * the task lock, so write the string carefully.
> 	 * Readers without a lock may see incomplete new
> 	 * names but are safe from non-terminating string reads.
> 	 */
> 	memset(tsk->comm, 0, TASK_COMM_LEN);
> 	wmb();
> 	strlcpy(tsk->comm, buf, sizeof(tsk->comm));
> 	task_unlock(tsk);
> 
> but afaics this wmb() buys absolutely nothing if we race with the
> reader doing, say,
> 
> 	printk("my name is %s\n", current->comm);
> 
> Afaics, this wmb()
> 
> 	- can't prevent from printing the mixture of the old/new data
> 
> 	- is not needed to make strcpy(somewhere, task->comm) safe,
> 	  the final char is always '0', we never change it.
> 
> 	- adds the unnecessary confusion

I agree -- I cannot see how this wmb() can help.

> > [... snip a lot of good ideas ...]
> 
> Thanks a lot, Paul ;)

Glad you liked them.  ;-)

							Thanx, Paul