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

[Oleg Nesterov <oleg@redhat.com>]

On 06/29, Artem Bityutskiy wrote:
>
> On Wed, 2010-06-23 at 21:41 +0200, Oleg Nesterov wrote:
> > On 06/23, Kees Cook wrote:
> > >
> > > @@ -956,7 +957,15 @@ void set_task_comm(struct task_struct *tsk, char *buf)
> > >  	 */
> > >  	memset(tsk->comm, 0, TASK_COMM_LEN);
> > >  	wmb();
> >
> > Off-topic. I'd wish I could understand this barrier. Since the lockless
> > reader doesn't do rmb() I don't see how this can help.
>
> This wmb() looks wrong to me as well. To achieve what the comment in
> this function says, it should be smp_wmb() and we should have smp_rmb()
> in the reading side, AFAIU.
>
> >  OTOH, I don't
> > understand why it is needed, we never change ->comm[TASK_COMM_LEN-1] == '0'.
>
> I think the idea was that readers can see incomplete names, but not
> messed up names, consisting of old and new ones.

OK, agreed, comm[TASK_COMM_LEN-1] == '0' can't help to avoid the
messed names.

But nether can help smp_rmb() in the reading (lockless) side?

Say,

	printk("comm=%s\n", current->comm);

if we add rmb() before printk, it can't make any difference. The lockless
code should do something like

	get_comm_lockless(char *to, char *comm)
	{
		while (*comm++ = *to++)
			rmb();
	}

to ensure it sees the result of

	memset(tsk->comm, 0, TASK_COMM_LEN);
	wmb();
	strcpy(tsk->comm, buf);

in the right order.

otherwise printk("comm=%s\n", current->comm) can read, say, comm[1]
before set_task_comm->memset(), and comm[0] after set_task_comm->strcpy().

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)

Oleg.