On 05/28, Eric W. Biederman wrote:
>
> Oleg Nesterov <o...@redhat.com> writes:
>
> > proc_task_readdir() verifies that the result of get_proc_task()
> > is pid_alive() and thus its ->group_leader is fine too. However
> > this is not necessarily true after rcu_read_unlock(), we need
> > to recheck this after first_tid() does rcu_read_lock() again.
>
> I agree with you but you are missing something critical from your
> explanation. If a process has been passed through __unhash_process
> then task->thread_group.next (aka next_thread) returns a pointer to the
> process that was it's next thread in the thread group. Importantly
> that pointer is only guaranteed to point to valid memory until the rcu
> grace period expires.
I tried to explain this below, in 1-4 steps... But OK, agreed, this
should be explained more clearly.
I'll update the changelog.
> > Note that we need 2. and 3. only because of get_nr_threads() check,
> > and this check was supposed to be optimization only.
>
> An optimization and denial of service attack prevention. It keeps us
> spinning for nearly unbounded amounts of time in the rcu critical
> section.
I do not really think we need this check to prevent the DoS attacks.
The main loop does while_each_thread(), so it will stop after
nr_threads iterations. And a user can always do llseek to trigger
the "full" scan.
But this is off-topic, and
> But I agree it should not be needed from this part of
> correctness.
Yes.
> >
> > - /* If nr exceeds the number of threads there is nothing todo */
> > pos = NULL;
> > + /* If nr exceeds the number of threads there is nothing todo */
>
> Moving the comment is just noise and makes for confusing reading of your
> patch.
Well, I think this makes the code look a bit better. Without this change
the code will be
/* If nr exceeds the number of threads there is nothing todo */
pos = NULL;
if (nr && nr >= get_nr_threads(leader))
goto out;
/* It could be unhashed before we take rcu lock */
if (!pid_alive(leader))
goto out;
and the comments explaining the checks are not "simmetrical". But I won't
argue, I'll update the patch and remove it. 3/3 changes this code anyway.
Oleg.
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