Messages in this thread | | | Date | Wed, 29 May 2013 14:30:09 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH 1/3] proc: first_tid: fix the potential use-after-free |
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On 05/28, Eric W. Biederman wrote: > > Oleg Nesterov <oleg@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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