Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: Question about kill a process group | Date | Thu, 12 May 2022 00:53:16 +0200 |
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On Wed, May 11 2022 at 13:33, Eric W. Biederman wrote: > Thomas Gleixner <tglx@linutronix.de> writes: >> So unless the number of PIDs for a user is limited this _is_ an >> unpriviledged DoS vector. > > After having slept on this a bit it finally occurred to me the > semi-obvious solution to this issue is to convert tasklist_lock > from a rw-spinlock to rw-semaphore. The challenge is finding > the users (tty layer?) that generate signals from interrupt > context and redirect that signal generation.
From my outdated notes where I looked at this before:
[soft]interrupt context which acquires tasklist_lock: sysrq-e send_sig_all() sysrq-i send_sig_all() sysrq-n normalize_rt_tasks()
tasklist_lock nesting into other locks: fs/fcntl.c: send_sigio(), send_sigurg()
send_sigurg() is called from the network stack ...
Some very obscure stuff in arch/ia64/kernel/mca.c which is called from a DIE notifier.
Plus quite a bunch of read_lock() instances which nest inside rcu_read_lock() held sections.
This is probably incomplete, but the scope of the problem has been greatly reduced vs. the point where I looked at it last time a couple of years ago. But that's still a herculean task.
> Once signals holding tasklist_lock are no longer generated from > interrupt context irqs no longer need to be disabled and > after verifying tasklist_lock isn't held under any other spinlocks > it can be converted to a semaphore.
Going to take a while. :)
> It won't help the signal delivery times, but it should reduce > the effect on the rest of the system, and prevent watchdogs from > firing.
The signal delivery time itself is the least of the worries, but this still prevents any other operations which require tasklist_lock from making progress for quite some time, i.e. fork/exec for unrelated processes/users will have to wait too. So you converted the 'visible' DoS to an 'invisible' one.
The real problem is that the scope of tasklist_lock is too broad for most use cases. That does not change when you actually can convert it to a rwsem. The underlying problem still persists.
Let's take a step back and look what most sane use cases (sysrq-* is not in that category) require:
Preventing that tasks are added or removed
Do they require that any task is added or removed? No.
They require to prevent add/remove for the intended scope.
That's the thing we need to focus on: reducing the protection scope.
If we can segment the protection for the required scope of e.g. kill(2) then we still can let unrelated processes/tasks make progress and just inflict the damage on the affected portion of processes/tasks.
For example:
read_lock(&tasklist_lock); for_each_process(p) { if (task_pid_vnr(p) > 1 && !same_thread_group(p, current)) {
group_send_sig_info(...., p); } } read_unlock(&tasklist_lock);
same_thread_group() does:
return p->signal == current->signal;
Ideally we can do:
read_lock(&tasklist_lock); prevent_add_remove(current->signal); read_unlock(&tasklist_lock);
rcu_read_lock(); for_each_process(p) { if (task_pid_vnr(p) > 1 && !same_thread_group(p, current)) {
group_send_sig_info(...., p); } } rcu_read_unlock();
allow_add_remove(current->signal);
Where prevent_add_remove() sets a state which has to be waited for to be cleared by anything which wants to add/remove a task in that scope or change $relatedtask->signal until allow_add_remove() removes that blocker. I'm sure it's way more complicated, but you get the idea.
If we find a solution to this scope reduction problem, then it will not only squash the issue which started this discussion. This will have a benefit in general.
Thanks,
tglx
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