Messages in this thread |  | | Date | Tue, 16 Jan 2018 22:13:41 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH 3/4] tty: Iterate only thread group leaders in __do_SAK() |
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On 01/16, Kirill Tkhai wrote: > > On 15.01.2018 23:51, Oleg Nesterov wrote: > > > >> kill: > >> - force_sig(SIGKILL, p); > >> + send_sig(SIGKILL, p, 1); > > > > Agreed, I didn't actually want to use force_sig(SIGKILL), copy-and-paste error. > > force_sig() is still safe under tasklist_lock as release_task() unhashes a task > from the lists and destroys sighand at the same time under it. So, it seems > there is no a problem :)
I didn't mean it is unsafe. The problem is that force_sig() replaced send_sig() to avoid tasklist_lock which we no longer take in send-signal paths. Another problem is that it differs from send_sig(SIGKILL) used in other places and this difference (ability to kill SIGNAL_UNKILLABLE tasks) was added by accident, that was my point.
> Anyway, we could use send_sig_info(SIGKILL, SEND_SIG_FORCED, p) instead of that > in the patch like you suggested below.
Probably, but this needs another/separate change.
> Also we skip global init on session iteration. This could be useful for debugging, > when init is "/bin/bash" and some task started on top of bash is hunged.
We will need this only after we use SEND_SIG_FORCED, send_sig(SIGKILL) won't kill init.
> > This looks strange, and probably unintentional. So it seems yoou should start > > with "revert 20ac94378 [PATCH] do_SAK: Don't recursively take the tasklist_lock" ? > > The original reason for that commit has gone a long ago. > > If we revert it, lock_task_sighand() will be nested with task_lock().
This is safe. lock_task_sighand() is irq-safe (just like ->siglock) and it is actually used in irqs. Thus it is safe to use it under task_lock() which doesn't disable irqs.
And,
> Yeah, it's not for > a long time, next commit will change that.
Yes, there is no reason to send SIGKILL under task_lock().
> > At the same time, I do not know if we actually want to kill sub-namespace inits > > or not. If yes, we can use SEND_SIG_FORCED (better than ugly force_sig()) but > > skip the global init. But this will need yet another change. > > From https://www.kernel.org/doc/Documentation/SAK.txt: > > "An operating system's Secure Attention Key is a security tool which is > provided as protection against trojan password capturing programs. It > is an undefeatable way of killing all programs which could be > masquerading as login applications" > > It seems, since not privileged user is able to create pid_ns to start > a "trojan password capturing program", we have to kill sub-namespace inits too.
Agreed, that is why I suggested SEND_SIG_FORCED.
However. this is the user-visible change and who knows, perhaps it is too late to change the current behaviour. So I think we should do this after cleanups, this way we can easily revert it later in (unlikely) case someone complains.
But, Kirill, this is up to you, I won't insist.
Oleg.
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