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SubjectRe: [PATCH 3/4] tty: Iterate only thread group leaders in __do_SAK()
On 17.01.2018 00:13, Oleg Nesterov wrote:
> 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.

Ok, as there is no single right answer on this problem, let's skip it for this patchset.

>> 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
>> "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.

Thanks, Oleg. I've sent v2 "[PATCH v2 0/3] tty: Make __do_SAK() less greedy in regard to tasklist_lock"
taking into account all of your comments.

(It seems this theme is not interesting for most people. I've removed them from CC in v2,
to reduce people mail traffic)


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