linux-kernel - Re: [PATCH v3 00/13] Virtually mapped stacks with guard pages (x86, core)

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Message-ID: <CA+55aFwHUw+6xEc-M+mZvwOxCsBD6zZDwyAQ-a_xupaH8py7cA@mail.gmail.com>
Date:	Thu, 23 Jun 2016 10:44:08 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Andy Lutomirski <luto@...nel.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Nadav Amit <nadav.amit@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Brian Gerst <brgerst@...il.com>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Jann Horn <jann@...jh.net>,
	Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [PATCH v3 00/13] Virtually mapped stacks with guard pages (x86, core)

On Thu, Jun 23, 2016 at 10:03 AM, Oleg Nesterov <oleg@...hat.com> wrote:
>
> Let me quote my previous email ;)
>
>         And we can't free/nullify it when the parent/debuger reaps a zombie,
>         say, mark_oom_victim() expects that get_task_struct() protects
>         thread_info as well.
>
> probably we can fix all such users though...

TIF_MEMDIE is indeed a potential problem, but I don't think
mark_oom_victim() is actually problematic.

mark_oom_victim() is called with either "current", or with a victim
that still has its mm and signal pointer (and the task is locked). So
the lifetime is already guaranteed - or that code is already very very
buggy, since it follows tsk->signal and tsk->mm

So as far as I can tell, that's all fine.

That said, by now it would actually in many ways be great if we could
get rid of thread_info entirely. The historical reasons for
thread_info have almost all been subsumed by the percpu area.

The reason for thread_info originally was

 - we used to find the task_struct by just masking the stack pointer
(long long ago). When the task struct grew too big, we kept just the
critical pieces and some arch-specific stuff and , called it
"thread_info", and moved the rest to an external allocation and added
the pointer to it.

 - the really crticial stuff we didn't want to follow a pointer for,
so things like preempt_count etc were in thread_info

 - but they were *so* critical that PeterZ (at my prodding) moved
those things to percpu caches that get updated at schedule time
instead

so these days, thread_info has almost nothing really critical in it
any more. There's the thread-local flags, yes, but they could stay or
easily be moved to the task_struct or get similar per-cpu fixup as
preempt_count did a couple of years ago. The only annoyance is the few
remaining entry code assembly sequences, but I suspect they would
actually become simpler with a per-cpu thing, and with Andy's cleanups
they are pretty insignificant these days. There seems to be exactly
two uses of ASM_THREAD_INFO(TI_flags,.. left.

So I suspect that it would

 (a) already be possible to just free the stack and thread info at
release time, because any rcu users will already be doing task_lock()
and check mm etc.

 (b) it probably would be a nice cleanup to try to make it even more
obviously safe by just shrinking thread_info more (or even getting rid
of it entirely, but that may be too painful because there are other
architectures that may depend on it more).

I dunno. Looking at what remains of thread_info, it really doesn't
seem very critical.

The thread_info->tsk pointer, that was one of the most critical issues
and the main raison d'être of the thread_info, has been replaced on
x86 by just using the per-cpu "current_task". Yes,.there are probably
more than a few "ti->task" users left for legacy reasons, harking back
to when the thread-info was cheaper to access, but it shouldn't be a
big deal.

                  Linus