linux-kernel - Re: ptrace && fpu_lazy

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Mon, 16 Apr 2012 00:38:08 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Chuck Ebbert <chuckebbert.lk@...il.com>,
	Jan Kratochvil <jan.kratochvil@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: ptrace && fpu_lazy_restore

On 04/14, Linus Torvalds wrote:
>
> So I actually think that I would prefer the patch that invalidates the
> FPU caches more aggressively. Sure, we don't really *need* to
> invalidate if we're just reading, but I'd almost prefer to just have
> it done once in "init_fpu()".

Agreed. I'll send your patch back to you tomorrow.

> The only case where we care about the FPU caches remaining is actually
> the nice normal "we just switched tasks through normal scheduling".

Yes. And there is another case when fpu_lazy_restore() returns the
false positive.

Suppose that fpu_owner_task exits on CPU_0, and then fork() reuses
its task_struct. The new child is still fpu_owner_task and this is
obviously wrong (unless of course another thread uses fpu).

Initially I thought this should be fixed too, but it seems that
"p->fpu_counter = 0" in copy_thread() saves us.

This looks a bit fragile... And could you confirm this is really
fine?

Btw, do we really need this "old->thread.fpu.last_cpu = ~0" in
the "else" branch of switch_fpu_prepare()? Just curious, I guees
this doesn't matter since we reset old->fpu_counter. But if we
can remove this line, then perhaps we can another optimization.

Oleg.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/