linux-kernel - Re: soft lockup when passing vvar address to write(2)

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Date:	Sat, 5 Mar 2016 18:59:07 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: soft lockup when passing vvar address to write(2)

On Mar 5, 2016 1:04 AM, "Ingo Molnar" <mingo@...nel.org> wrote:
>
>
> * Thomas Gleixner <tglx@...utronix.de> wrote:
>
> > On Fri, 4 Mar 2016, Andy Lutomirski wrote:
> > > Thomas, I still think we should consider just deleting the HPET vclock
> > > code and accept the syscall overhead on systems that are stuck using
> > > HPET.  If fast syscalls are available (which should include every
> > > system with HPET, unless there are some 32-bit AMD systems lying
> > > around), then the overhead in a syscall is *tiny* compared to the code
> > > of the HPET read itself.
> >
> > No objection from my side, really.
>
> Seconded. HPET hardware overhead is typically horrifically large in any case, no
> need to memory map it and expose hardware breakages to user-space ...

I'll do it for 4.7.

>
> It's also a (mild) security hole: a well-known HPET address can be abused as a
> statistical trampoline periodically cycling through 'dangerous' instruction
> values.

That weakness has closed for quite a while -- it's mapped NX and it's
randomized.

I'm also not planning to revert the mapping security improvement --
even if we remove the HPET code, it still applies to kvmclock and to
anything else that gets added in the future.  It's also very little
code.

--Andy