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

[Andy Lutomirski]

On Mar 5, 2016 1:04 AM, "Ingo Molnar" <mingo@xxxxxxxxxx> wrote:
>
>
> * Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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