Re: [PATCH] x86: mce: fix kernel panic when check_interval is changed

[Borislav Petkov]

On Fri, Feb 23, 2018 at 07:13:50PM +0900, Seunghun Han wrote:
> I am Seunghun Han and a senior security researcher at National Security
> Research Institute of South Korea.
> 
> I found a critical security issue which can make kernel panic in userspace.
> After analyzing the issue carefully, I found that MCE driver in the kernel
> has a problem which can be occurred in SMP environment.
> 
> The check_interval file in
> /sys/devices/system/machinecheck/machinecheck<cpu number> directory is a
> global timer value for MCE polling. If it is changed by one CPU, MCE driver
> in kernel calls mce_restart() function and broadcasts the event to other

Right, so I'm thinking that doing that per-CPU configuration doesn't
make a whole lot of sense. It is not something that needs to happen very
often and it is done globally anyway.

So what we should do here, IMO, is make mce_restart() grab a mutex and
thus serialize all those sysfs writes. It will naturally also slow down
any hammering from userspace which we should not allow anyway.

Tony, what do you think?

-- 
Regards/Gruss,
    Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.