RE: [PATCH v4 1/3] x86/mce: Use is_copy_from_user() to determine copy-from-user context

["Luck, Tony" <tony.luck@intel.com>]

> > is_copy_from_user() decodes the instruction that took the trap. It looks for
> > MOV, MOVZ and MOVS instructions to find the source address, and then
> > checks whether that's user (< TASK_SIZE_MAX) or kernel.
>
> You mean there's absolutely nothing else like, say, some epbf or some other
> hackery we tend to do in the kernel (or we will do in the future) which won't
> create the exact same two conditions:
>
> - one of the three insns
> - user mem read
>
> and it would cause a recovery action.
>
> Perhaps it still might be the proper thing to do even then but it does sound
> fishy and unclean to me.
>
> Nothing beats the explicit markup we had until recently...

Every "user mem read" needs to have an extable[] recovery entry
attached to the IP of the instruction  (to handle the much more common
#PF for page-not-present). All those places already have to deal with
the possibility that the #PF can't be recovered. The #MC handling is
really just a small extension.

As for "explicit markup" I don't think it would be better to decorate
every get_user() and copy_from_user() with some "this one can
recover from #MC" 

Note also that "what we had recently" was fragile, broke, and resulted
in this regression.

-Tony