Re: [PATCH v3 3/4] uaccess: Check no rescheduling function is called in unsafe region

Peter Zijlstra
2019-02-13T14:41:53Z

On Wed, Feb 13, 2019 at 02:39:22PM +0000, Julien Thierry wrote:
> Hi Peter,
>
> On 13/02/2019 14:25, Peter Zijlstra wrote:
> > On Wed, Feb 13, 2019 at 02:00:26PM +0000, Will Deacon wrote:
> >> The difference is because getting preempted in the sequence above is
> >> triggered off the back of an interrupt. On arm64, and I think also on x86,
> >> the user access state (SMAP or PAN) is saved and restored across exceptions
> >> but not across context switch.
> >
> > A quick reading of the SDM seems to suggest the SMAP state is part of
> > EFLAGS, which is context switched just fine AFAIK.
> >
> I fail to see where this is happening when looking at the switch_to()
> logic in x86_64.

Yeah, me too.. we obviously preserve EFLAGS for user context, but for
kernel-kernel switches we do not seem to preserve it :-(