[PATCH v5 06/30] arm64: context switch POR_EL0 register

[Will Deacon]

On Fri, Aug 23, 2024 at 05:41:06PM +0100, Catalin Marinas wrote:
> On Fri, Aug 23, 2024 at 03:45:32PM +0100, Will Deacon wrote:
> > On Thu, Aug 22, 2024 at 04:10:49PM +0100, Joey Gouly wrote:
> > > +static void permission_overlay_switch(struct task_struct *next)
> > > +{
> > > +	if (!system_supports_poe())
> > > +		return;
> > > +
> > > +	current->thread.por_el0 = read_sysreg_s(SYS_POR_EL0);
> > > +	if (current->thread.por_el0 != next->thread.por_el0) {
> > > +		write_sysreg_s(next->thread.por_el0, SYS_POR_EL0);
> > > +		/* ISB required for kernel uaccess routines when chaning POR_EL0 */
> > 
> > nit: typo "chaning".
> > 
> > But more substantially, is this just to prevent spurious faults in the
> > context of a new thread using a stale value for POR_EL0?
> 
> Not just prevent faults but enforce the permissions from the new
> thread's POR_EL0. The kernel may continue with a uaccess routine from
> here, we can't tell.

Hmm, I wondered if that was the case. It's a bit weird though, because:

  - There's a window between switch_mm() and switch_to() where you might
    reasonably expect to be able to execute uaccess routines

  - kthread_use_mm() doesn't/can't look at this at all

  - GUP obviously doesn't care

So what do we actually gain by having the uaccess routines honour this?

Will