On Fri, Aug 23, 2024 at 07:40:52PM +0100, Catalin Marinas wrote:
> On Fri, Aug 23, 2024 at 06:08:36PM +0100, Will Deacon wrote:
> > 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
>
> I don't think we can have any uaccess between these two switches (a
> uaccess could fault, that's a pretty weird state between these two).
>
> > - kthread_use_mm() doesn't/can't look at this at all
>
> No, but a kthread would have it's own, most permissive, POR_EL0.
>
> > - GUP obviously doesn't care
> >
> > So what do we actually gain by having the uaccess routines honour this?
>
> I guess where it matters is more like not accidentally faulting because
> the previous thread had more restrictive permissions.
That's what I wondered initially, but won't the fault handler retry in
that case?
Will
