On Tue, Sep 03, 2024 at 03:54:13PM +0100, Joey Gouly wrote:
> On Mon, Sep 02, 2024 at 08:08:08PM +0100, Catalin Marinas wrote:
> > On Tue, Aug 27, 2024 at 12:38:04PM +0100, Will Deacon wrote:
> > > 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.
> > [...]
> > > > > 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?
> >
> > Yes, it will retry and this should be fine (I assume you are only
> > talking about the dropping ISB in the context switch).
> >
> > For the case of running with a more permissive stale POR_EL0, arguably it's
> > slightly more predictable for the user but, OTOH, some syscalls like
> > readv() could be routed through GUP with no checks. As with MTE, we
> > don't guarantee uaccesses honour the user permissions.
> >
> > That said, at some point we should sanitise this path anyway and have a
> > single ISB at the end. In the meantime, I'm fine with dropping the ISB
> > here.
> >
>
> commit 3141fb86bee8d48ae47cab1594dad54f974a8899
> Author: Joey Gouly <joey.gouly@xxxxxxx>
> Date: Tue Sep 3 15:47:26 2024 +0100
>
> fixup! arm64: context switch POR_EL0 register
>
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index a3a61ecdb165..c224b0955f1a 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -515,11 +515,8 @@ static void permission_overlay_switch(struct task_struct *next)
> return;
>
> current->thread.por_el0 = read_sysreg_s(SYS_POR_EL0);
> - if (current->thread.por_el0 != next->thread.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 */
> - isb();
> - }
> }
What about the one in flush_poe()? I'm inclined to drop that as well.
> Will, do you want me to re-send the series with this and the permissions
> diff from the other thread [1],
> or you ok with applying them when you pull it in?
I'll have a crack now, but if it fails miserably then I'll let you know.
Will
