Subject: Re: [PATCH 18/18] arm64: implement Shadow Call Stack

From: Mark Rutland <[email-masked]>
Date: Mon, 21 Oct 2019 17:49:04 +0100

On Fri, Oct 18, 2019 at 10:35:49AM -0700, Sami Tolvanen wrote:
> On Fri, Oct 18, 2019 at 10:23 AM Mark Rutland <[email-masked]> wrote:
> > I think scs_save() would better live in assembly in cpu_switch_to(),
> > where we switch the stack and current. It shouldn't matter whether
> > scs_load() is inlined or not, since the x18 value _should_ be invariant
> > from the PoV of the task.
> 
> Note that there's also a call to scs_save in cpu_die, because the
> current task's shadow stack pointer is only stored in x18 and we don't
> want to lose it.
> 
> > We just need to add a TSK_TI_SCS to asm-offsets.c, and then insert a
> > single LDR at the end:
> >
> >         mov     sp, x9
> >         msr     sp_el0, x1
> > #ifdef CONFIG_SHADOW_CALL_STACK
> >         ldr     x18, [x1, TSK_TI_SCS]
> > #endif
> >         ret
> 
> TSK_TI_SCS is already defined, so yes, we could move this to
> cpu_switch_to. I would still prefer to have the overflow check that's
> in scs_thread_switch though.

The only bit that I think needs to be in cpu_switch_to() is the install
of the next task's shadow addr into x18.

Having a separate scs_check_overflow() sounds fine to me, as that only
has to read from the shadow stack. IIUC that's also for the prev task,
not next, in the current patches.

Thanks,
Mark.

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