Re: [PATCH] arm64: mte: Do not service syscalls after async tag fault

[Peter Collingbourne <pcc@google.com>]

On Wed, Feb 12, 2020 at 3:09 AM Catalin Marinas <catalin.marinas@arm.com> wrote:
>
> On Thu, Dec 19, 2019 at 05:36:39PM -0800, Peter Collingbourne wrote:
> > When entering the kernel after an async tag fault due to a syscall, rather
> > than for another reason (e.g. preemption), we don't want to service the
> > syscall as it may mask the tag fault. Rewind the PC to the svc instruction
> > in order to give a userspace signal handler an opportunity to handle the
> > fault and resume, and skip all other syscall processing.
> >
> > Signed-off-by: Peter Collingbourne <pcc@google.com>
> > ---
> [...]
> >  arch/arm64/kernel/syscall.c | 22 +++++++++++++++++++---
> >  1 file changed, 19 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
> > index 9a9d98a443fc..49ea9bb47190 100644
> > --- a/arch/arm64/kernel/syscall.c
> > +++ b/arch/arm64/kernel/syscall.c
> > @@ -95,13 +95,29 @@ static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
> >  {
> >       unsigned long flags = current_thread_info()->flags;
> >
> > -     regs->orig_x0 = regs->regs[0];
> > -     regs->syscallno = scno;
> > -
> >       cortex_a76_erratum_1463225_svc_handler();
> >       local_daif_restore(DAIF_PROCCTX);
> >       user_exit();
> >
> > +#ifdef CONFIG_ARM64_MTE
> > +     if (flags & _TIF_MTE_ASYNC_FAULT) {
> > +             /*
> > +              * We entered the kernel after an async tag fault due to a
> > +              * syscall, rather than for another reason (e.g. preemption).
> > +              * In this case, we don't want to service the syscall as it may
> > +              * mask the tag fault. Rewind the PC to the svc instruction in
> > +              * order to give a userspace signal handler an opportunity to
> > +              * handle the fault and resume, and skip all other syscall
> > +              * processing.
> > +              */
> > +             regs->pc -= 4;
> > +             return;
> > +     }
> > +#endif
> > +
> > +     regs->orig_x0 = regs->regs[0];
> > +     regs->syscallno = scno;
>
> I'm slightly worried about the interaction with single-step, other
> signals. It might be better if we just use the existing syscall
> restarting mechanism. Untested diff below:
>
> -------------------8<-------------------------------
> diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
> index a12c0c88d345..db25f5d6a07c 100644
> --- a/arch/arm64/kernel/syscall.c
> +++ b/arch/arm64/kernel/syscall.c
> @@ -102,6 +102,16 @@ static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
>         local_daif_restore(DAIF_PROCCTX);
>         user_exit();
>
> +       if (system_supports_mte() && (flags & _TIF_MTE_ASYNC_FAULT)) {
> +               /*
> +                * Process the asynchronous tag check fault before the actual
> +                * syscall. do_notify_resume() will send a signal to userspace
> +                * before the syscall is restarted.
> +                */
> +               regs->regs[0] = -ERESTARTNOINTR;
> +               return;
> +       }
> +
>         if (has_syscall_work(flags)) {
>                 /* set default errno for user-issued syscall(-1) */
>                 if (scno == NO_SYSCALL)

That works for me, and I verified that my small test program as well
as some larger unit tests behave as expected.

Tested-by: Peter Collingbourne <pcc@google.com>


Peter