On Tue, Feb 18, 2020 at 01:59:34PM -0800, Peter Collingbourne wrote:
> On Wed, Feb 12, 2020 at 3:09 AM Catalin Marinas <catalin.marinas@xxxxxxx> 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@xxxxxxxxxx>
> > > ---
> > [...]
> > > 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@xxxxxxxxxx>
Thanks Peter.
--
Catalin
