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)
