[PATCH 16/21] arm64: entry: Implement EL1t exception handlers for overflow stack

[Will Deacon <will@kernel.org>]

In preparation for using EL1t to handle exceptions taken whilst running
on the overflow stack, implement some simple wrappers around the EL1h
handlers which perform a best-effort stack overflow check before
proceeding.

Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
---
 arch/arm64/kernel/entry-common.c | 43 +++++++++++++++++++++++++++++---
 1 file changed, 39 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
index 72c03ccea59f..9738142780df 100644
--- a/arch/arm64/kernel/entry-common.c
+++ b/arch/arm64/kernel/entry-common.c
@@ -327,10 +327,45 @@ static void debug_exception_exit(struct pt_regs *regs)
 }
 NOKPROBE_SYMBOL(debug_exception_exit);
 
-UNHANDLED(el1t, 64, sync)
-UNHANDLED(el1t, 64, irq)
-UNHANDLED(el1t, 64, fiq)
-UNHANDLED(el1t, 64, error)
+static void noinstr el1t_64_check_overflow_stack(struct pt_regs *regs)
+{
+	unsigned long sp = kernel_stack_pointer(regs) - sizeof(*regs);
+	unsigned long ovf_stack = (unsigned long)this_cpu_ptr(overflow_stack);
+
+	/*
+	 * We're in big trouble if we've overflowed the overflow stack
+	 * so perform a best-effort check before we proceed. If our SP
+	 * is outside of the overflow stack for this CPU then presumably
+	 * we're already corrupting memory, so park ourselves here in an
+	 * attempt to contain the damage.
+	 */
+	if (sp < ovf_stack || sp > ovf_stack + OVERFLOW_STACK_SIZE)
+		cpu_park_loop();
+}
+
+asmlinkage void noinstr el1t_64_sync_handler(struct pt_regs *regs)
+{
+	el1t_64_check_overflow_stack(regs);
+	el1h_64_sync_handler(regs);
+}
+
+asmlinkage void noinstr el1t_64_irq_handler(struct pt_regs *regs)
+{
+	el1t_64_check_overflow_stack(regs);
+	el1h_64_irq_handler(regs);
+}
+
+asmlinkage void noinstr el1t_64_fiq_handler(struct pt_regs *regs)
+{
+	el1t_64_check_overflow_stack(regs);
+	el1h_64_fiq_handler(regs);
+}
+
+asmlinkage void noinstr el1t_64_error_handler(struct pt_regs *regs)
+{
+	el1t_64_check_overflow_stack(regs);
+	el1h_64_error_handler(regs);
+}
 
 static void noinstr el1_abort(struct pt_regs *regs, unsigned long esr)
 {
-- 
2.55.0.979.g7e5102b832-goog