[PATCH 2/3] x86/entry/64: Teach idtentry to use the IRQ stack

[Andy Lutomirski <luto@kernel.org>]

We don't specifically need IST for things like kprobes, but we do
want to avoid rare, surprising extra stack usage if a kprobe hits
with a deep stack.

Teach idtentry to use the IRQ stack for selected entries.

This implementation uses the IRQ stack even if we entered from user
mode.  This disallows tricks like ist_begin_non_atomic.  If we ever
need such a trick in one of these entries, we can rework this.  For
now, let's keep it simple.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/entry/entry_64.S | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 5f7df8949fa7..ce72beba6045 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -739,13 +739,17 @@ apicinterrupt IRQ_WORK_VECTOR			irq_work_interrupt		smp_irq_work_interrupt
  */
 #define CPU_TSS_IST(x) PER_CPU_VAR(cpu_tss) + (TSS_ist + ((x) - 1) * 8)
 
-.macro idtentry sym do_sym has_error_code:req paranoid=0 shift_ist=-1
+.macro idtentry sym do_sym has_error_code:req irqstack=0 paranoid=0 shift_ist=-1
 ENTRY(\sym)
 	/* Sanity check */
 	.if \shift_ist != -1 && \paranoid == 0
 	.error "using shift_ist requires paranoid=1"
 	.endif
 
+	.if \irqstack && \paranoid
+	.error "using irqstack requires !paranoid"
+	.endif
+
 	ASM_CLAC
 	PARAVIRT_ADJUST_EXCEPTION_FRAME
 
@@ -787,8 +791,16 @@ ENTRY(\sym)
 	subq	$EXCEPTION_STKSZ, CPU_TSS_IST(\shift_ist)
 	.endif
 
+	.if \irqstack
+	ENTER_IRQ_STACK old_rsp=%rcx
+	.endif
+
 	call	\do_sym
 
+	.if \irqstack
+	LEAVE_IRQ_STACK
+	.endif
+
 	.if \shift_ist != -1
 	addq	$EXCEPTION_STKSZ, CPU_TSS_IST(\shift_ist)
 	.endif
-- 
2.4.3