ARM: implement IRQ stacks

Now that we no longer rely on the stack pointer to access the current
task struct or thread info, we can implement support for IRQ stacks
cleanly as well.

Define a per-CPU IRQ stack and switch to this stack when taking an IRQ,
provided that we were not already using that stack in the interrupted
context. This is never the case for IRQs taken from user space, but ones
taken while running in the kernel could fire while one taken from user
space has not completed yet.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Keith Packard <keithpac@amazon.com>
Acked-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Marc Zyngier <maz@kernel.org>
Tested-by: Vladimir Murzin <vladimir.murzin@arm.com> # ARMv7M

arch/arm/Kconfig

@@ -1166,6 +1166,10 @@ config CURRENT_POINTER_IN_TPIDRURO
 	def_bool y
 	depends on SMP && CPU_32v6K && !CPU_V6
 
+config IRQSTACKS
+	def_bool y
+	depends on GENERIC_IRQ_MULTI_HANDLER && THREAD_INFO_IN_TASK
+
 config ARM_CPU_TOPOLOGY
 	bool "Support cpu topology definition"
 	depends on SMP && CPU_V7

arch/arm/include/asm/assembler.h

@@ -86,6 +86,10 @@
 
 #define IMM12_MASK 0xfff
 
+/* the frame pointer used for stack unwinding */
+ARM(	fpreg	.req	r11	)
+THUMB(	fpreg	.req	r7	)
+
 /*
  * Enable and disable interrupts
  */

arch/arm/kernel/entry-armv.S

@@ -36,10 +36,58 @@
 /*
  * Interrupt handling.
  */
-	.macro	irq_handler
+	.macro	irq_handler, from_user:req
 #ifdef CONFIG_GENERIC_IRQ_MULTI_HANDLER
 	mov	r0, sp
+#ifdef CONFIG_IRQSTACKS
+	mov_l	r2, irq_stack_ptr	@ Take base address
+	mrc	p15, 0, r3, c13, c0, 4	@ Get CPU offset
+#ifdef CONFIG_UNWINDER_ARM
+	mov	fpreg, sp		@ Preserve original SP
+#else
+	mov	r8, fp			@ Preserve original FP
+	mov	r9, sp			@ Preserve original SP
+#endif
+	ldr	sp, [r2, r3]		@ Load SP from per-CPU var
+	.if	\from_user == 0
+UNWIND(	.setfp	fpreg, sp		)
+	@
+	@ If we took the interrupt while running in the kernel, we may already
+	@ be using the IRQ stack, so revert to the original value in that case.
+	@
+	subs	r2, sp, r0		@ SP above bottom of IRQ stack?
+	rsbscs	r2, r2, #THREAD_SIZE	@ ... and below the top?
+	movcs	sp, r0			@ If so, revert to incoming SP
+
+#ifndef CONFIG_UNWINDER_ARM
+	@
+	@ Inform the frame pointer unwinder where the next frame lives
+	@
+	movcc	lr, pc			@ Make LR point into .entry.text so
+					@ that we will get a dump of the
+					@ exception stack for this frame.
+#ifdef CONFIG_CC_IS_GCC
+	movcc	ip, r0			@ Store the old SP in the frame record.
+	stmdbcc	sp!, {fp, ip, lr, pc}	@ Push frame record
+	addcc	fp, sp, #12
+#else
+	stmdbcc	sp!, {fp, lr}		@ Push frame record
+	movcc	fp, sp
+#endif // CONFIG_CC_IS_GCC
+#endif // CONFIG_UNWINDER_ARM
+	.endif
+#endif // CONFIG_IRQSTACKS
+
 	bl	generic_handle_arch_irq
+
+#ifdef CONFIG_IRQSTACKS
+#ifdef CONFIG_UNWINDER_ARM
+	mov	sp, fpreg		@ Restore original SP
+#else
+	mov	fp, r8			@ Restore original FP
+	mov	sp, r9			@ Restore original SP
+#endif // CONFIG_UNWINDER_ARM
+#endif // CONFIG_IRQSTACKS
 #else
 	arch_irq_handler_default
 #endif
@@ -199,7 +247,7 @@ ENDPROC(__dabt_svc)
 	.align	5
 __irq_svc:
 	svc_entry
-	irq_handler
+	irq_handler from_user=0
 
 #ifdef CONFIG_PREEMPTION
 	ldr	r8, [tsk, #TI_PREEMPT]		@ get preempt count
@@ -426,7 +474,7 @@ ENDPROC(__dabt_usr)
 __irq_usr:
 	usr_entry
 	kuser_cmpxchg_check
-	irq_handler
+	irq_handler from_user=1
 	get_thread_info tsk
 	mov	why, #0
 	b	ret_to_user_from_irq

arch/arm/kernel/irq.c

@@ -43,6 +43,25 @@
 
 unsigned long irq_err_count;
 
+#ifdef CONFIG_IRQSTACKS
+
+asmlinkage DEFINE_PER_CPU_READ_MOSTLY(u8 *, irq_stack_ptr);
+
+static void __init init_irq_stacks(void)
+{
+	u8 *stack;
+	int cpu;
+
+	for_each_possible_cpu(cpu) {
+		stack = (u8 *)__get_free_pages(GFP_KERNEL, THREAD_SIZE_ORDER);
+		if (WARN_ON(!stack))
+			break;
+		per_cpu(irq_stack_ptr, cpu) = &stack[THREAD_SIZE];
+	}
+}
+
+#endif
+
 int arch_show_interrupts(struct seq_file *p, int prec)
 {
 #ifdef CONFIG_FIQ
@@ -101,6 +120,10 @@ void __init init_IRQ(void)
 {
 	int ret;
 
+#ifdef CONFIG_IRQSTACKS
+	init_irq_stacks();
+#endif
+
 	if (IS_ENABLED(CONFIG_OF) && !machine_desc->init_irq)
 		irqchip_init();
 	else

arch/arm/kernel/traps.c

@@ -66,6 +66,19 @@ void dump_backtrace_entry(unsigned long where, unsigned long from,
 {
 	unsigned long end = frame + 4 + sizeof(struct pt_regs);
 
+	if (IS_ENABLED(CONFIG_UNWINDER_FRAME_POINTER) &&
+	    IS_ENABLED(CONFIG_CC_IS_GCC) &&
+	    end > ALIGN(frame, THREAD_SIZE)) {
+		/*
+		 * If we are walking past the end of the stack, it may be due
+		 * to the fact that we are on an IRQ or overflow stack. In this
+		 * case, we can load the address of the other stack from the
+		 * frame record.
+		 */
+		frame = ((unsigned long *)frame)[-2] - 4;
+		end = frame + 4 + sizeof(struct pt_regs);
+	}
+
 #ifdef CONFIG_KALLSYMS
 	printk("%s[<%08lx>] (%ps) from [<%08lx>] (%pS)\n",
 		loglvl, where, (void *)where, from, (void *)from);
@@ -278,7 +291,7 @@ static int __die(const char *str, int err, struct pt_regs *regs)
 
 	if (!user_mode(regs) || in_interrupt()) {
 		dump_mem(KERN_EMERG, "Stack: ", regs->ARM_sp,
-			 THREAD_SIZE + (unsigned long)task_stack_page(tsk));
+			 ALIGN(regs->ARM_sp, THREAD_SIZE));
 		dump_backtrace(regs, tsk, KERN_EMERG);
 		dump_instr(KERN_EMERG, regs);
 	}

arch/arm/lib/backtrace-clang.S

@@ -197,6 +197,14 @@ finished_setup:
 
 		cmp	sv_fp, frame		@ next frame must be
 		mov	frame, sv_fp		@ above the current frame
+#ifdef CONFIG_IRQSTACKS
+		@
+		@ Kernel stacks may be discontiguous in memory. If the next
+		@ frame is below the previous frame, accept it as long as it
+		@ lives in kernel memory.
+		@
+		cmpls	sv_fp, #PAGE_OFFSET
+#endif
 		bhi	for_each_frame
 
 1006:		adr	r0, .Lbad

arch/arm/lib/backtrace.S

@@ -98,6 +98,14 @@ for_each_frame:	tst	frame, mask		@ Check for address exceptions
 
 		cmp	sv_fp, frame		@ next frame must be
 		mov	frame, sv_fp		@ above the current frame
+#ifdef CONFIG_IRQSTACKS
+		@
+		@ Kernel stacks may be discontiguous in memory. If the next
+		@ frame is below the previous frame, accept it as long as it
+		@ lives in kernel memory.
+		@
+		cmpls	sv_fp, #PAGE_OFFSET
+#endif
 		bhi	for_each_frame
 
 1006:		adr	r0, .Lbad