powerpc/irq: Perform stack_overflow detection after switching to IRQ stack

When KASAN is enabled, as shown by the Oops below, the 2k limit is not
enough to allow stack dump after a stack overflow detection when
CONFIG_DEBUG_STACKOVERFLOW is selected:

	do_IRQ: stack overflow: 1984
	CPU: 0 PID: 126 Comm: systemd-udevd Not tainted 5.18.0-gentoo-PMacG4 #1
	Call Trace:
	Oops: Kernel stack overflow, sig: 11 [#1]
	BE PAGE_SIZE=4K MMU=Hash SMP NR_CPUS=2 PowerMac
	Modules linked in: sr_mod cdrom radeon(+) ohci_pci(+) hwmon i2c_algo_bit drm_ttm_helper ttm drm_dp_helper snd_aoa_i2sbus snd_aoa_soundbus snd_pcm ehci_pci snd_timer ohci_hcd snd ssb ehci_hcd 8250_pci soundcore drm_kms_helper pcmcia 8250 pcmcia_core syscopyarea usbcore sysfillrect 8250_base sysimgblt serial_mctrl_gpio fb_sys_fops usb_common pkcs8_key_parser fuse drm drm_panel_orientation_quirks configfs
	CPU: 0 PID: 126 Comm: systemd-udevd Not tainted 5.18.0-gentoo-PMacG4 #1
	NIP:  c02e5558 LR: c07eb3bc CTR: c07f46a8
	REGS: e7fe9f50 TRAP: 0000   Not tainted  (5.18.0-gentoo-PMacG4)
	MSR:  00001032 <ME,IR,DR,RI>  CR: 44a14824  XER: 20000000

	GPR00: c07eb3bc eaa1c000 c26baea0 eaa1c0a0 00000008 00000000 c07eb3bc eaa1c010
	GPR08: eaa1c0a8 04f3f3f3 f1f1f1f1 c07f4c84 44a14824 0080f7e4 00000005 00000010
	GPR16: 00000025 eaa1c154 eaa1c158 c0dbad64 00000020 fd543810 eaa1c0a0 eaa1c29e
	GPR24: c0dbad44 c0db8740 05ffffff fd543802 eaa1c150 c0c9a3c0 eaa1c0a0 c0c9a3c0
	NIP [c02e5558] kasan_check_range+0xc/0x2b4
	LR [c07eb3bc] format_decode+0x80/0x604
	Call Trace:
	[eaa1c000] [c07eb3bc] format_decode+0x80/0x604 (unreliable)
	[eaa1c070] [c07f4dac] vsnprintf+0x128/0x938
	[eaa1c110] [c07f5788] sprintf+0xa0/0xc0
	[eaa1c180] [c0154c1c] __sprint_symbol.constprop.0+0x170/0x198
	[eaa1c230] [c07ee71c] symbol_string+0xf8/0x260
	[eaa1c430] [c07f46d0] pointer+0x15c/0x710
	[eaa1c4b0] [c07f4fbc] vsnprintf+0x338/0x938
	[eaa1c550] [c00e8fa0] vprintk_store+0x2a8/0x678
	[eaa1c690] [c00e94e4] vprintk_emit+0x174/0x378
	[eaa1c6d0] [c00ea008] _printk+0x9c/0xc0
	[eaa1c750] [c000ca94] show_stack+0x21c/0x260
	[eaa1c7a0] [c07d0bd4] dump_stack_lvl+0x60/0x90
	[eaa1c7c0] [c0009234] __do_IRQ+0x170/0x174
	[eaa1c800] [c0009258] do_IRQ+0x20/0x34
	[eaa1c820] [c00045b4] HardwareInterrupt_virt+0x108/0x10c
...

As the detection is asynchronously performed at IRQs, we could be long
after the limit has been crossed, so increasing the limit would not
solve the problem completely.

In order to be sure that there is enough stack space for the stack
dump, do it after the switch to the IRQ stack. That way it is sure
that the stack is large enough, unless the IRQ stack has been
overfilled in which case the end of life is close.
Reported-by: Erhard Furtner <erhard_f@mailbox.org>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/c215d714329f475b431a6193369035aadfc0d182.1654769775.git.christophe.leroy@csgroup.eu

arch/powerpc/kernel/irq.c

@@ -185,14 +185,12 @@ u64 arch_irq_stat_cpu(unsigned int cpu)
 	return sum;
 }
 
-static inline void check_stack_overflow(void)
+static inline void check_stack_overflow(unsigned long sp)
 {
-	long sp;
-
 	if (!IS_ENABLED(CONFIG_DEBUG_STACKOVERFLOW))
 		return;
 
-	sp = current_stack_pointer & (THREAD_SIZE - 1);
+	sp &= THREAD_SIZE - 1;
 
 	/* check for stack overflow: is there less than 1/4th free? */
 	if (unlikely(sp < THREAD_SIZE / 4)) {
@@ -222,12 +220,14 @@ static __always_inline void call_do_softirq(const void *sp)
 
 DEFINE_STATIC_CALL_RET0(ppc_get_irq, *ppc_md.get_irq);
 
-static void __do_irq(struct pt_regs *regs)
+static void __do_irq(struct pt_regs *regs, unsigned long oldsp)
 {
 	unsigned int irq;
 
 	trace_irq_entry(regs);
 
+	check_stack_overflow(oldsp);
+
 	/*
 	 * Query the platform PIC for the interrupt & ack it.
 	 *
@@ -255,6 +255,7 @@ static __always_inline void call_do_irq(struct pt_regs *regs, void *sp)
 	/* Temporarily switch r1 to sp, call __do_irq() then restore r1. */
 	asm volatile (
 		 PPC_STLU "	%%r1, %[offset](%[sp])	;"
+		"mr		%%r4, %%r1		;"
 		"mr		%%r1, %[sp]		;"
 		"bl		%[callee]		;"
 		 PPC_LL "	%%r1, 0(%%r1)		;"
@@ -280,11 +281,9 @@ void __do_IRQ(struct pt_regs *regs)
 	irqsp = hardirq_ctx[raw_smp_processor_id()];
 	sirqsp = softirq_ctx[raw_smp_processor_id()];
 
-	check_stack_overflow();
-
 	/* Already there ? */
 	if (unlikely(cursp == irqsp || cursp == sirqsp)) {
-		__do_irq(regs);
+		__do_irq(regs, current_stack_pointer);
 		set_irq_regs(old_regs);
 		return;
 	}