[PATCH] x86: fix early boot crash on gcc-10

[Sergei Trofimovich <slyfox@gentoo.org>]

The change fixes boot failure on physical machine where kernel
is built with gcc-10 with stack-protector enabled by default:

```
Kernel panic — not syncing: stack-protector: Kernel stack is corrupted in: start_secondary+0x191/0x1a0
CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.6.0-rc5—00235—gfffb08b37df9 #139
Hardware name: Gigabyte Technology Co., Ltd. To be filled by O.E.M./H77M—D3H, BIOS F12 11/14/2013
Call Trace:
  dump_stack+0x71/0xa0
  panic+0x107/0x2b8
  ? start_secondary+0x191/0x1a0
  __stack_chk_fail+0x15/0x20
  start_secondary+0x191/0x1a0
  secondary_startup_64+0xa4/0xb0
-—-[ end Kernel panic — not syncing: stack—protector: Kernel stack is corrupted in: start_secondary+0x191
```

This happens because `start_secondary()` is responsible for setting
up initial stack canary value in `smpboot.c`, but nothing prevents
gcc from inserting stack canary into `start_secondary()` itself
before `boot_init_stack_canary()` call.

The fix passes `-fno-stack-protector` to avoid any early stack
checks in `start_secondary()` or inlined functions into it.

Tested the change by successfully booting the machine.

A few similar crashes on VMs:
- https://bugzilla.redhat.com/show_bug.cgi?id=1796780
- http://rglinuxtech.com/?p=2694

CC: Jakub Jelinek <jakub@redhat.com>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Ingo Molnar <mingo@redhat.com>
CC: Borislav Petkov <bp@alien8.de>
CC: "H. Peter Anvin" <hpa@zytor.com>
CC: Andy Lutomirski <luto@kernel.org>
CC: x86@kernel.org
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
---
 arch/x86/kernel/Makefile | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index 9b294c13809a..da9f4ea9bf4c 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -11,6 +11,12 @@ extra-y	+= vmlinux.lds
 
 CPPFLAGS_vmlinux.lds += -U$(UTS_MACHINE)
 
+# smpboot's init_secondary initializes stack canary.
+# Make sure we don't emit stack checks before it's
+# initialized.
+nostackp := $(call cc-option, -fno-stack-protector)
+CFLAGS_smpboot.o := $(nostackp)
+
 ifdef CONFIG_FUNCTION_TRACER
 # Do not profile debug and lowlevel utilities
 CFLAGS_REMOVE_tsc.o = -pg
-- 
2.25.1