[PATCH v2] x86: fix early boot crash on gcc-10

[Sergei Trofimovich]

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

```
Kernel panic â 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-rc5â00235âgfffb08b37df9 #139
Hardware name: Gigabyte Technology Co., Ltd. To be filled by O.E.M./H77MâD3H, 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
-â-[ end Kernel panic â not syncing: stackâprotector: 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 inhibits stack canary check foa single `start_secondary()`
function.

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@xxxxxxxxxx>
CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxxxxx>
CC: Borislav Petkov <bp@xxxxxxxxx>
CC: "H. Peter Anvin" <hpa@xxxxxxxxx>
CC: Andy Lutomirski <luto@xxxxxxxxxx>
CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CC: Michael Matz <matz@xxxxxxx>
CC: x86@xxxxxxxxxx
Signed-off-by: Sergei Trofimovich <slyfox@xxxxxxxxxx>
---
 arch/x86/kernel/smpboot.c      | 5 ++++-
 include/linux/compiler-gcc.h   | 1 +
 include/linux/compiler_types.h | 4 ++++
 3 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 69881b2d446c..99a4cb631a64 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -207,8 +207,11 @@ static int cpu0_logical_apicid;
 static int enable_start_cpu0;
 /*
  * Activate a secondary processor.
+ *
+ * Note: 'boot_init_stack_canary' changes canary value. Omit
+ * stack protection to avoid canary check (and boot) failure.
  */
-static void notrace start_secondary(void *unused)
+static void __no_stack_protector notrace start_secondary(void *unused)
 {
 	/*
 	 * Don't put *anything* except direct CPU state initialization
diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index d7ee4c6bad48..fb67c743138c 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -172,3 +172,4 @@
 #endif
 
 #define __no_fgcse __attribute__((optimize("-fno-gcse")))
+#define __no_stack_protector __attribute__((optimize("-fno-stack-protector")))
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 72393a8c1a6c..9d5de1ea0b03 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -212,6 +212,10 @@ struct ftrace_likely_data {
 #define asm_inline asm
 #endif
 
+#ifndef __no_stack_protector
+# define __no_stack_protector
+#endif
+
 #ifndef __no_fgcse
 # define __no_fgcse
 #endif
-- 
2.26.0