A single fix for early boot crashes of kernels built with gcc10 and

stack protector enabled.
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Merge tag 'x86_urgent_for_v5.7-rc7' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 fix from Borislav Petkov:
 "A single fix for early boot crashes of kernels built with gcc10 and
  stack protector enabled"

* tag 'x86_urgent_for_v5.7-rc7' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86: Fix early boot crash on gcc-10, third try

arch/x86/include/asm/stackprotector.h

@@ -55,8 +55,13 @@
 /*
  * Initialize the stackprotector canary value.
  *
- * NOTE: this must only be called from functions that never return,
+ * NOTE: this must only be called from functions that never return
  * and it must always be inlined.
+ *
+ * In addition, it should be called from a compilation unit for which
+ * stack protector is disabled. Alternatively, the caller should not end
+ * with a function call which gets tail-call optimized as that would
+ * lead to checking a modified canary value.
  */
 static __always_inline void boot_init_stack_canary(void)
 {

arch/x86/kernel/smpboot.c

@@ -266,6 +266,14 @@ static void notrace start_secondary(void *unused)
 
 	wmb();
 	cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
+
+	/*
+	 * Prevent tail call to cpu_startup_entry() because the stack protector
+	 * guard has been changed a couple of function calls up, in
+	 * boot_init_stack_canary() and must not be checked before tail calling
+	 * another function.
+	 */
+	prevent_tail_call_optimization();
 }
 
 /**

arch/x86/xen/smp_pv.c

@@ -93,6 +93,7 @@ asmlinkage __visible void cpu_bringup_and_idle(void)
 	cpu_bringup();
 	boot_init_stack_canary();
 	cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
+	prevent_tail_call_optimization();
 }
 
 void xen_smp_intr_free_pv(unsigned int cpu)

include/linux/compiler.h

@@ -356,4 +356,10 @@ static inline void *offset_to_ptr(const int *off)
 /* &a[0] degrades to a pointer: a different type from an array */
 #define __must_be_array(a)	BUILD_BUG_ON_ZERO(__same_type((a), &(a)[0]))
 
+/*
+ * This is needed in functions which generate the stack canary, see
+ * arch/x86/kernel/smpboot.c::start_secondary() for an example.
+ */
+#define prevent_tail_call_optimization()	mb()
+
 #endif /* __LINUX_COMPILER_H */

init/main.c

@@ -1038,6 +1038,8 @@ asmlinkage __visible void __init start_kernel(void)
 
 	/* Do the rest non-__init'ed, we're now alive */
 	arch_call_rest_init();
+
+	prevent_tail_call_optimization();
 }
 
 /* Call all constructor functions linked into the kernel. */