Merge tag 'gcc-plugins-v4.21-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux

Pull gcc-plugins update from Kees Cook:
 "Both arm and arm64 are gaining per-task stack canaries (to match x86),
  but arm is being done with a gcc plugin, hence it going through the
  gcc-plugins tree.

  New gcc-plugin:

   - Enable per-task stack protector for ARM (Ard Biesheuvel)"

* tag 'gcc-plugins-v4.21-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux:
  ARM: smp: add support for per-task stack canaries

arch/arm/include/asm/stackprotector.h

@@ -6,8 +6,10 @@
  * the stack frame and verifying that it hasn't been overwritten when
  * returning from the function.  The pattern is called stack canary
  * and gcc expects it to be defined by a global variable called
- * "__stack_chk_guard" on ARM.  This unfortunately means that on SMP
- * we cannot have a different canary value per task.
+ * "__stack_chk_guard" on ARM.  This prevents SMP systems from using a
+ * different value for each task unless we enable a GCC plugin that
+ * replaces these symbol references with references to each task's own
+ * value.
  */
 
 #ifndef _ASM_STACKPROTECTOR_H
@@ -16,6 +18,8 @@
 #include <linux/random.h>
 #include <linux/version.h>
 
+#include <asm/thread_info.h>
+
 extern unsigned long __stack_chk_guard;
 
 /*
@@ -33,7 +37,11 @@ static __always_inline void boot_init_stack_canary(void)
 	canary ^= LINUX_VERSION_CODE;
 
 	current->stack_canary = canary;
+#ifndef CONFIG_STACKPROTECTOR_PER_TASK
 	__stack_chk_guard = current->stack_canary;
+#else
+	current_thread_info()->stack_canary = current->stack_canary;
+#endif
 }
 
 #endif	/* _ASM_STACKPROTECTOR_H */

arch/arm/include/asm/thread_info.h

@@ -53,6 +53,9 @@ struct thread_info {
 	struct task_struct	*task;		/* main task structure */
 	__u32			cpu;		/* cpu */
 	__u32			cpu_domain;	/* cpu domain */
+#ifdef CONFIG_STACKPROTECTOR_PER_TASK
+	unsigned long		stack_canary;
+#endif
 	struct cpu_context_save	cpu_context;	/* cpu context */
 	__u32			syscall;	/* syscall number */
 	__u8			used_cp[16];	/* thread used copro */