xtensa: enable stack protector

The implementation is adopted from the ARM arch. GCC 7.3, 8 or newer is
required for building the xtensa kernel with SSP.

Signed-off-by: Max Filippov <[e-mail]>

Documentation/features/debug/stackprotector/arch-support.txt

···
     |          um: | TODO |
     |   unicore32: | TODO |
     |         x86: |  ok  |
-    |      xtensa: | TODO |
+    |      xtensa: |  ok  |
     -----------------------

arch/xtensa/Kconfig

···
 	select GENERIC_IRQ_SHOW
 	select GENERIC_PCI_IOMAP
 	select GENERIC_SCHED_CLOCK
+	select HAVE_CC_STACKPROTECTOR
 	select HAVE_DEBUG_KMEMLEAK
 	select HAVE_DMA_API_DEBUG
 	select HAVE_DMA_CONTIGUOUS

arch/xtensa/boot/lib/Makefile

···
 CFLAGS_REMOVE_inffast.o = -pg
 endif
 
+CFLAGS_REMOVE_inflate.o += -fstack-protector -fstack-protector-strong
+CFLAGS_REMOVE_zmem.o += -fstack-protector -fstack-protector-strong
+CFLAGS_REMOVE_inftrees.o += -fstack-protector -fstack-protector-strong
+CFLAGS_REMOVE_inffast.o += -fstack-protector -fstack-protector-strong
 
 quiet_cmd_copy_zlib = COPY    $@
       cmd_copy_zlib = cat $< > $@

arch/xtensa/include/asm/stackprotector.h

···
+/*
+ * GCC stack protector support.
+ *
+ * (This is directly adopted from the ARM implementation)
+ *
+ * Stack protector works by putting predefined pattern at the start of
+ * 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 Xtensa.  This unfortunately means that on SMP
+ * we cannot have a different canary value per task.
+ */
+
+#ifndef _ASM_STACKPROTECTOR_H
+#define _ASM_STACKPROTECTOR_H 1
+
+#include <linux/random.h>
+#include <linux/version.h>
+
+extern unsigned long __stack_chk_guard;
+
+/*
+ * Initialize the stackprotector canary value.
+ *
+ * NOTE: this must only be called from functions that never return,
+ * and it must always be inlined.
+ */
+static __always_inline void boot_init_stack_canary(void)
+{
+	unsigned long canary;
+
+	/* Try to get a semi random initial value. */
+	get_random_bytes(&canary, sizeof(canary));
+	canary ^= LINUX_VERSION_CODE;
+
+	current->stack_canary = canary;
+	__stack_chk_guard = current->stack_canary;
+}
+
+#endif	/* _ASM_STACKPROTECTOR_H */

arch/xtensa/kernel/asm-offsets.c

···
 	DEFINE(TASK_PID, offsetof (struct task_struct, pid));
 	DEFINE(TASK_THREAD, offsetof (struct task_struct, thread));
 	DEFINE(TASK_THREAD_INFO, offsetof (struct task_struct, stack));
+#ifdef CONFIG_CC_STACKPROTECTOR
+	DEFINE(TASK_STACK_CANARY, offsetof(struct task_struct, stack_canary));
+#endif
 	DEFINE(TASK_STRUCT_SIZE, sizeof (struct task_struct));
 
 	/* offsets in thread_info struct */

arch/xtensa/kernel/entry.S

···
 	s32i	a1, a2, THREAD_SP	# save stack pointer
 #endif
 
+#if defined(CONFIG_CC_STACKPROTECTOR) && !defined(CONFIG_SMP)
+	movi	a6, __stack_chk_guard
+	l32i	a8, a3, TASK_STACK_CANARY
+	s32i	a8, a6, 0
+#endif
+
 	/* Disable ints while we manipulate the stack pointer. */
 
 	irq_save a14, a3

arch/xtensa/kernel/process.c

···
 EXPORT_SYMBOL(pm_power_off);
 
 
+#ifdef CONFIG_CC_STACKPROTECTOR
+#include <linux/stackprotector.h>
+unsigned long __stack_chk_guard __read_mostly;
+EXPORT_SYMBOL(__stack_chk_guard);
+#endif
+
 #if XTENSA_HAVE_COPROCESSORS
 
 void coprocessor_release_all(struct thread_info *ti)