xen: common: add ability to enable stack protector

Both GCC and Clang support -fstack-protector feature, which add stack
canaries to functions where stack corruption is possible. This patch
makes general preparations to enable this feature on different
supported architectures:

 - Added CONFIG_HAS_STACK_PROTECTOR option so each architecture
   can enable this feature individually
 - Added user-selectable CONFIG_STACK_PROTECTOR option
 - Implemented code that sets up random stack canary and a basic
   handler for stack protector failures

Stack guard value is initialized in two phases:

1. Pre-defined randomly-selected value.

2. Own implementation of linear congruent random number generator. It
relies on get_cycles() being available very early. If get_cycles()
returns zero, it would leave pre-defined value from the previous step.

boot_stack_chk_guard_setup() is declared as always_inline to ensure
that it will not trigger stack protector by itself. And of course,
caller should ensure that stack protection code will not be reached
later. It is possible to call the same function from an ASM code by
introducing simple trampoline in stack-protector.c, but right now
there is no use case for such trampoline.

As __stack_chk_fail() is not called by Xen source code directly, and
only called by compiler-generated code, it does not needed to be
declared separately. So we need separate MISRA deviation for it.

Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

docs/misra/safe.json

@@ -108,6 +108,14 @@
         },
         {
             "id": "SAF-13-safe",
+            "analyser": {
+                "eclair": "MC3A2.R8.4"
+            },
+            "name": "Rule 8.4: compiler-called function",
+            "text": "A function, all invocations of which are compiler generated, does not need to have a visible declaration prior to its definition."
+        },
+        {
+            "id": "SAF-14-safe",
             "analyser": {},
             "name": "Sentinel",
             "text": "Next ID to be used"

xen/Makefile

@@ -435,7 +435,11 @@ else
 CFLAGS_UBSAN :=
 endif
 
+ifeq ($(CONFIG_STACK_PROTECTOR),y)
+CFLAGS += -fstack-protector
+else
 CFLAGS += -fno-stack-protector
+endif
 
 ifeq ($(CONFIG_LTO),y)
 CFLAGS += -flto

xen/common/Kconfig

@@ -100,6 +100,9 @@ config HAS_PMAP
 config HAS_SCHED_GRANULARITY
 	bool
 
+config HAS_STACK_PROTECTOR
+	bool
+
 config HAS_UBSAN
 	bool
 
@@ -233,6 +236,18 @@ config SPECULATIVE_HARDEN_LOCK
 
 endmenu
 
+menu "Other hardening"
+
+config STACK_PROTECTOR
+	bool "Stack protector"
+	depends on HAS_STACK_PROTECTOR
+	help
+	  Enable the Stack Protector compiler hardening option. This inserts a
+	  canary value in the stack frame of functions, and performs an integrity
+	  check on function exit.
+
+endmenu
+
 config DIT_DEFAULT
 	bool "Data Independent Timing default"
 	depends on HAS_DIT

xen/common/Makefile

@@ -46,6 +46,7 @@ obj-y += shutdown.o
 obj-y += softirq.o
 obj-y += smp.o
 obj-y += spinlock.o
+obj-$(CONFIG_STACK_PROTECTOR) += stack-protector.o
 obj-y += stop_machine.o
 obj-y += symbols.o
 obj-y += tasklet.o

xen/common/stack-protector.c

@@ -0,0 +1,22 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#include <xen/init.h>
+#include <xen/lib.h>
+#include <xen/random.h>
+#include <xen/time.h>
+
+/*
+ * Initial value is chosen by a fair dice roll.
+ * It will be updated during boot process.
+ */
+#if BITS_PER_LONG == 32
+unsigned long __ro_after_init __stack_chk_guard = 0xdd2cc927UL;
+#else
+unsigned long __ro_after_init __stack_chk_guard = 0x2d853605a4d9a09cUL;
+#endif
+
+/* SAF-13-safe compiler-called function */
+void noreturn __stack_chk_fail(void)
+{
+    dump_execution_state();
+    panic("Stack Protector integrity violation identified\n");
+}

xen/include/xen/stack-protector.h

@@ -0,0 +1,39 @@
+#ifndef __XEN_STACK_PROTECTOR_H__
+#define __XEN_STACK_PROTECTOR_H__
+
+extern unsigned long __stack_chk_guard;
+
+/*
+ * This function should be called from a C function that escapes stack
+ * canary tracking (by calling reset_stack_and_jump() for example).
+ */
+static always_inline void boot_stack_chk_guard_setup(void)
+{
+#ifdef CONFIG_STACK_PROTECTOR
+
+    /*
+     * Linear congruent generator (X_n+1 = X_n * a + c).
+     *
+     * Constant is taken from "Tables Of Linear Congruential
+     * Generators Of Different Sizes And Good Lattice Structure" by
+     * Pierre L’Ecuyer.
+     */
+#if BITS_PER_LONG == 32
+    const unsigned long a = 2891336453UL;
+#else
+    const unsigned long a = 2862933555777941757UL;
+#endif
+    const unsigned long c = 1;
+
+    unsigned long cycles = get_cycles();
+
+    /* Use the initial value if we can't generate random one */
+    if ( !cycles )
+        return;
+
+    __stack_chk_guard = cycles * a + c;
+
+#endif	/* CONFIG_STACK_PROTECTOR */
+}
+
+#endif	/* __XEN_STACK_PROTECTOR_H__ */