[PATCH v6 2/4] xen: common: add ability to enable stack protector

[Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>]

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 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 inline, so it can be
called from C code. Of course, in this case, caller should ensure that
stack protection code will not be reached. It is possible to call the
same function from ASM code by introducing simple trampoline in
stack-protector.c, but right now there is no use case for such
trampoline.

Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>

---

Changes in v6:
- boot_stack_chk_guard_setup() moved to stack-protector.h
- Removed Andrew's r-b tag

Changes in v5:

- Fixed indentation
- Added stack-protector.h
---
 xen/Makefile                      |  4 +++
 xen/common/Kconfig                | 15 +++++++++++
 xen/common/Makefile               |  1 +
 xen/common/stack-protector.c      | 21 +++++++++++++++
 xen/include/xen/stack-protector.h | 43 +++++++++++++++++++++++++++++++
 5 files changed, 84 insertions(+)
 create mode 100644 xen/common/stack-protector.c
 create mode 100644 xen/include/xen/stack-protector.h

diff --git a/xen/Makefile b/xen/Makefile
index a0c774ab7d..48bc17c418 100644
--- a/xen/Makefile
+++ b/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
diff --git a/xen/common/Kconfig b/xen/common/Kconfig
index 6166327f4d..bd53dae43c 100644
--- a/xen/common/Kconfig
+++ b/xen/common/Kconfig
@@ -83,6 +83,9 @@ config HAS_PMAP
 config HAS_SCHED_GRANULARITY
        bool
 
+config HAS_STACK_PROTECTOR
+       bool
+
 config HAS_UBSAN
        bool
 
@@ -216,6 +219,18 @@ 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
diff --git a/xen/common/Makefile b/xen/common/Makefile
index cba3b32733..8adbf6a3b5 100644
--- a/xen/common/Makefile
+++ b/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
diff --git a/xen/common/stack-protector.c b/xen/common/stack-protector.c
new file mode 100644
index 0000000000..9089294d30
--- /dev/null
+++ b/xen/common/stack-protector.c
@@ -0,0 +1,21 @@
+/* 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
+
+void asmlinkage __stack_chk_fail(void)
+{
+    dump_execution_state();
+    panic("Stack Protector integrity violation identified\n");
+}
diff --git a/xen/include/xen/stack-protector.h 
b/xen/include/xen/stack-protector.h
new file mode 100644
index 0000000000..b758a8cb3d
--- /dev/null
+++ b/xen/include/xen/stack-protector.h
@@ -0,0 +1,43 @@
+#ifndef __XEN_STACK_PROTECTOR_H__
+#define __XEN_STACK_PROTECTOR_H__
+
+#ifdef CONFIG_STACK_PROTECTOR
+
+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 inline void boot_stack_chk_guard_setup(void)
+{
+    /*
+     * 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;
+}
+
+#else
+
+static inline void boot_stack_chk_guard_setup(void) {};
+
+#endif
+
+#endif /* __XEN_STACK_PROTECTOR_H__ */
-- 
2.47.1