[PATCH v4 4/5] x86/entry: Enable random_kstack_offset support

[Kees Cook <keescook@xxxxxxxxxxxx>]

Allow for a randomized stack offset on a per-syscall basis, with roughly
5 bits of entropy.

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 arch/x86/Kconfig        |  1 +
 arch/x86/entry/common.c | 11 +++++++++++
 2 files changed, 12 insertions(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 6a0cc524882d..57c2a458150e 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -156,6 +156,7 @@ config X86
 	select HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD if X86_64
 	select HAVE_ARCH_USERFAULTFD_WP         if X86_64 && USERFAULTFD
 	select HAVE_ARCH_VMAP_STACK		if X86_64
+	select HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET
 	select HAVE_ARCH_WITHIN_STACK_FRAMES
 	select HAVE_ASM_MODVERSIONS
 	select HAVE_CMPXCHG_DOUBLE
diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index bd3f14175193..681125bbf09a 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -26,6 +26,7 @@
 #include <linux/livepatch.h>
 #include <linux/syscalls.h>
 #include <linux/uaccess.h>
+#include <linux/randomize_kstack.h>
 
 #ifdef CONFIG_XEN_PV
 #include <xen/xen-ops.h>
@@ -240,6 +241,13 @@ static void __prepare_exit_to_usermode(struct pt_regs *regs)
 	lockdep_assert_irqs_disabled();
 	lockdep_sys_exit();
 
+	/*
+	 * x86_64 stack alignment means 3 bits are ignored, so keep
+	 * the top 5 bits. x86_32 needs only 2 bits of alignment, so
+	 * the top 6 bits will be used.
+	 */
+	choose_random_kstack_offset(rdtsc() & 0xFF);
+
 	cached_flags = READ_ONCE(ti->flags);
 
 	if (unlikely(cached_flags & EXIT_TO_USERMODE_LOOP_FLAGS))
@@ -346,6 +354,7 @@ __visible noinstr void do_syscall_64(unsigned long nr, struct pt_regs *regs)
 {
 	struct thread_info *ti;
 
+	add_random_kstack_offset();
 	enter_from_user_mode();
 	instrumentation_begin();
 
@@ -409,6 +418,7 @@ static void do_syscall_32_irqs_on(struct pt_regs *regs)
 /* Handles int $0x80 */
 __visible noinstr void do_int80_syscall_32(struct pt_regs *regs)
 {
+	add_random_kstack_offset();
 	enter_from_user_mode();
 	instrumentation_begin();
 
@@ -467,6 +477,7 @@ __visible noinstr long do_fast_syscall_32(struct pt_regs *regs)
 	 */
 	regs->ip = landing_pad;
 
+	add_random_kstack_offset();
 	enter_from_user_mode();
 	instrumentation_begin();
 
-- 
2.25.1