s390/entry: add support for syscall stack randomization

This adds support for adding a random offset to the stack while handling
syscalls. The patch uses get_tod_clock_fast() as this is considered good
enough and has much less performance penalty compared to using
get_random_int(). The patch also adds randomization in pgm_check_handler()
as the sigreturn/rt_sigreturn system calls might be called from there.

Signed-off-by: Sven Schnelle <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Heiko Carstens <[email protected]>

Author: svens-s390

arch/s390/Kconfig

@@ -137,6 +137,7 @@ config S390
 	select HAVE_ARCH_JUMP_LABEL_RELATIVE
 	select HAVE_ARCH_KASAN
 	select HAVE_ARCH_KASAN_VMALLOC
+	select HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET
 	select HAVE_ARCH_SECCOMP_FILTER
 	select HAVE_ARCH_SOFT_DIRTY
 	select HAVE_ARCH_TRACEHOOK

arch/s390/include/asm/entry-common.h

@@ -4,9 +4,11 @@
 
 #include <linux/sched.h>
 #include <linux/audit.h>
+#include <linux/randomize_kstack.h>
 #include <linux/tracehook.h>
 #include <linux/processor.h>
 #include <linux/uaccess.h>
+#include <asm/timex.h>
 #include <asm/fpu/api.h>
 
 #define ARCH_EXIT_TO_USER_MODE_WORK (_TIF_GUARDED_STORAGE | _TIF_PER_TRAP)
@@ -48,6 +50,14 @@ static __always_inline void arch_exit_to_user_mode(void)
 
 #define arch_exit_to_user_mode arch_exit_to_user_mode
 
+static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs,
+						  unsigned long ti_work)
+{
+	choose_random_kstack_offset(get_tod_clock_fast() & 0xff);
+}
+
+#define arch_exit_to_user_mode_prepare arch_exit_to_user_mode_prepare
+
 static inline bool on_thread_stack(void)
 {
 	return !(((unsigned long)(current->stack) ^ current_stack_pointer()) & ~(THREAD_SIZE - 1));

arch/s390/kernel/syscall.c

@@ -142,6 +142,7 @@ void do_syscall(struct pt_regs *regs)
 
 void noinstr __do_syscall(struct pt_regs *regs, int per_trap)
 {
+	add_random_kstack_offset();
 	enter_from_user_mode(regs);
 
 	memcpy(&regs->gprs[8], S390_lowcore.save_area_sync, 8 * sizeof(unsigned long));

arch/s390/kernel/traps.c

@@ -17,6 +17,7 @@
 #include "asm/ptrace.h"
 #include <linux/kprobes.h>
 #include <linux/kdebug.h>
+#include <linux/randomize_kstack.h>
 #include <linux/extable.h>
 #include <linux/ptrace.h>
 #include <linux/sched.h>
@@ -301,6 +302,7 @@ void noinstr __do_pgm_check(struct pt_regs *regs)
 	unsigned int trapnr, syscall_redirect = 0;
 	irqentry_state_t state;
 
+	add_random_kstack_offset();
 	regs->int_code = *(u32 *)&S390_lowcore.pgm_ilc;
 	regs->int_parm_long = S390_lowcore.trans_exc_code;