[PATCH] s390/entry: add support for syscall stack randomization

Sven Schnelle
2021-04-29T09:15:19Z

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 <svens@linux.ibm.com>
---
arch/s390/Kconfig | 1 +
arch/s390/include/asm/entry-common.h | 10 ++++++++++
arch/s390/kernel/syscall.c | 1 +
arch/s390/kernel/traps.c | 2 ++
4 files changed, 14 insertions(+)

diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index c1ff874e6c2e..1900428ce557 100644
--- a/arch/s390/Kconfig
+++ b/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
diff --git a/arch/s390/include/asm/entry-common.h b/arch/s390/include/asm/entry-common.h
index 9cceb26ed63f..baa8005090c3 100644
--- a/arch/s390/include/asm/entry-common.h
+++ b/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));
diff --git a/arch/s390/kernel/syscall.c b/arch/s390/kernel/syscall.c
index bc8e650e377d..4e5cc7d2364e 100644
--- a/arch/s390/kernel/syscall.c
+++ b/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));
diff --git a/arch/s390/kernel/traps.c b/arch/s390/kernel/traps.c
index 63021d484626..8dd23c703718 100644
--- a/arch/s390/kernel/traps.c
+++ b/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;

--
2.25.1