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

[Kees Cook <keescook@xxxxxxxxxxxx>]

On Thu, Apr 29, 2021 at 11:14:51AM +0200, Sven Schnelle wrote:
> 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

Nice! :)

> 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.

Ah, interesting. Is this path to syscalls unique to s390? (As in, should
x86 and arm64 gain coverage over a path that got missed?)

> Signed-off-by: Sven Schnelle <svens@xxxxxxxxxxxxx>
> ---
>  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);

What's the stack alignment on s390? Or, better question, what's the
expected number of entropy bits?

> +}
> +
> +#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;


-- 
Kees Cook