Re: [PATCH -next] powerpc: add support for syscall stack randomization

[Nicholas Piggin <npiggin@gmail.com>]

Excerpts from Xiu Jianfeng's message of May 5, 2022 9:19 pm:
> Add support for adding a random offset to the stack while handling
> syscalls. This patch uses mftb() instead of get_random_int() for better
> performance.

Hey, very nice.

> 
> Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
> ---
>  arch/powerpc/Kconfig            | 1 +
>  arch/powerpc/kernel/interrupt.c | 3 +++
>  2 files changed, 4 insertions(+)
> 
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 5fc9153927ac..7e04c9f80cbc 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -192,6 +192,7 @@ config PPC
>  	select HAVE_ARCH_KASAN			if PPC32 && PPC_PAGE_SHIFT <= 14
>  	select HAVE_ARCH_KASAN_VMALLOC		if PPC32 && PPC_PAGE_SHIFT <= 14
>  	select HAVE_ARCH_KFENCE			if PPC_BOOK3S_32 || PPC_8xx || 40x
> +	select HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET
>  	select HAVE_ARCH_KGDB
>  	select HAVE_ARCH_MMAP_RND_BITS
>  	select HAVE_ARCH_MMAP_RND_COMPAT_BITS	if COMPAT
> diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
> index 784ea3289c84..459385769721 100644
> --- a/arch/powerpc/kernel/interrupt.c
> +++ b/arch/powerpc/kernel/interrupt.c
> @@ -4,6 +4,7 @@
>  #include <linux/err.h>
>  #include <linux/compat.h>
>  #include <linux/sched/debug.h> /* for show_regs */
> +#include <linux/randomize_kstack.h>
>  
>  #include <asm/kup.h>
>  #include <asm/cputime.h>
> @@ -82,6 +83,7 @@ notrace long system_call_exception(long r3, long r4, long r5,
>  
>  	kuap_lock();
>  
> +	add_random_kstack_offset();
>  	regs->orig_gpr3 = r3;
>  
>  	if (IS_ENABLED(CONFIG_PPC_IRQ_SOFT_MASK_DEBUG))

This looks like the right place. I wonder why other interrupts don't
get the same treatment. Userspace can induce the kernel to take a 
synchronous interrupt, or wait for async ones. Smaller surface area 
maybe but certain instruction emulation for example could result in
significant logic that depends on user state. Anyway that's for
hardening gurus to ponder.

> @@ -405,6 +407,7 @@ interrupt_exit_user_prepare_main(unsigned long ret, struct pt_regs *regs)
>  
>  	/* Restore user access locks last */
>  	kuap_user_restore(regs);
> +	choose_random_kstack_offset(mftb() & 0xFF);
>  
>  	return ret;
>  }

So this seems to be what x86 and s390 do, but why are we choosing a
new offset for every interrupt when it's only used on a syscall?
I would rather you do what arm64 does and just choose the offset
at the end of system_call_exception.

I wonder why the choose is separated from the add? I guess it's to
avoid a data dependency for stack access on an expensive random
function, so that makes sense (a comment would be nice in the
generic code).

I don't actually know if mftb() is cheaper here than a RNG. It
may not be conditioned all that well either. I would be tempted
to measure. 64-bit *may* be able to use a bit more than 256
bytes of stack too -- we have 16 byte alignment minimum so this
gives only 4 bits of randomness AFAIKS.

Thanks,
Nick