Re: [PATCH -next, v2] powerpc: add support for syscall stack randomization

[Christophe Leroy]

Le 16/05/2022 à 09:32, Xiu Jianfeng a écrit :
> 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.
> 
> In order to avoid unconditional stack canaries on syscall entry (due to
> the use of alloca()), also disable stack protector to avoid triggering
> needless checks and slowing down the entry path. As there is no general
> way to control stack protector coverage with a function attribute, this
> must be disabled at the compilation unit level.
> 
> Signed-off-by: Xiu Jianfeng <xiujianfeng@xxxxxxxxxx>
> 

Did you see this patch generates sparse warnings ?

See 
https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20220516073225.112875-1-xiujianfeng@xxxxxxxxxx/

It is not directly related to this patch but that's worth a patch to fix it.

> ---
> Changes in v2:
>    -move choose choose_random_kstack_offset() to the end of system_call_exception
>    -allow full 6 (10) bits of entropy
>    -disable stack-protector for interrupt.c
> ---
>   arch/powerpc/Kconfig            |  1 +
>   arch/powerpc/kernel/Makefile    |  7 +++++++
>   arch/powerpc/kernel/interrupt.c | 19 ++++++++++++++++++-
>   3 files changed, 26 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 98309eeae09c..2f0019a0054e 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/Makefile b/arch/powerpc/kernel/Makefile
> index 4ddd161aef32..5c5e85b8229b 100644
> --- a/arch/powerpc/kernel/Makefile
> +++ b/arch/powerpc/kernel/Makefile
> @@ -40,6 +40,13 @@ CFLAGS_cputable.o += -DDISABLE_BRANCH_PROFILING
>   CFLAGS_btext.o += -DDISABLE_BRANCH_PROFILING
>   endif
>   
> +#ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
> +# Remove stack protector to avoid triggering unneeded stack canary
> +# checks due to randomize_kstack_offset.
> +CFLAGS_REMOVE_interrupt.o = -fstack-protector -fstack-protector-strong
> +CFLAGS_interrupt.o += -fno-stack-protector
> +#endif
> +
>   obj-y				:= cputable.o syscalls.o \
>   				   irq.o align.o signal_$(BITS).o pmc.o vdso.o \
>   				   process.o systbl.o idle.o \
> diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
> index 784ea3289c84..d7cdcb6fc336 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>
> @@ -78,10 +79,12 @@ notrace long system_call_exception(long r3, long r4, long r5,
>   				   long r6, long r7, long r8,
>   				   unsigned long r0, struct pt_regs *regs)
>   {
> +	long ret;
>   	syscall_fn f;
>   
>   	kuap_lock();
>   
> +	add_random_kstack_offset();
>   	regs->orig_gpr3 = r3;
>   
>   	if (IS_ENABLED(CONFIG_PPC_IRQ_SOFT_MASK_DEBUG))
> @@ -229,7 +232,21 @@ notrace long system_call_exception(long r3, long r4, long r5,
>   		f = (void *)sys_call_table[r0];
>   	}
>   
> -	return f(r3, r4, r5, r6, r7, r8);
> +	ret = f(r3, r4, r5, r6, r7, r8);
> +	/*
> +	 * Ultimately, this value will get limited by KSTACK_OFFSET_MAX(),
> +	 * so the maximum stack offset is 1k bytes(10 bits).
> +	 *
> +	 * The actual entropy will be further reduced by the compiler when
> +	 * applying stack alignment constraints: the powerpc architecture
> +	 * may have two kinds of stack alignment(16-bytes and 8-bytes).
> +	 *
> +	 * So the resulting 6 or 7 bits of entropy is seen in SP[9:4] or SP[9:3].
> +	 *
> +	 */
> +	choose_random_kstack_offset(mftb());
> +
> +	return ret;
>   }
>   
>   static notrace void booke_load_dbcr0(void)