Subject: Re: [PATCH] powerpc/32: Clear volatile regs on syscall exit

From: Kees Cook <[email-masked]>
Date: Wed, 23 Feb 2022 11:34:08 -0800

On Wed, Feb 23, 2022 at 06:11:36PM +0100, Christophe Leroy wrote:
> Commit a82adfd5c7cb ("hardening: Introduce CONFIG_ZERO_CALL_USED_REGS")
> added zeroing of used registers at function exit.
> 
> At the time being, PPC64 clears volatile registers on syscall exit but
> PPC32 doesn't do it for performance reason.
> 
> Add that clearing in PPC32 syscall exit as well, but only when
> CONFIG_ZERO_CALL_USED_REGS is selected.
> 
> On an 8xx, the null_syscall selftest gives:
> - Without CONFIG_ZERO_CALL_USED_REGS		: 288 cycles
> - With CONFIG_ZERO_CALL_USED_REGS		: 305 cycles
> - With CONFIG_ZERO_CALL_USED_REGS + this patch	: 319 cycles
> 
> Note that (independent of this patch), with pmac32_defconfig,
> vmlinux size is as follows with/without CONFIG_ZERO_CALL_USED_REGS:
> 
>    text	   	data	    bss	    dec	    hex		filename
> 9578869		2525210	 194400	12298479	bba8ef	vmlinux.without
> 10318045	2525210  194400	13037655	c6f057	vmlinux.with
> 
> That is a 7.7% increase on text size, 6.0% on overall size.
> 
> Signed-off-by: Christophe Leroy <[email-masked]>
> ---
>  arch/powerpc/kernel/entry_32.S | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S
> index 7748c278d13c..199f23092c02 100644
> --- a/arch/powerpc/kernel/entry_32.S
> +++ b/arch/powerpc/kernel/entry_32.S
> @@ -151,6 +151,21 @@ syscall_exit_finish:
>  	bne	3f
>  	mtcr	r5
>  
> +#ifdef CONFIG_ZERO_CALL_USED_REGS
> +	/* Zero volatile regs that may contain sensitive kernel data */
> +	li	r0,0
> +	li	r4,0
> +	li	r5,0
> +	li	r6,0
> +	li	r7,0
> +	li	r8,0
> +	li	r9,0
> +	li	r10,0
> +	li	r11,0
> +	li	r12,0
> +	mtctr	r0
> +	mtxer	r0
> +#endif

I think this should probably be unconditional -- if this is actually
leaking kernel pointers (or data) that's pretty bad. :|

If you really want to leave it build-time selectable, maybe add a new
config that gets "select"ed by CONFIG_ZERO_CALL_USED_REGS?

(And you may want to consider wiping all "unused" registers at syscall
entry as well.)

-Kees

>  1:	lwz	r2,GPR2(r1)
>  	lwz	r1,GPR1(r1)
>  	rfi
> -- 
> 2.34.1

-- 
Kees Cook