linux-kernel - Re: [PATCH] powerpc/32: Clear volatile regs on syscall exit

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Message-ID: <YhadiVbwao/p2N7o@lt-gp.iram.es>
Date:   Wed, 23 Feb 2022 21:48:09 +0100
From:   Gabriel Paubert <paubert@...m.es>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Kees Cook <keescook@...omium.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc/32: Clear volatile regs on syscall exit

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 <christophe.leroy@...roup.eu>
> ---
>  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

Here, I'm almost sure that on some processors, it would be better to
separate mtctr form mtxer. mtxer is typically very expensive (pipeline
flush) but I don't know what's the best ordering for the average core.

And what about lr? Should it also be cleared?

	Gabriel

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