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

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Message-ID: <5c223dac-8434-a6ae-7d92-d2bd213ef320@csgroup.eu>
Date:   Thu, 24 Feb 2022 06:41:39 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Gabriel Paubert <paubert@...m.es>
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" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] powerpc/32: Clear volatile regs on syscall exit



Le 23/02/2022 à 21:48, Gabriel Paubert a écrit :
> 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.

In the 8xx, CTR and LR are handled by the BPU as any other reg (Latency 
1 blocage 1).
AFAIU, XER is serialize + 1

> 
> And what about lr? Should it also be cleared?

LR is restored from stack.

Christophe