linux-kernel - Re: [PATCH] powerpc/chrp: Fix enter_rtas() with CONFIG_VMAP

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Date:   Mon, 17 Feb 2020 07:40:05 +0100
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Michael Neuling <mikey@...ling.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc/chrp: Fix enter_rtas() with CONFIG_VMAP_STACK



Le 16/02/2020 à 23:40, Michael Neuling a écrit :
> On Fri, 2020-02-14 at 08:33 +0000, Christophe Leroy wrote:
>> With CONFIG_VMAP_STACK, data MMU has to be enabled
>> to read data on the stack.
> 
> Can you describe what goes wrong without this? Some oops message? rtas blows up?
> Get corrupt data?

Larry reported a machine check. Or in fact, he reported a Oops in 
kprobe_handler(), that Oops being a bug in kprobe_handle() triggered by 
this machine check.

By converting a VM address to a phys-like address as if is was linear 
mem, you get in the dark. Either there is some physical memory at that 
address and you corrupt it. Or there is none and you get a machine check.

> 
> Also can you say what you're actually doing (ie turning on MSR[DR])

Euh ... I'm saying that data MMU has to be enabled, so I'm enabling it.

> 
> 
>> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
>> ---
>>   arch/powerpc/kernel/entry_32.S | 9 +++++++--
>>   1 file changed, 7 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S
>> index 0713daa651d9..bc056d906b51 100644
>> --- a/arch/powerpc/kernel/entry_32.S
>> +++ b/arch/powerpc/kernel/entry_32.S
>> @@ -1354,12 +1354,17 @@ _GLOBAL(enter_rtas)
>>   	mtspr	SPRN_SRR0,r8
>>   	mtspr	SPRN_SRR1,r9
>>   	RFI
>> -1:	tophys(r9,r1)
>> +1:	tophys_novmstack r9, r1
>> +#ifdef CONFIG_VMAP_STACK
>> +	li	r0, MSR_KERNEL & ~MSR_IR	/* can take DTLB miss */
> 
> You're potentially turning on more than MSR DR here. This should be clear in the
> commit message.

Am I ?

At the time of the RFI just above, SRR1 contains the value of r9 which 
has been set 2 lines before to MSR_KERNEL & ~(MSR_IR|MSR_DR).

What should be clear in the commit message ?

> 
>> +	mtmsr	r0
>> +	isync
>> +#endif
>>   	lwz	r8,INT_FRAME_SIZE+4(r9)	/* get return address */
>>   	lwz	r9,8(r9)	/* original msr value */
>>   	addi	r1,r1,INT_FRAME_SIZE
>>   	li	r0,0
>> -	tophys(r7, r2)
>> +	tophys_novmstack r7, r2
>>   	stw	r0, THREAD + RTAS_SP(r7)
>>   	mtspr	SPRN_SRR0,r8
>>   	mtspr	SPRN_SRR1,r9

Christophe