linux-kernel - Re: [PATCH] powerpc/8xx: Fix clearing of bits 20-23 in ITLB miss

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Date:   Sat, 15 Feb 2020 11:17:27 +0100
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Leonardo Bras <leonardo@...ux.ibm.com>,
        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/8xx: Fix clearing of bits 20-23 in ITLB miss



Le 15/02/2020 à 07:28, Leonardo Bras a écrit :
> On Sun, 2020-02-09 at 18:14 +0000, Christophe Leroy wrote:
>> In ITLB miss handled the line supposed to clear bits 20-23 on the
>> L2 ITLB entry is buggy and does indeed nothing, leading to undefined
>> value which could allow execution when it shouldn't.
>>
>> Properly do the clearing with the relevant instruction.
>>
>> Fixes: 74fabcadfd43 ("powerpc/8xx: don't use r12/SPRN_SPRG_SCRATCH2 in TLB Miss handlers")
>> Cc: stable@...r.kernel.org
>> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
>> ---
>>   arch/powerpc/kernel/head_8xx.S | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/powerpc/kernel/head_8xx.S b/arch/powerpc/kernel/head_8xx.S
>> index 9922306ae512..073a651787df 100644
>> --- a/arch/powerpc/kernel/head_8xx.S
>> +++ b/arch/powerpc/kernel/head_8xx.S
>> @@ -256,7 +256,7 @@ InstructionTLBMiss:
>>   	 * set.  All other Linux PTE bits control the behavior
>>   	 * of the MMU.
>>   	 */
>> -	rlwimi	r10, r10, 0, 0x0f00	/* Clear bits 20-23 */
>> +	rlwinm	r10, r10, 0, ~0x0f00	/* Clear bits 20-23 */
>>   	rlwimi	r10, r10, 4, 0x0400	/* Copy _PAGE_EXEC into bit 21 */
>>   	ori	r10, r10, RPN_PATTERN | 0x200 /* Set 22 and 24-27 */
>>   	mtspr	SPRN_MI_RPN, r10	/* Update TLB entry */
> 
> Looks a valid change.
> rlwimi  r10, r10, 0, 0x0f00 means:
> r10 = ((r10 << 0) & 0x0f00) | (r10 & ~0x0f00) which ends up being
> r10 = r10
> 
> On ISA, rlwinm is recommended for clearing high order bits.
> rlwinm  r10, r10, 0, ~0x0f00 means:
> r10 = (r10 << 0) & ~0x0f00
> 
> Which does exactly what the comments suggests.
> 
> FWIW:
> Reviwed-by: Leonardo Bras <leonardo@...ux.ibm.com>
> 

I guess you mean

Reviewed-by: Leonardo Bras <leonardo@...ux.ibm.com>