linux-kernel - Re: [PATCH] KVM: arm64: ptdump: Fix exec attribute printing

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Message-ID: <261de805-2914-4172-85b4-69b5a5b1edf6@arm.com>
Date: Tue, 5 Aug 2025 05:10:39 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Mark Rutland <mark.rutland@....com>,
 Wei-Lin Chang <r09922117@...e.ntu.edu.tw>
Cc: linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
 linux-kernel@...r.kernel.org, Marc Zyngier <maz@...nel.org>,
 Oliver Upton <oliver.upton@...ux.dev>, Joey Gouly <joey.gouly@....com>,
 Sebastian Ene <sebastianene@...gle.com>,
 Suzuki K Poulose <suzuki.poulose@....com>, Zenghui Yu
 <yuzenghui@...wei.com>, Catalin Marinas <catalin.marinas@....com>,
 Will Deacon <will@...nel.org>
Subject: Re: [PATCH] KVM: arm64: ptdump: Fix exec attribute printing



On 04/08/25 8:52 PM, Mark Rutland wrote:
> On Mon, Aug 04, 2025 at 08:41:35PM +0800, Wei-Lin Chang wrote:
>> Hi Anshuman,
>>
>> On Sun, Aug 03, 2025 at 07:33:04PM +0530, Anshuman Khandual wrote:
>>>
>>>
>>> On 02/08/25 4:10 PM, Wei-Lin Chang wrote:
>>>> Currently the guest stage-2 page table dump has the executable attribute
>>>> printed in reverse, showing "X" for a non-executable region and showing
>>>> " " for an executable one. This is caused by misjudgement of which
>>>> string gets printed for the executable and non-executable case. Fix it
>>>> by swapping the two strings.
>>>>
>>>> Signed-off-by: Wei-Lin Chang <r09922117@...e.ntu.edu.tw>
>>>> ---
>>>>  arch/arm64/kvm/ptdump.c | 4 ++--
>>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/arch/arm64/kvm/ptdump.c b/arch/arm64/kvm/ptdump.c
>>>> index 098416d7e5c25..99fc13f1c11fb 100644
>>>> --- a/arch/arm64/kvm/ptdump.c
>>>> +++ b/arch/arm64/kvm/ptdump.c
>>>> @@ -44,8 +44,8 @@ static const struct ptdump_prot_bits stage2_pte_bits[] = {
>>>>  	}, {
>>>>  		.mask	= KVM_PTE_LEAF_ATTR_HI_S2_XN | PTE_VALID,
>>>>  		.val	= PTE_VALID,
>>>> -		.set	= " ",
>>>> -		.clear	= "X",
>>>> +		.set	= "X",
>>>> +		.clear	= " ",
>>>>  	}, {
> 
> I think the big problem here is that we've included the 'PTE_VALID' bit
> in the mask. We don't bother with that for the Stage-1 ptdump code, e.g.
> 
>         {
>                 .mask   = PTE_PXN,
>                 .val    = PTE_PXN,
>                 .set    = "NX",
>                 .clear  = "x ",
>         },
> 
> .... so do we actually need to take the PTE_VALID bit into account here? Do
> invalid Stage-2 entries have anything we don't want to report?
> 
> ... or can we change the Stage-2 ptdump code to have:
> 
> 	{
> 		.mask	= KVM_PTE_LEAF_ATTR_HI_S2_XN,
> 		.val	= KVM_PTE_LEAF_ATTR_HI_S2_XN,
> 		.set	= "NX",
> 		.clear	= "x ",
> 	},
> 
> ... and match the Stage-1 code?
> 
> Otherwise, maybe we can add a separate valid-only filter.

Agreed.

PTE_VALID should be removed from all these existing filters here
and then add it back as a separate filter like Stage-1. Ohh, but
it is already in there. Checking for PTE_VALID along with other
intended filter masks does not make sense.

> 
>>>>  		.mask	= KVM_PTE_LEAF_ATTR_LO_S2_AF | PTE_VALID,
>>>>  		.val	= KVM_PTE_LEAF_ATTR_LO_S2_AF | PTE_VALID,
>>>
>>> Is not KVM_PTE_LEAF_ATTR_HI_S2_XN already in the reverse semantics aka
>>> XN (Execute Never). Hence when KVM_PTE_LEAF_ATTR_HI_S2_XN macro is set
>>> that means the entry is not executable and vice versa.
>>
>> Yes you are correct. However in dump_prot() we have:
>>
>> if ((st->current_prot & bits->mask) == bits->val)
>> 	s = bits->set;
>> else
>> 	s = bits->clear;
>>
>> Analysis:
>>
>> 1. region is executable:
>>     - st->current_prot == PTE_VALID (ignore other bits)
>>     - st->current_prot & bits->mask gets PTE_VALID
>>     - if condition is true (.val is PTE_VALID)
>>     - prints bits->set
>>
>> 2. region is not executable:
>>     - st->current_prot == KVM_PTE_LEAF_ATTR_HI_S2_XN | PTE_VALID
>>     - st->current_prot & bits->mask gets (KVM_PTE_LEAF_ATTR_HI_S2_XN | PTE_VALID)
>>     - if condition is false
>>     - prints bits->clear
> 
> As above, this is because the PTE_VALID bit has been placed into the
> mask, and that's not how the ptdump code was intended to be used.
> 
>> Therefore we want .set = "X", and .clear = " ".
> 
> That'll work around the problem, but I'm not sure that's the right fix.
> If nothing else, it's *very* confusing.
> 
> Mark.