linux-kernel - Re: [PATCH] x86/mm: fix lookup_address() to handle physical memory holes in direct mapping

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Date: Mon, 1 Jul 2024 12:57:58 -0500
From: "Kalra, Ashish" <ashish.kalra@....com>
To: Jürgen Groß <jgross@...e.com>,
 dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
 tglx@...utronix.de, mingo@...hat.com, bp@...en8.de
Cc: x86@...nel.org, hpa@...or.com, kirill.shutemov@...ux.intel.com,
 rick.p.edgecombe@...el.com, mhklinux@...look.com, peterx@...hat.com,
 linux-kernel@...r.kernel.org, thomas.lendacky@....com,
 linux-coco@...ts.linux.dev, jroedel@...e.de
Subject: Re: [PATCH] x86/mm: fix lookup_address() to handle physical memory
 holes in direct mapping


On 6/29/2024 5:20 AM, Jürgen Groß wrote:
> On 28.06.24 22:52, Ashish Kalra wrote:
>> From: Ashish Kalra <ashish.kalra@....com>
>>
>> lookup_address_in_pgd_attr() at pte level it is simply returning
>> pte_offset_kernel() and there does not seem to be a check for
>> returning NULL if pte_none().
>>
>> Fix lookup_address_in_pgd_attr() to add check for pte_none()
>> after pte_offset_kernel() and return NULL if it is true.
>
> Please have a look at the comment above lookup_address(). You should not
> break the documented behavior without verifying that no caller is relying
> on the current behavior. If this is fine, please update the comment.
>
>
I don't get that, in this case the PTE does not exist, so as per the comments here lookup_address() should have returned NULL.

Thanks, Ashish

>
>>
>> Signed-off-by: Ashish Kalra <ashish.kalra@....com>
>> ---
>>   arch/x86/mm/pat/set_memory.c | 7 ++++++-
>>   1 file changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
>> index 443a97e515c0..be8b5bf3bc3f 100644
>> --- a/arch/x86/mm/pat/set_memory.c
>> +++ b/arch/x86/mm/pat/set_memory.c
>> @@ -672,6 +672,7 @@ pte_t *lookup_address_in_pgd_attr(pgd_t *pgd, unsigned long address,
>>       p4d_t *p4d;
>>       pud_t *pud;
>>       pmd_t *pmd;
>> +    pte_t *pte;
>>         *level = PG_LEVEL_256T;
>>       *nx = false;
>> @@ -717,7 +718,11 @@ pte_t *lookup_address_in_pgd_attr(pgd_t *pgd, unsigned long address,
>>       *nx |= pmd_flags(*pmd) & _PAGE_NX;
>>       *rw &= pmd_flags(*pmd) & _PAGE_RW;
>>   -    return pte_offset_kernel(pmd, address);
>> +    pte = pte_offset_kernel(pmd, address);
>> +    if (pte_none(*pte))
>> +        return NULL;
>> +
>> +    return pte;
>>   }
>>     /*
>