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

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Date: Sat, 29 Jun 2024 12:20:23 +0200
From: Jürgen Groß <jgross@...e.com>
To: Ashish Kalra <Ashish.Kalra@....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 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.


Juergen

> 
> 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;
>   }
>   
>   /*