Re: [PATCH] arm64/mm: Check the requested PFN range during memoroy removal

[Anshuman Khandual]

On 20/07/26 7:36 AM, Richard Cheng wrote:
> prevent_memory_remove_notifier() advances pfn while checking whether the
> range contains early memory. After the loop, pfn points to end_pfn, but
> it is passed to can_unmap_without_split(). This checks the range
> immediately after the requested memory instead of the range being
> offlined.

can_unmap_without_split() ends up checking 'a memory range' after
the actual range being offlined, because pfn could have gone past
end_pfn by the time it exists the loop. This is wrong and was not
intended.

> 
> Consequently, valid requests can be rejected with shifted address range,
> while an unsafe request can be accepted when the following range is
> unmapped. This was observed with CXL DAX memory, where the final memory
> block was incorrectly allowed offline.

Agreed.
> 
> Pass arg->start_pfn so the leaf-split check examines the requested

Pass arg->start_pfn into can_unmap_without_split().
> range.

Although the explanations above are correct, the wording in the
commit message could have been better.
> 
> Fixes: 95a58852b0e5 ("arm64/mm: Reject memory removal that splits a kernel leaf mapping")

Correct commit ID for attribution.

> Signed-off-by: Richard Cheng <icheng@xxxxxxxxxx>
> ---
> The bug occurred on a machine with CXL Type-3 device.
> Branch: cxl-next
> 
> Before 95a58852b0e5:
> 
> """
> $ sudo echo offline > memory557056/state
> write error: Operation not permitted
> $ sudo dmesg
> ---[snip]---
> [440008000000 440010000000] splits a leaf entry in linear map
> 
> $ sudo echo offline > memory558079/state
> $ cat memory558079/state
> offline
> """
> 
> After:
> """
> $ sudo echo offline > memory557056/state
> write error: Operation not permitted
> [440000000000 440008000000] splits a leaf entry in linear map
> 
> $ sudo echo offline > memory558079/state
> write error: Operation not permitted
> [441ff8000000 442000000000] splits a leaf entry in linear map

Seems exactly as would have been expected.
> 
> My fix corrects the checked range and prevents the false acceptance.

> 
> Best regards,
> Richard Cheng.
> ---
>  arch/arm64/mm/mmu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index a25d8beacc83..18a8b0d3714e 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -2194,7 +2194,7 @@ static int prevent_memory_remove_notifier(struct notifier_block *nb,
>  		}
>  	}
>  
> -	if (!can_unmap_without_split(pfn, arg->nr_pages))
> +	if (!can_unmap_without_split(arg->start_pfn, arg->nr_pages))
>  		return NOTIFY_BAD;
>  
>  	return NOTIFY_OK;
> 
> base-commit: 5ca04f3ba91f1773bbd5da6d9c654ccc1ba7831d

Reviewed-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>