Re: [PATCH] arm64/ptdump: Ensure memory hotplug is prevented during ptdump_check_wx()

[Ryan Roberts <ryan.roberts@arm.com>]

On 09/06/2025 05:12, Anshuman Khandual wrote:
> The arm64 page table dump code can race with concurrent modification of the
> kernel page tables. When a leaf entries are modified concurrently, the dump
> code may log stale or inconsistent information for a VA range, but this is
> otherwise not harmful.
> 
> When intermediate levels of table are freed, the dump code will continue to
> use memory which has been freed and potentially reallocated for another
> purpose. In such cases, the dump code may dereference bogus addresses,
> leading to a number of potential problems.
> 
> This problem was fixed for ptdump_show() earlier via commit 'bf2b59f60ee1
> ("arm64/mm: Hold memory hotplug lock while walking for kernel page table
> dump")' but a same was missed for ptdump_check_wx() which faced the race
> condition as well. Let's just take the memory hotplug lock while executing
> ptdump_check_wx().
> 
> Cc: stable@vger.kernel.org
> Fixes: bbd6ec605c0f ("arm64/mm: Enable memory hot remove")
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-kernel@vger.kernel.org
> Reported-by: Dev Jain <dev.jain@arm.com>
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>

Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>

> ---
> This patch applies on v6.16-rc1
> 
> Dev Jain found this via code inspection.
> 
>  arch/arm64/mm/ptdump.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/mm/ptdump.c b/arch/arm64/mm/ptdump.c
> index 421a5de806c62..551f80d41e8d2 100644
> --- a/arch/arm64/mm/ptdump.c
> +++ b/arch/arm64/mm/ptdump.c
> @@ -328,7 +328,7 @@ static struct ptdump_info kernel_ptdump_info __ro_after_init = {
>  	.mm		= &init_mm,
>  };
>  
> -bool ptdump_check_wx(void)
> +static bool __ptdump_check_wx(void)
>  {
>  	struct ptdump_pg_state st = {
>  		.seq = NULL,
> @@ -367,6 +367,16 @@ bool ptdump_check_wx(void)
>  	}
>  }
>  
> +bool ptdump_check_wx(void)
> +{
> +	bool ret;
> +
> +	get_online_mems();
> +	ret = __ptdump_check_wx();
> +	put_online_mems();
> +	return ret;
> +}
> +
>  static int __init ptdump_init(void)
>  {
>  	u64 page_offset = _PAGE_OFFSET(vabits_actual);