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

Ryan Roberts
2025-06-09T11:10:32Z

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);