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

Will Deacon
2025-06-18T11:36:40Z

On Fri, Jun 13, 2025 at 10:39:02AM +0530, Anshuman Khandual wrote:
>
>
> On 12/06/25 8:28 PM, Will Deacon wrote:
> > On Mon, Jun 09, 2025 at 05:12:14AM +0100, 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().
> >
> > How do other architectures (e.g. x86) handle this? I don't see any usage
> > of {get,put}_online_mems() over there. Should this be moved into the core
> > code?
>
> Memory hot remove on arm64 unmaps kernel linear and vmemmap mapping while
> also freeing page table pages if those become empty. Although this might
> not be true for all other architectures, which might just unmap affected
> kernel regions but does not tear down the kernel page table.

... that sounds like something we should be able to give a definitive
answer to?

Will