Re: [PATCH] x86/kaslr: P2PDMA is one of a class of ZONE_DEVICE-KASLR collisions

Balbir Singh
2025-11-08T02:40:03Z

On 11/8/25 13:32, Dan Williams wrote:
> Commit 7ffb791423c7 ("x86/kaslr: Reduce KASLR entropy on most x86 systems")
> is too narrow. ZONE_DEVICE, in general, lets any physical address be added
> to the direct-map. I.e. not only ACPI hotplug ranges, CXL Memory Windows,
> or EFI Specific Purpose Memory, but also any PCI MMIO range for the
> CONFIG_DEVICE_PRIVATE and CONFIG_PCI_P2PDMA cases.
>
> A potential path to recover entropy would be to walk ACPI and determine the
> limits for hotplug and PCI MMIO before kernel_randomize_memory(). On
> smaller systems that could yield some KASLR address bits. This needs
> additional investigation to determine if some limited ACPI table scanning
> can happen this early without an open coded solution like
> arch/x86/boot/compressed/acpi.c needs to deploy.
>
> Cc: Balbir Singh <balbirs@nvidia.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Kees Cook <kees@kernel.org>
> Cc: Bjorn Helgaas <bhelgaas@google.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Logan Gunthorpe <logang@deltatee.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Mike Rapoport <rppt@kernel.org>
> Cc: Suren Baghdasaryan <surenb@google.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: "Yasunori Gotou (Fujitsu)" <y-goto@fujitsu.com>
> Fixes: 7ffb791423c7 ("x86/kaslr: Reduce KASLR entropy on most x86 systems")
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>

P2PDMA requires ZONE_DEVICE, Most distros have P2PDMA enabled, you mention
smaller devices - are you referring to kernels/distros where P2PDMA is not
enabled and only ZONE_DEVICE is?

Balbir