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

Dave Jiang
2026-01-06T01:11:27Z

On 12/1/25 2:29 PM, dan.j.williams@intel.com wrote:
> Dave Hansen wrote:
>> The subject probably wants to be something along the lines of:
>>
>> x86/kaslr: Recognize all ZONE_DEVICE users as physaddr consumers
>
> ...works for me.
>
>>
>> On 11/7/25 18:32, 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.
>>
>> This should probably also mention the fact that:
>>
>> config PCI_P2PDMA
>> depends on ZONE_DEVICE
>>
>> It would also be nice to point out how the "too narrow" check had an
>> impact on real ZONE_DEVICE but !PCI_P2PDMA users. This isn't just a
>> theoretical problem, right?
>
> Yasunori filled in a detail [1] that I did not have when creating the
> patch, specifically that when he enountered the CXL collision with KASLR
> he was running on a kernel before commit 7ffb791423c7 ("x86/kaslr:
> Reduce KASLR entropy on most x86 systems").
>
> Either way, a pre-7ffb791423c7 kernel and a kernel with
> CONFIG_PCI_P2PDMA=n would fail the same way. Yasunori confirmed that
> current kernel with CONFIG_PCI_P2PDMA=y, or this patch solved the
> problem for him.
>
> See below for a reworked patch with these changes.
>
> [1]: http://lore.kernel.org/OS9PR01MB124215C4182B59D590049B99390CCA@OS9PR01MB12421.jpnprd01.prod.outlook.com
>>
>>> 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.
>>
>> Yeah, a more flexible runtime solution would be highly preferred over
>> the existing solution built around config options. But this is really
>> orthogonal to the bug fix here.
>>
>> With the changelog fixes above:
>>
>> Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
>>
>> Oh, and does this need to be cc:stable@?
>
> Yes, especially because it would create a dependency on 7ffb791423c7
> also being backported and that would have helped Yasunori avoid this
> problem (for CONFIG_PCI_P2PDMA=y builds at least).
>
> -- >8 --
> From d2f4b9ac915ce35e2ec842548ae1ccb4f1690b04 Mon Sep 17 00:00:00 2001
> From: Dan Williams <dan.j.williams@intel.com>
> Date: Thu, 6 Nov 2025 15:13:50 -0800
> Subject: [PATCH v2] x86/kaslr: Recognize all ZONE_DEVICE users as physaddr
> consumers
>
> Commit 7ffb791423c7 ("x86/kaslr: Reduce KASLR entropy on most x86 systems")
> is too narrow. The effect being mitigated in that commit is caused by
> ZONE_DEVICE which PCI_P2PDMA has a dependency. 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 DEVICE_PRIVATE and PCI_P2PDMA cases. Update
> the mitigation, limit KASLR entropy, to apply in all ZONE_DEVICE=y cases.
>
> Distro kernels typically have PCI_P2PDMA=y, so the practical exposure of
> this problem is limited to the PCI_P2PDMA=n case.
>
> 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: 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>
> Fixes: 7ffb791423c7 ("x86/kaslr: Reduce KASLR entropy on most x86 systems")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> Reviewed-by: Balbir Singh <balbirs@nvidia.com>
> Tested-by: Yasunori Goto <y-goto@fujitsu.com>
> Acked-by: Dave Hansen <dave.hansen@linux.intel.com>

Applied to cxl/fixes
269031b15c1433ff39e30fa7ea3ab8f0be9d6ae2

DJ

> ---
> drivers/pci/Kconfig | 6 ------
> mm/Kconfig | 12 ++++++++----
> arch/x86/mm/kaslr.c | 10 +++++-----
> 3 files changed, 13 insertions(+), 15 deletions(-)
>
> diff --git a/drivers/pci/Kconfig b/drivers/pci/Kconfig
> index f94f5d384362..47e466946bed 100644
> --- a/drivers/pci/Kconfig
> +++ b/drivers/pci/Kconfig
> @@ -207,12 +207,6 @@ config PCI_P2PDMA
> P2P DMA transactions must be between devices behind the same root
> port.
>
> - Enabling this option will reduce the entropy of x86 KASLR memory
> - regions. For example - on a 46 bit system, the entropy goes down
> - from 16 bits to 15 bits. The actual reduction in entropy depends
> - on the physical address bits, on processor features, kernel config
> - (5 level page table) and physical memory present on the system.
> -
> If unsure, say N.
>
> config PCI_LABEL
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 0e26f4fc8717..d17ebcc1a029 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -1128,10 +1128,14 @@ config ZONE_DEVICE
> Device memory hotplug support allows for establishing pmem,
> or other device driver discovered memory regions, in the
> memmap. This allows pfn_to_page() lookups of otherwise
> - "device-physical" addresses which is needed for using a DAX
> - mapping in an O_DIRECT operation, among other things.
> -
> - If FS_DAX is enabled, then say Y.
> + "device-physical" addresses which is needed for DAX, PCI_P2PDMA, and
> + DEVICE_PRIVATE features among others.
> +
> + Enabling this option will reduce the entropy of x86 KASLR memory
> + regions. For example - on a 46 bit system, the entropy goes down
> + from 16 bits to 15 bits. The actual reduction in entropy depends
> + on the physical address bits, on processor features, kernel config
> + (5 level page table) and physical memory present on the system.
>
> #
> # Helpers to mirror range of the CPU page tables of a process into device page
> diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
> index 3c306de52fd4..834641c6049a 100644
> --- a/arch/x86/mm/kaslr.c
> +++ b/arch/x86/mm/kaslr.c
> @@ -115,12 +115,12 @@ void __init kernel_randomize_memory(void)
>
> /*
> * Adapt physical memory region size based on available memory,
> - * except when CONFIG_PCI_P2PDMA is enabled. P2PDMA exposes the
> - * device BAR space assuming the direct map space is large enough
> - * for creating a ZONE_DEVICE mapping in the direct map corresponding
> - * to the physical BAR address.
> + * except when CONFIG_ZONE_DEVICE is enabled. ZONE_DEVICE wants to map
> + * any physical address into the direct-map. KASLR wants to reliably
> + * steal some physical address bits. Those design choices are in direct
> + * conflict.
> */
> - if (!IS_ENABLED(CONFIG_PCI_P2PDMA) && (memory_tb < kaslr_regions[0].size_tb))
> + if (!IS_ENABLED(CONFIG_ZONE_DEVICE) && (memory_tb < kaslr_regions[0].size_tb))
> kaslr_regions[0].size_tb = memory_tb;
>
> /*