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vk4qOpP2fojKMxKc= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 11/11/25 10:34, dan.j.williams@intel.com wrote: > Balbir Singh wrote: >> 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 >>> Cc: Ingo Molnar >>> Cc: Kees Cook >>> Cc: Bjorn Helgaas >>> Cc: Peter Zijlstra >>> Cc: Andy Lutomirski >>> Cc: Logan Gunthorpe >>> Cc: Andrew Morton >>> Cc: David Hildenbrand >>> Cc: Lorenzo Stoakes >>> Cc: "Liam R. Howlett" >>> Cc: Vlastimil Babka >>> Cc: Mike Rapoport >>> Cc: Suren Baghdasaryan >>> Cc: Michal Hocko >>> Cc: "Yasunori Gotou (Fujitsu)" >>> Fixes: 7ffb791423c7 ("x86/kaslr: Reduce KASLR entropy on most x86 systems") >>> Signed-off-by: Dan Williams >> >> 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? > > There are 2 considerations > > - Occasions where P2PDMA is disabled, but ZONE_DEVICE is enabled. > > I started looking at this after a report about CXL failures with KASLR. > I do not have the kernel configuration for that end user report, but I > can only a imagine it was indeed a case of CONFIG_PCI_P2PDMA=n and > CONFIG_DEV_DAX_CXL=y > > - Occasions where ZONE_DEVICE and memory hotplug are enabled, but ACPI > does not publish any hotplug or CXL memory ranges, and BIOS did not > find any large PCI devices at initial scan. > > In this case even the default 10 TB padding is overkill and more address > bits could be consumed for KASLR entropy. Makes sense! Reviewed-by: Balbir Singh