Re: [PATCH v2] x86/kaslr: Revisit entropy when CONFIG_PCI_P2PDMA is enabled

[Kees Cook]

On Fri, Feb 07, 2025 at 10:42:34AM +1100, Balbir Singh wrote:
> When CONFIG_PCI_P2PDMA is enabled, it maps the PFN's via a
> ZONE_DEVICE mapping using devm_memremap_pages(). The mapped
> virtual address range corresponds to the pci_resource_start()
> of the BAR address and size corresponding to the BAR length.
> 
> When KASLR is enabled, the direct map range of the kernel is
> reduced to the size of physical memory plus additional padding.
> If the BAR address is beyond this limit, PCI peer to peer DMA
> mappings fail.
> 
> Fix this by not shrinking the size of direct map when CONFIG_PCI_P2PDMA
> is enabled. This reduces the total available entropy, but it's
> better than the current work around of having to disable KASLR
> completely.
> 
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Kees Cook <kees@xxxxxxxxxx>
> Cc: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
> Link: https://lore.kernel.org/lkml/20250206023201.1481957-1-balbirs@xxxxxxxxxx/
> 
> Signed-off-by: Balbir Singh <balbirs@xxxxxxxxxx>

Thanks for the update!

Reviewed-by: Kees Cook <kees@xxxxxxxxxx>

-- 
Kees Cook