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

[Kees Cook]

On Fri, Feb 07, 2025 at 09:23:50AM +1100, Balbir Singh wrote:
> On 2/7/25 08:46, Kees Cook wrote:
> > On Fri, Feb 07, 2025 at 08:22:23AM +1100, Balbir Singh wrote:
> >> On 2/7/25 06:59, Kees Cook wrote:
> >>> On Thu, Feb 06, 2025 at 09:10:58AM +0100, Peter Zijlstra wrote:
> >>>> On Thu, Feb 06, 2025 at 01:32:01PM +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.
> >>>
> >>> So, just to restate my understanding: this is about only the direct map
> >>> (i.e. kaslr_region[0]). The notes (which I think should be left in the
> >>> commit log) say that the entropy dropped from 49 TiB (46 bits) to 20 TiB
> >>> (45 bits). If I'm reading right, the offset granularity is in PUD_SIZE
> >>> (30 bits) steps, so the entropy is going from 16 bits to 15 bits. I don't
> >>> see any general problem with that. Especially if the alternative is 0
> >>> bits of entropy. :)
> >>>
> >>
> >> Yes, this is about the direct map (kaslr_region[0]) and the data is from my
> >> system which has 46 bits of physical address. On larger systems with LA57
> >> the drop might be higher. I am happy to repost the patch with my testing notes
> >> in the commit log, if you think it's useful to have in the commit log.
> > 
> > Actually, it might be more useful to note it in the CONFIG help
> > text? "This may reduce direct map ASLR entropy by 1 bit with 46 physical
> > bits, X bits with YY physical bits, [etc...]"
> > 
> 
> I worded it as follows in drivers/pci/Kconfig
> 
> 	  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.
> 
> Does that seem reasonable?

Yeah, I like it. Thanks!

-- 
Kees Cook