linux-kernel - Re: [PATCH v2] x86/kaslr: Revisit entropy when CONFIG_PCI

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Message-ID: <202502061700.5C52C6D453@keescook>
Date: Thu, 6 Feb 2025 17:00:59 -0800
From: Kees Cook <kees@...nel.org>
To: Balbir Singh <balbirs@...dia.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	apopple@...dia.com, jhubbard@...dia.com, jgg@...dia.com,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH v2] x86/kaslr: Revisit entropy when CONFIG_PCI_P2PDMA is
 enabled

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@...ux.intel.com>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: Kees Cook <kees@...nel.org>
> Cc: Bjorn Helgaas <bhelgaas@...gle.com>
> Link: https://lore.kernel.org/lkml/20250206023201.1481957-1-balbirs@nvidia.com/
> 
> Signed-off-by: Balbir Singh <balbirs@...dia.com>

Thanks for the update!

Reviewed-by: Kees Cook <kees@...nel.org>

-- 
Kees Cook