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

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Message-ID: <8a3cef5a-145b-40f3-a6e4-16dfec76fa2b@nvidia.com>
Date: Wed, 19 Feb 2025 12:11:50 +1100
From: Balbir Singh <balbirs@...dia.com>
To: Kees Cook <kees@...nel.org>
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 2/7/25 12:00, Kees Cook wrote:
> 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>
> 

Thank you Kees! 

I wanted to request that we pick this up in linux-next via the relevant
subtree for further testing

Balbir Singh