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

From: Bjorn Helgaas
Date: Fri Feb 21 2025 - 17:43:22 EST


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>

Acked-by: Bjorn Helgaas <bhelgaas@xxxxxxxxxx> # drivers/pci/Kconfig

> ---
> Changelog v2
> - Add information about entropy drop when PCI_P2PDMA is
> selected
>
> Testing:
>
> commit 0483e1fa6e09d ("x86/mm: Implement ASLR for kernel memory regions") mentions the
> problems that the following problems need to be addressed.
>
> 1 The three target memory sections are never at the same place between
> boots.
> 2 The physical memory mapping can use a virtual address not aligned on
> the PGD page table.
> 3 Have good entropy early at boot before get_random_bytes is available.
> 4 Add optional padding for memory hotplug compatibility.
>
> Ran an automated test to ensure that (1) holds true across several
> iterations of automated reboot testing. 2, 3 and 4 are not impacted
> by this patch.
>
> Manual Testing on a system where the problem reproduces
>
> 1. With KASLR
>
> Hotplug memory [0x240000000000-0x242000000000] exceeds maximum addressable range [0x0-0xaffffffffff]
> ------------[ cut here ]------------
> 2. With the fixes
>
> added peer-to-peer DMA memory 0x240000000000-0x241fffffffff
>
> KASLR is still enabled as seen by kaslr_offset() (difference
> between __START_KERNEL and _stext)
> 3. Without the fixes and KASLR disabled
>
>
> arch/x86/mm/kaslr.c | 10 ++++++++--
> drivers/pci/Kconfig | 6 ++++++
> 2 files changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
> index 11a93542d198..3c306de52fd4 100644
> --- a/arch/x86/mm/kaslr.c
> +++ b/arch/x86/mm/kaslr.c
> @@ -113,8 +113,14 @@ void __init kernel_randomize_memory(void)
> memory_tb = DIV_ROUND_UP(max_pfn << PAGE_SHIFT, 1UL << TB_SHIFT) +
> CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING;
>
> - /* Adapt physical memory region size based on available memory */
> - if (memory_tb < kaslr_regions[0].size_tb)
> + /*
> + * Adapt physical memory region size based on available memory,
> + * except when CONFIG_PCI_P2PDMA is enabled. P2PDMA exposes the
> + * device BAR space assuming the direct map space is large enough
> + * for creating a ZONE_DEVICE mapping in the direct map corresponding
> + * to the physical BAR address.
> + */
> + if (!IS_ENABLED(CONFIG_PCI_P2PDMA) && (memory_tb < kaslr_regions[0].size_tb))
> kaslr_regions[0].size_tb = memory_tb;
>
> /*
> diff --git a/drivers/pci/Kconfig b/drivers/pci/Kconfig
> index 2fbd379923fd..5c3054aaec8c 100644
> --- a/drivers/pci/Kconfig
> +++ b/drivers/pci/Kconfig
> @@ -203,6 +203,12 @@ config PCI_P2PDMA
> P2P DMA transactions must be between devices behind the same root
> port.
>
> + 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.
> +
> If unsure, say N.
>
> config PCI_LABEL
> --
> 2.48.1
>