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x86/kaslr: Reduce KASLR entropy on most x86 systems

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When CONFIG_PCI_P2PDMA=y (which is basically enabled on all
large x86 distros), 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 the direct map when
CONFIG_PCI_P2PDMA=y.

This reduces the total available entropy, but it's better than
the current work around of having to disable KASLR completely.

[ mingo: Clarified the changelog to point out the broad impact ... ]

Signed-off-by: Balbir Singh <balbirs@nvidia.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Acked-by: Bjorn Helgaas <bhelgaas@google.com> # drivers/pci/Kconfig
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>
Link: https://lore.kernel.org/lkml/20250206023201.1481957-1-balbirs@nvidia.com/
Link: https://lore.kernel.org/r/20250206234234.1912585-1-balbirs@nvidia.com
--
 arch/x86/mm/kaslr.c | 10 ++++++++--
 drivers/pci/Kconfig |  6 ++++++
 2 files changed, 14 insertions(+), 2 deletions(-)
This commit is contained in:
Balbir Singh 2025-02-07 10:42:34 +11:00 committed by Ingo Molnar
parent 3fcae7771f
commit 7ffb791423
2 changed files with 14 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

10
arch/x86/mm/kaslr.c
View File

@ -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;
/*

6
drivers/pci/Kconfig
View File

@ -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