The following commit has been merged into the x86/mm branch of tip: Commit-ID: 4816f6361fffb172c04e702c9af3f8aa80962cea Gitweb: https://git.kernel.org/tip/4816f6361fffb172c04e702c9af3f8aa80962cea Author: Balbir Singh <balbirs@xxxxxxxxxx> AuthorDate: Fri, 07 Feb 2025 10:42:34 +11:00 Committer: Ingo Molnar <mingo@xxxxxxxxxx> CommitterDate: Fri, 21 Feb 2025 17:08:26 +01:00 x86/kaslr: Reduce KASLR entropy on most x86 systems 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@xxxxxxxxxx> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> Reviewed-by: Kees Cook <kees@xxxxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Andy Lutomirski <luto@xxxxxxxxxx> Link: https://lore.kernel.org/lkml/20250206023201.1481957-1-balbirs@xxxxxxxxxx/ Link: https://lore.kernel.org/r/20250206234234.1912585-1-balbirs@xxxxxxxxxx -- arch/x86/mm/kaslr.c | 10 ++++++++-- drivers/pci/Kconfig | 6 ++++++ 2 files changed, 14 insertions(+), 2 deletions(-) --- 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 11a9354..3c306de 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 2fbd379..5c3054a 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
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