From: Balbir Singh balbirs@nvidia.com
[ Upstream commit 7ffb791423c7c518269a9aad35039ef824a40adb ]
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(-) Signed-off-by: Sasha Levin sashal@kernel.org --- 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 230f1dee4f095..e0b0ec0f82457 100644 --- a/arch/x86/mm/kaslr.c +++ b/arch/x86/mm/kaslr.c @@ -109,8 +109,14 @@ 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 e9ae66cc4189b..a3927daebeb02 100644 --- a/drivers/pci/Kconfig +++ b/drivers/pci/Kconfig @@ -180,6 +180,12 @@ 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