x86/kaslr: Reduce KASLR entropy on most x86 systems

[ 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>

diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
index 37db264866b6489ef99325d2fe1604d6cd3c9885..2ef1951ce1fd67eacdc1c23572390a8dc70260be 100644 (file)
--- a/arch/x86/mm/kaslr.c
+++ b/arch/x86/mm/kaslr.c
@@ -96,8 +96,14 @@ 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 43e615aa12ffac88e50733771f1737cb2e549525..8ed3bf14f0ce5236174f06f4d366fd4098dbba4d 100644 (file)
--- a/drivers/pci/Kconfig
+++ b/drivers/pci/Kconfig
@@ -176,6 +176,12 @@ 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