x86/kaslr: Reduce KASLR entropy on most x86 systems

JIRA: https://issues.redhat.com/browse/RHEL-110657

This patch is a backport of the following upstream commit:
commit 7ffb791
Author: Balbir Singh <balbirs@nvidia.com>
Date:   Fri Feb 7 10:42:34 2025 +1100

    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@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
    --

Signed-off-by: Ricardo Robaina <rrobaina@redhat.com>

Author: rprobaina

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;
 
 	/*

drivers/pci/Kconfig

@@ -206,6 +206,12 @@ 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