1 files changed, 181 insertions, 0 deletions
diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
new file mode 100644
index 000000000..37db26486
--- /dev/null
+++ b/arch/x86/mm/kaslr.c
@@ -0,0 +1,181 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * This file implements KASLR memory randomization for x86_64. It randomizes
+ * the virtual address space of kernel memory regions (physical memory
+ * mapping, vmalloc & vmemmap) for x86_64. This security feature mitigates
+ * exploits relying on predictable kernel addresses.
+ *
+ * Entropy is generated using the KASLR early boot functions now shared in
+ * the lib directory (originally written by Kees Cook). Randomization is
+ * done on PGD & P4D/PUD page table levels to increase possible addresses.
+ * The physical memory mapping code was adapted to support P4D/PUD level
+ * virtual addresses. This implementation on the best configuration provides
+ * 30,000 possible virtual addresses in average for each memory region.
+ * An additional low memory page is used to ensure each CPU can start with
+ * a PGD aligned virtual address (for realmode).
+ *
+ * The order of each memory region is not changed. The feature looks at
+ * the available space for the regions based on different configuration
+ * options and randomizes the base and space between each. The size of the
+ * physical memory mapping is the available physical memory.
+ */
+
+#include <linux/kernel.h>
+#include <linux/init.h>
+#include <linux/random.h>
+#include <linux/memblock.h>
+#include <linux/pgtable.h>
+
+#include <asm/setup.h>
+#include <asm/kaslr.h>
+
+#include "mm_internal.h"
+
+#define TB_SHIFT 40
+
+/*
+ * The end address could depend on more configuration options to make the
+ * highest amount of space for randomization available, but that's too hard
+ * to keep straight and caused issues already.
+ */
+static const unsigned long vaddr_end = CPU_ENTRY_AREA_BASE;
+
+/*
+ * Memory regions randomized by KASLR (except modules that use a separate logic
+ * earlier during boot). The list is ordered based on virtual addresses. This
+ * order is kept after randomization.
+ */
+static __initdata struct kaslr_memory_region {
+	unsigned long *base;
+	unsigned long size_tb;
+} kaslr_regions[] = {
+	{ &page_offset_base, 0 },
+	{ &vmalloc_base, 0 },
+	{ &vmemmap_base, 0 },
+};
+
+/* Get size in bytes used by the memory region */
+static inline unsigned long get_padding(struct kaslr_memory_region *region)
+{
+	return (region->size_tb << TB_SHIFT);
+}
+
+/* Initialize base and padding for each memory region randomized with KASLR */
+void __init kernel_randomize_memory(void)
+{
+	size_t i;
+	unsigned long vaddr_start, vaddr;
+	unsigned long rand, memory_tb;
+	struct rnd_state rand_state;
+	unsigned long remain_entropy;
+	unsigned long vmemmap_size;
+
+	vaddr_start = pgtable_l5_enabled() ? __PAGE_OFFSET_BASE_L5 : __PAGE_OFFSET_BASE_L4;
+	vaddr = vaddr_start;
+
+	/*
+	 * These BUILD_BUG_ON checks ensure the memory layout is consistent
+	 * with the vaddr_start/vaddr_end variables. These checks are very
+	 * limited....
+	 */
+	BUILD_BUG_ON(vaddr_start >= vaddr_end);
+	BUILD_BUG_ON(vaddr_end != CPU_ENTRY_AREA_BASE);
+	BUILD_BUG_ON(vaddr_end > __START_KERNEL_map);
+
+	if (!kaslr_memory_enabled())
+		return;
+
+	kaslr_regions[0].size_tb = 1 << (MAX_PHYSMEM_BITS - TB_SHIFT);
+	kaslr_regions[1].size_tb = VMALLOC_SIZE_TB;
+
+	/*
+	 * Update Physical memory mapping to available and
+	 * add padding if needed (especially for memory hotplug support).
+	 */
+	BUG_ON(kaslr_regions[0].base != &page_offset_base);
+	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)
+		kaslr_regions[0].size_tb = memory_tb;
+
+	/*
+	 * Calculate the vmemmap region size in TBs, aligned to a TB
+	 * boundary.
+	 */
+	vmemmap_size = (kaslr_regions[0].size_tb << (TB_SHIFT - PAGE_SHIFT)) *
+			sizeof(struct page);
+	kaslr_regions[2].size_tb = DIV_ROUND_UP(vmemmap_size, 1UL << TB_SHIFT);
+
+	/* Calculate entropy available between regions */
+	remain_entropy = vaddr_end - vaddr_start;
+	for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++)
+		remain_entropy -= get_padding(&kaslr_regions[i]);
+
+	prandom_seed_state(&rand_state, kaslr_get_random_long("Memory"));
+
+	for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++) {
+		unsigned long entropy;
+
+		/*
+		 * Select a random virtual address using the extra entropy
+		 * available.
+		 */
+		entropy = remain_entropy / (ARRAY_SIZE(kaslr_regions) - i);
+		prandom_bytes_state(&rand_state, &rand, sizeof(rand));
+		entropy = (rand % (entropy + 1)) & PUD_MASK;
+		vaddr += entropy;
+		*kaslr_regions[i].base = vaddr;
+
+		/*
+		 * Jump the region and add a minimum padding based on
+		 * randomization alignment.
+		 */
+		vaddr += get_padding(&kaslr_regions[i]);
+		vaddr = round_up(vaddr + 1, PUD_SIZE);
+		remain_entropy -= entropy;
+	}
+}
+
+void __meminit init_trampoline_kaslr(void)
+{
+	pud_t *pud_page_tramp, *pud, *pud_tramp;
+	p4d_t *p4d_page_tramp, *p4d, *p4d_tramp;
+	unsigned long paddr, vaddr;
+	pgd_t *pgd;
+
+	pud_page_tramp = alloc_low_page();
+
+	/*
+	 * There are two mappings for the low 1MB area, the direct mapping
+	 * and the 1:1 mapping for the real mode trampoline:
+	 *
+	 * Direct mapping: virt_addr = phys_addr + PAGE_OFFSET
+	 * 1:1 mapping:    virt_addr = phys_addr
+	 */
+	paddr = 0;
+	vaddr = (unsigned long)__va(paddr);
+	pgd = pgd_offset_k(vaddr);
+
+	p4d = p4d_offset(pgd, vaddr);
+	pud = pud_offset(p4d, vaddr);
+
+	pud_tramp = pud_page_tramp + pud_index(paddr);
+	*pud_tramp = *pud;
+
+	if (pgtable_l5_enabled()) {
+		p4d_page_tramp = alloc_low_page();
+
+		p4d_tramp = p4d_page_tramp + p4d_index(paddr);
+
+		set_p4d(p4d_tramp,
+			__p4d(_KERNPG_TABLE | __pa(pud_page_tramp)));
+
+		trampoline_pgd_entry =
+			__pgd(_KERNPG_TABLE | __pa(p4d_page_tramp));
+	} else {
+		trampoline_pgd_entry =
+			__pgd(_KERNPG_TABLE | __pa(pud_page_tramp));
+	}
+}