arm64: kaslr: Adjust randomization range dynamically

Currently, we base the KASLR randomization range on a rough estimate of
the available space in the upper VA region: the lower 1/4th has the
module region and the upper 1/4th has the fixmap, vmemmap and PCI I/O
ranges, and so we pick a random location in the remaining space in the
middle.

Once we enable support for 5-level paging with 4k pages, this no longer
works: the vmemmap region, being dimensioned to cover a 52-bit linear
region, takes up so much space in the upper VA region (the size of which
is based on a 48-bit VA space for compatibility with non-LVA hardware)
that the region above the vmalloc region takes up more than a quarter of
the available space.

So instead of a heuristic, let's derive the randomization range from the
actual boundaries of the vmalloc region.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20231213084024.2367360-16-ardb@google.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>

Author: ardbiesheuvel

arch/arm64/kernel/image-vars.h

@@ -36,6 +36,8 @@ PROVIDE(__pi___memcpy			= __pi_memcpy);
 PROVIDE(__pi___memmove			= __pi_memmove);
 PROVIDE(__pi___memset			= __pi_memset);
 
+PROVIDE(__pi_vabits_actual		= vabits_actual);
+
 #ifdef CONFIG_KVM
 
 /*

arch/arm64/kernel/pi/kaslr_early.c

@@ -14,6 +14,7 @@
 
 #include <asm/archrandom.h>
 #include <asm/memory.h>
+#include <asm/pgtable.h>
 
 /* taken from lib/string.c */
 static char *__strstr(const char *s1, const char *s2)
@@ -87,7 +88,7 @@ static u64 get_kaslr_seed(void *fdt)
 
 asmlinkage u64 kaslr_early_init(void *fdt)
 {
-	u64 seed;
+	u64 seed, range;
 
 	if (is_kaslr_disabled_cmdline(fdt))
 		return 0;
@@ -102,9 +103,9 @@ asmlinkage u64 kaslr_early_init(void *fdt)
 	/*
 	 * OK, so we are proceeding with KASLR enabled. Calculate a suitable
 	 * kernel image offset from the seed. Let's place the kernel in the
-	 * middle half of the VMALLOC area (VA_BITS_MIN - 2), and stay clear of
-	 * the lower and upper quarters to avoid colliding with other
-	 * allocations.
+	 * 'middle' half of the VMALLOC area, and stay clear of the lower and
+	 * upper quarters to avoid colliding with other allocations.
 	 */
-	return BIT(VA_BITS_MIN - 3) + (seed & GENMASK(VA_BITS_MIN - 3, 0));
+	range = (VMALLOC_END - KIMAGE_VADDR) / 2;
+	return range / 2 + (((__uint128_t)range * seed) >> 64);
 }