[PATCH v3 10/60] arm64: kaslr: Adjust randomization range dynamically

Ard Biesheuvel
2023-03-07T14:08:05Z

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>
---
arch/arm64/kernel/image-vars.h | 2 ++
arch/arm64/kernel/pi/kaslr_early.c | 11 ++++++-----
2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/kernel/image-vars.h b/arch/arm64/kernel/image-vars.h
index 8309197c0ebd4a8e..b5906f8e18d7eb8d 100644
--- a/arch/arm64/kernel/image-vars.h
+++ b/arch/arm64/kernel/image-vars.h
@@ -34,6 +34,8 @@ PROVIDE(__pi___memcpy = __pi_memcpy);
PROVIDE(__pi___memmove = __pi_memmove);
PROVIDE(__pi___memset = __pi_memset);

+PROVIDE(__pi_vabits_actual = vabits_actual);
+
#ifdef CONFIG_KVM

/*
diff --git a/arch/arm64/kernel/pi/kaslr_early.c b/arch/arm64/kernel/pi/kaslr_early.c
index 17bff6e399e46b0b..b9e0bb4bc6a9766f 100644
--- a/arch/arm64/kernel/pi/kaslr_early.c
+++ b/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);
}
--
2.39.2