arm64: add support for kernel ASLR

This adds support for KASLR is implemented, based on entropy provided by
the bootloader in the /chosen/kaslr-seed DT property. Depending on the size
of the address space (VA_BITS) and the page size, the entropy in the
virtual displacement is up to 13 bits (16k/2 levels) and up to 25 bits (all
4 levels), with the sidenote that displacements that result in the kernel
image straddling a 1GB/32MB/512MB alignment boundary (for 4KB/16KB/64KB
granule kernels, respectively) are not allowed, and will be rounded up to
an acceptable value.

If CONFIG_RANDOMIZE_MODULE_REGION_FULL is enabled, the module region is
randomized independently from the core kernel. This makes it less likely
that the location of core kernel data structures can be determined by an
adversary, but causes all function calls from modules into the core kernel
to be resolved via entries in the module PLTs.

If CONFIG_RANDOMIZE_MODULE_REGION_FULL is not enabled, the module region is
randomized by choosing a page aligned 128 MB region inside the interval
[_etext - 128 MB, _stext + 128 MB). This gives between 10 and 14 bits of
entropy (depending on page size), independently of the kernel randomization,
but still guarantees that modules are within the range of relative branch
and jump instructions (with the caveat that, since the module region is
shared with other uses of the vmalloc area, modules may need to be loaded
further away if the module region is exhausted)

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 173 insertions, 0 deletions

diff --git a/arch/arm64/kernel/kaslr.c b/arch/arm64/kernel/kaslr.c
new file mode 100644
index 000000000000..8b32a1f8f09f
--- /dev/null
+++ b/arch/arm64/kernel/kaslr.c
@@ -0,0 +1,173 @@
+/*
+ * Copyright (C) 2016 Linaro Ltd <ard.biesheuvel@linaro.org>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+
+#include <linux/crc32.h>
+#include <linux/init.h>
+#include <linux/libfdt.h>
+#include <linux/mm_types.h>
+#include <linux/sched.h>
+#include <linux/types.h>
+
+#include <asm/fixmap.h>
+#include <asm/kernel-pgtable.h>
+#include <asm/memory.h>
+#include <asm/mmu.h>
+#include <asm/pgtable.h>
+#include <asm/sections.h>
+
+u64 __read_mostly module_alloc_base;
+
+static __init u64 get_kaslr_seed(void *fdt)
+{
+	int node, len;
+	u64 *prop;
+	u64 ret;
+
+	node = fdt_path_offset(fdt, "/chosen");
+	if (node < 0)
+		return 0;
+
+	prop = fdt_getprop_w(fdt, node, "kaslr-seed", &len);
+	if (!prop || len != sizeof(u64))
+		return 0;
+
+	ret = fdt64_to_cpu(*prop);
+	*prop = 0;
+	return ret;
+}
+
+static __init const u8 *get_cmdline(void *fdt)
+{
+	static __initconst const u8 default_cmdline[] = CONFIG_CMDLINE;
+
+	if (!IS_ENABLED(CONFIG_CMDLINE_FORCE)) {
+		int node;
+		const u8 *prop;
+
+		node = fdt_path_offset(fdt, "/chosen");
+		if (node < 0)
+			goto out;
+
+		prop = fdt_getprop(fdt, node, "bootargs", NULL);
+		if (!prop)
+			goto out;
+		return prop;
+	}
+out:
+	return default_cmdline;
+}
+
+extern void *__init __fixmap_remap_fdt(phys_addr_t dt_phys, int *size,
+				       pgprot_t prot);
+
+/*
+ * This routine will be executed with the kernel mapped at its default virtual
+ * address, and if it returns successfully, the kernel will be remapped, and
+ * start_kernel() will be executed from a randomized virtual offset. The
+ * relocation will result in all absolute references (e.g., static variables
+ * containing function pointers) to be reinitialized, and zero-initialized
+ * .bss variables will be reset to 0.
+ */
+u64 __init kaslr_early_init(u64 dt_phys)
+{
+	void *fdt;
+	u64 seed, offset, mask, module_range;
+	const u8 *cmdline, *str;
+	int size;
+
+	/*
+	 * Set a reasonable default for module_alloc_base in case
+	 * we end up running with module randomization disabled.
+	 */
+	module_alloc_base = (u64)_etext - MODULES_VSIZE;
+
+	/*
+	 * Try to map the FDT early. If this fails, we simply bail,
+	 * and proceed with KASLR disabled. We will make another
+	 * attempt at mapping the FDT in setup_machine()
+	 */
+	early_fixmap_init();
+	fdt = __fixmap_remap_fdt(dt_phys, &size, PAGE_KERNEL);
+	if (!fdt)
+		return 0;
+
+	/*
+	 * Retrieve (and wipe) the seed from the FDT
+	 */
+	seed = get_kaslr_seed(fdt);
+	if (!seed)
+		return 0;
+
+	/*
+	 * Check if 'nokaslr' appears on the command line, and
+	 * return 0 if that is the case.
+	 */
+	cmdline = get_cmdline(fdt);
+	str = strstr(cmdline, "nokaslr");
+	if (str == cmdline || (str > cmdline && *(str - 1) == ' '))
+		return 0;
+
+	/*
+	 * OK, so we are proceeding with KASLR enabled. Calculate a suitable
+	 * kernel image offset from the seed. Let's place the kernel in the
+	 * lower half of the VMALLOC area (VA_BITS - 2).
+	 * Even if we could randomize at page granularity for 16k and 64k pages,
+	 * let's always round to 2 MB so we don't interfere with the ability to
+	 * map using contiguous PTEs
+	 */
+	mask = ((1UL << (VA_BITS - 2)) - 1) & ~(SZ_2M - 1);
+	offset = seed & mask;
+
+	/*
+	 * The kernel Image should not extend across a 1GB/32MB/512MB alignment
+	 * boundary (for 4KB/16KB/64KB granule kernels, respectively). If this
+	 * happens, increase the KASLR offset by the size of the kernel image.
+	 */
+	if ((((u64)_text + offset) >> SWAPPER_TABLE_SHIFT) !=
+	    (((u64)_end + offset) >> SWAPPER_TABLE_SHIFT))
+		offset = (offset + (u64)(_end - _text)) & mask;
+
+	if (IS_ENABLED(CONFIG_KASAN))
+		/*
+		 * KASAN does not expect the module region to intersect the
+		 * vmalloc region, since shadow memory is allocated for each
+		 * module at load time, whereas the vmalloc region is shadowed
+		 * by KASAN zero pages. So keep modules out of the vmalloc
+		 * region if KASAN is enabled.
+		 */
+		return offset;
+
+	if (IS_ENABLED(CONFIG_RANDOMIZE_MODULE_REGION_FULL)) {
+		/*
+		 * Randomize the module region independently from the core
+		 * kernel. This prevents modules from leaking any information
+		 * about the address of the kernel itself, but results in
+		 * branches between modules and the core kernel that are
+		 * resolved via PLTs. (Branches between modules will be
+		 * resolved normally.)
+		 */
+		module_range = VMALLOC_END - VMALLOC_START - MODULES_VSIZE;
+		module_alloc_base = VMALLOC_START;
+	} else {
+		/*
+		 * Randomize the module region by setting module_alloc_base to
+		 * a PAGE_SIZE multiple in the range [_etext - MODULES_VSIZE,
+		 * _stext) . This guarantees that the resulting region still
+		 * covers [_stext, _etext], and that all relative branches can
+		 * be resolved without veneers.
+		 */
+		module_range = MODULES_VSIZE - (u64)(_etext - _stext);
+		module_alloc_base = (u64)_etext + offset - MODULES_VSIZE;
+	}
+
+	/* use the lower 21 bits to randomize the base of the module region */
+	module_alloc_base += (module_range * (seed & ((1 << 21) - 1))) >> 21;
+	module_alloc_base &= PAGE_MASK;
+
+	return offset;
+}