arm64: module: move module randomization to module.c

When CONFIG_RANDOMIZE_BASE=y, module_alloc_base is a variable which is
configured by kaslr_module_init() in kaslr.c, and otherwise it is an
expression defined in module.h.

As kaslr_module_init() is no longer tightly coupled with the KASLR
initialization code, we can centralize this in module.c.

This patch moves kaslr_module_init() to module.c, making
module_alloc_base a static variable, and removing redundant includes from
kaslr.c. For the defintion of struct arm64_ftr_override we must include
<asm/cpufeature.h>, which was previously included transitively via
another header.

There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Cc: Will Deacon <will@kernel.org>
Tested-by: Shanker Donthineni <sdonthineni@nvidia.com>
Link: https://lore.kernel.org/r/20230530110328.2213762-5-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/module.c

@@ -15,13 +15,61 @@
 #include <linux/kernel.h>
 #include <linux/mm.h>
 #include <linux/moduleloader.h>
+#include <linux/random.h>
 #include <linux/scs.h>
 #include <linux/vmalloc.h>
+
 #include <asm/alternative.h>
 #include <asm/insn.h>
 #include <asm/scs.h>
 #include <asm/sections.h>
 
+static u64 __ro_after_init module_alloc_base = (u64)_etext - MODULES_VSIZE;
+
+#ifdef CONFIG_RANDOMIZE_BASE
+static int __init kaslr_module_init(void)
+{
+	u64 module_range;
+	u32 seed;
+
+	if (!kaslr_enabled())
+		return 0;
+
+	seed = get_random_u32();
+
+	if (IS_ENABLED(CONFIG_RANDOMIZE_MODULE_REGION_FULL)) {
+		/*
+		 * Randomize the module region over a 2 GB window covering the
+		 * kernel. This reduces the risk of modules leaking 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 = SZ_2G - (u64)(_end - _stext);
+		module_alloc_base = max((u64)_end - SZ_2G, (u64)MODULES_VADDR);
+	} 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 unless this region is exhausted
+		 * and we fall back to a larger 2GB window in module_alloc()
+		 * when ARM64_MODULE_PLTS is enabled.
+		 */
+		module_range = MODULES_VSIZE - (u64)(_etext - _stext);
+	}
+
+	/* 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 0;
+}
+subsys_initcall(kaslr_module_init)
+#endif
+
 void *module_alloc(unsigned long size)
 {
 	u64 module_alloc_end = module_alloc_base + MODULES_VSIZE;