arm64: add support for module PLTs

This adds support for emitting PLTs at module load time for relative
branches that are out of range. This is a prerequisite for KASLR, which
may place the kernel and the modules anywhere in the vmalloc area,
making it more likely that branch target offsets exceed the maximum
range of +/- 128 MB.

In this version, I removed the distinction between relocations against
.init executable sections and ordinary executable sections. The reason
is that it is hardly worth the trouble, given that .init.text usually
does not contain that many far branches, and this version now only
reserves PLT entry space for jump and call relocations against undefined
symbols (since symbols defined in the same module can be assumed to be
within +/- 128 MB)

For example, the mac80211.ko module (which is fairly sizable at ~400 KB)
built with -mcmodel=large gives the following relocation counts:

                    relocs    branches   unique     !local
  .text              3925       3347       518        219
  .init.text           11          8         7          1
  .exit.text            4          4         4          1
  .text.unlikely       81         67        36         17

('unique' means branches to unique type/symbol/addend combos, of which
!local is the subset referring to undefined symbols)

IOW, we are only emitting a single PLT entry for the .init sections, and
we are better off just adding it to the core PLT section instead.
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/Kconfig

@@ -395,6 +395,7 @@ config ARM64_ERRATUM_843419
 	bool "Cortex-A53: 843419: A load or store might access an incorrect address"
 	depends on MODULES
 	default y
+	select ARM64_MODULE_CMODEL_LARGE
 	help
 	  This option builds kernel modules using the large memory model in
 	  order to avoid the use of the ADRP instruction, which can cause
@@ -778,6 +779,14 @@ config ARM64_UAO
 	  regular load/store instructions if the cpu does not implement the
 	  feature.
 
+config ARM64_MODULE_CMODEL_LARGE
+	bool
+
+config ARM64_MODULE_PLTS
+	bool
+	select ARM64_MODULE_CMODEL_LARGE
+	select HAVE_MOD_ARCH_SPECIFIC
+
 endmenu
 
 menu "Boot options"

arch/arm64/Makefile

@@ -43,10 +43,14 @@ endif
 
 CHECKFLAGS	+= -D__aarch64__
 
-ifeq ($(CONFIG_ARM64_ERRATUM_843419), y)
+ifeq ($(CONFIG_ARM64_MODULE_CMODEL_LARGE), y)
 KBUILD_CFLAGS_MODULE	+= -mcmodel=large
 endif
 
+ifeq ($(CONFIG_ARM64_MODULE_PLTS),y)
+KBUILD_LDFLAGS_MODULE	+= -T $(srctree)/arch/arm64/kernel/module.lds
+endif
+
 # Default value
 head-y		:= arch/arm64/kernel/head.o
 

arch/arm64/include/asm/module.h

@@ -20,4 +20,15 @@
 
 #define MODULE_ARCH_VERMAGIC	"aarch64"
 
+#ifdef CONFIG_ARM64_MODULE_PLTS
+struct mod_arch_specific {
+	struct elf64_shdr	*plt;
+	int			plt_num_entries;
+	int			plt_max_entries;
+};
+#endif
+
+u64 module_emit_plt_entry(struct module *mod, const Elf64_Rela *rela,
+			  Elf64_Sym *sym);
+
 #endif /* __ASM_MODULE_H */

arch/arm64/kernel/Makefile

@@ -30,6 +30,7 @@ arm64-obj-$(CONFIG_COMPAT)		+= sys32.o kuser32.o signal32.o 	\
 					   ../../arm/kernel/opcodes.o
 arm64-obj-$(CONFIG_FUNCTION_TRACER)	+= ftrace.o entry-ftrace.o
 arm64-obj-$(CONFIG_MODULES)		+= arm64ksyms.o module.o
+arm64-obj-$(CONFIG_ARM64_MODULE_PLTS)	+= module-plts.o
 arm64-obj-$(CONFIG_PERF_EVENTS)		+= perf_regs.o perf_callchain.o
 arm64-obj-$(CONFIG_HW_PERF_EVENTS)	+= perf_event.o
 arm64-obj-$(CONFIG_HAVE_HW_BREAKPOINT)	+= hw_breakpoint.o

arch/arm64/kernel/module-plts.c

+/*
+ * Copyright (C) 2014-2016 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/elf.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/sort.h>
+
+struct plt_entry {
+	/*
+	 * A program that conforms to the AArch64 Procedure Call Standard
+	 * (AAPCS64) must assume that a veneer that alters IP0 (x16) and/or
+	 * IP1 (x17) may be inserted at any branch instruction that is
+	 * exposed to a relocation that supports long branches. Since that
+	 * is exactly what we are dealing with here, we are free to use x16
+	 * as a scratch register in the PLT veneers.
+	 */
+	__le32	mov0;	/* movn	x16, #0x....			*/
+	__le32	mov1;	/* movk	x16, #0x...., lsl #16		*/
+	__le32	mov2;	/* movk	x16, #0x...., lsl #32		*/
+	__le32	br;	/* br	x16				*/
+};
+
+u64 module_emit_plt_entry(struct module *mod, const Elf64_Rela *rela,
+			  Elf64_Sym *sym)
+{
+	struct plt_entry *plt = (struct plt_entry *)mod->arch.plt->sh_addr;
+	int i = mod->arch.plt_num_entries;
+	u64 val = sym->st_value + rela->r_addend;
+
+	/*
+	 * We only emit PLT entries against undefined (SHN_UNDEF) symbols,
+	 * which are listed in the ELF symtab section, but without a type
+	 * or a size.
+	 * So, similar to how the module loader uses the Elf64_Sym::st_value
+	 * field to store the resolved addresses of undefined symbols, let's
+	 * borrow the Elf64_Sym::st_size field (whose value is never used by
+	 * the module loader, even for symbols that are defined) to record
+	 * the address of a symbol's associated PLT entry as we emit it for a
+	 * zero addend relocation (which is the only kind we have to deal with
+	 * in practice). This allows us to find duplicates without having to
+	 * go through the table every time.
+	 */
+	if (rela->r_addend == 0 && sym->st_size != 0) {
+		BUG_ON(sym->st_size < (u64)plt || sym->st_size >= (u64)&plt[i]);
+		return sym->st_size;
+	}
+
+	mod->arch.plt_num_entries++;
+	BUG_ON(mod->arch.plt_num_entries > mod->arch.plt_max_entries);
+
+	/*
+	 * MOVK/MOVN/MOVZ opcode:
+	 * +--------+------------+--------+-----------+-------------+---------+
+	 * | sf[31] | opc[30:29] | 100101 | hw[22:21] | imm16[20:5] | Rd[4:0] |
+	 * +--------+------------+--------+-----------+-------------+---------+
+	 *
+	 * Rd     := 0x10 (x16)
+	 * hw     := 0b00 (no shift), 0b01 (lsl #16), 0b10 (lsl #32)
+	 * opc    := 0b11 (MOVK), 0b00 (MOVN), 0b10 (MOVZ)
+	 * sf     := 1 (64-bit variant)
+	 */
+	plt[i] = (struct plt_entry){
+		cpu_to_le32(0x92800010 | (((~val      ) & 0xffff)) << 5),
+		cpu_to_le32(0xf2a00010 | ((( val >> 16) & 0xffff)) << 5),
+		cpu_to_le32(0xf2c00010 | ((( val >> 32) & 0xffff)) << 5),
+		cpu_to_le32(0xd61f0200)
+	};
+
+	if (rela->r_addend == 0)
+		sym->st_size = (u64)&plt[i];
+
+	return (u64)&plt[i];
+}
+
+#define cmp_3way(a,b)	((a) < (b) ? -1 : (a) > (b))
+
+static int cmp_rela(const void *a, const void *b)
+{
+	const Elf64_Rela *x = a, *y = b;
+	int i;
+
+	/* sort by type, symbol index and addend */
+	i = cmp_3way(ELF64_R_TYPE(x->r_info), ELF64_R_TYPE(y->r_info));
+	if (i == 0)
+		i = cmp_3way(ELF64_R_SYM(x->r_info), ELF64_R_SYM(y->r_info));
+	if (i == 0)
+		i = cmp_3way(x->r_addend, y->r_addend);
+	return i;
+}
+
+static bool duplicate_rel(const Elf64_Rela *rela, int num)
+{
+	/*
+	 * Entries are sorted by type, symbol index and addend. That means
+	 * that, if a duplicate entry exists, it must be in the preceding
+	 * slot.
+	 */
+	return num > 0 && cmp_rela(rela + num, rela + num - 1) == 0;
+}
+
+static unsigned int count_plts(Elf64_Sym *syms, Elf64_Rela *rela, int num)
+{
+	unsigned int ret = 0;
+	Elf64_Sym *s;
+	int i;
+
+	for (i = 0; i < num; i++) {
+		switch (ELF64_R_TYPE(rela[i].r_info)) {
+		case R_AARCH64_JUMP26:
+		case R_AARCH64_CALL26:
+			/*
+			 * We only have to consider branch targets that resolve
+			 * to undefined symbols. This is not simply a heuristic,
+			 * it is a fundamental limitation, since the PLT itself
+			 * is part of the module, and needs to be within 128 MB
+			 * as well, so modules can never grow beyond that limit.
+			 */
+			s = syms + ELF64_R_SYM(rela[i].r_info);
+			if (s->st_shndx != SHN_UNDEF)
+				break;
+
+			/*
+			 * Jump relocations with non-zero addends against
+			 * undefined symbols are supported by the ELF spec, but
+			 * do not occur in practice (e.g., 'jump n bytes past
+			 * the entry point of undefined function symbol f').
+			 * So we need to support them, but there is no need to
+			 * take them into consideration when trying to optimize
+			 * this code. So let's only check for duplicates when
+			 * the addend is zero: this allows us to record the PLT
+			 * entry address in the symbol table itself, rather than
+			 * having to search the list for duplicates each time we
+			 * emit one.
+			 */
+			if (rela[i].r_addend != 0 || !duplicate_rel(rela, i))
+				ret++;
+			break;
+		}
+	}
+	return ret;
+}
+
+int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
+			      char *secstrings, struct module *mod)
+{
+	unsigned long plt_max_entries = 0;
+	Elf64_Sym *syms = NULL;
+	int i;
+
+	/*
+	 * Find the empty .plt section so we can expand it to store the PLT
+	 * entries. Record the symtab address as well.
+	 */
+	for (i = 0; i < ehdr->e_shnum; i++) {
+		if (strcmp(".plt", secstrings + sechdrs[i].sh_name) == 0)
+			mod->arch.plt = sechdrs + i;
+		else if (sechdrs[i].sh_type == SHT_SYMTAB)
+			syms = (Elf64_Sym *)sechdrs[i].sh_addr;
+	}
+
+	if (!mod->arch.plt) {
+		pr_err("%s: module PLT section missing\n", mod->name);
+		return -ENOEXEC;
+	}
+	if (!syms) {
+		pr_err("%s: module symtab section missing\n", mod->name);
+		return -ENOEXEC;
+	}
+
+	for (i = 0; i < ehdr->e_shnum; i++) {
+		Elf64_Rela *rels = (void *)ehdr + sechdrs[i].sh_offset;
+		int numrels = sechdrs[i].sh_size / sizeof(Elf64_Rela);
+		Elf64_Shdr *dstsec = sechdrs + sechdrs[i].sh_info;
+
+		if (sechdrs[i].sh_type != SHT_RELA)
+			continue;
+
+		/* ignore relocations that operate on non-exec sections */
+		if (!(dstsec->sh_flags & SHF_EXECINSTR))
+			continue;
+
+		/* sort by type, symbol index and addend */
+		sort(rels, numrels, sizeof(Elf64_Rela), cmp_rela, NULL);
+
+		plt_max_entries += count_plts(syms, rels, numrels);
+	}
+
+	mod->arch.plt->sh_type = SHT_NOBITS;
+	mod->arch.plt->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
+	mod->arch.plt->sh_addralign = L1_CACHE_BYTES;
+	mod->arch.plt->sh_size = plt_max_entries * sizeof(struct plt_entry);
+	mod->arch.plt_num_entries = 0;
+	mod->arch.plt_max_entries = plt_max_entries;
+	return 0;
+}

arch/arm64/kernel/module.c

@@ -38,6 +38,21 @@ void *module_alloc(unsigned long size)
 				GFP_KERNEL, PAGE_KERNEL_EXEC, 0,
 				NUMA_NO_NODE, __builtin_return_address(0));
 
+	if (!p && IS_ENABLED(CONFIG_ARM64_MODULE_PLTS) &&
+	    !IS_ENABLED(CONFIG_KASAN))
+		/*
+		 * KASAN can only deal with module allocations being served
+		 * from the reserved module region, since the remainder of
+		 * the vmalloc region is already backed by zero shadow pages,
+		 * and punching holes into it is non-trivial. Since the module
+		 * region is not randomized when KASAN is enabled, it is even
+		 * less likely that the module region gets exhausted, so we
+		 * can simply omit this fallback in that case.
+		 */
+		p = __vmalloc_node_range(size, MODULE_ALIGN, VMALLOC_START,
+				VMALLOC_END, GFP_KERNEL, PAGE_KERNEL_EXEC, 0,
+				NUMA_NO_NODE, __builtin_return_address(0));
+
 	if (p && (kasan_module_alloc(p, size) < 0)) {
 		vfree(p);
 		return NULL;
@@ -361,6 +376,13 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,
 		case R_AARCH64_CALL26:
 			ovf = reloc_insn_imm(RELOC_OP_PREL, loc, val, 2, 26,
 					     AARCH64_INSN_IMM_26);
+
+			if (IS_ENABLED(CONFIG_ARM64_MODULE_PLTS) &&
+			    ovf == -ERANGE) {
+				val = module_emit_plt_entry(me, &rel[i], sym);
+				ovf = reloc_insn_imm(RELOC_OP_PREL, loc, val, 2,
+						     26, AARCH64_INSN_IMM_26);
+			}
 			break;
 
 		default:

arch/arm64/kernel/module.lds

+SECTIONS {
+	.plt (NOLOAD) : { BYTE(0) }
+}