arm64/efi: isolate EFI stub from the kernel proper

Since arm64 does not use a builtin decompressor, the EFI stub is built
into the kernel proper. So far, this has been working fine, but actually,
since the stub is in fact a PE/COFF relocatable binary that is executed
at an unknown offset in the 1:1 mapping provided by the UEFI firmware, we
should not be seamlessly sharing code with the kernel proper, which is a
position dependent executable linked at a high virtual offset.

So instead, separate the contents of libstub and its dependencies, by
putting them into their own namespace by prefixing all of its symbols
with __efistub. This way, we have tight control over what parts of the
kernel proper are referenced by the stub.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Matt Fleming <matt.fleming@intel.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

drivers/firmware/efi/libstub/Makefile

@@ -14,6 +14,8 @@ cflags-$(CONFIG_ARM64)		:= $(subst -pg,,$(KBUILD_CFLAGS))
 cflags-$(CONFIG_ARM)		:= $(subst -pg,,$(KBUILD_CFLAGS)) \
 				   -fno-builtin -fpic -mno-single-pic-base
 
+cflags-$(CONFIG_EFI_ARMSTUB)	+= -I$(srctree)/scripts/dtc/libfdt
+
 KBUILD_CFLAGS			:= $(cflags-y) \
 				   $(call cc-option,-ffreestanding) \
 				   $(call cc-option,-fno-stack-protector)
@@ -22,7 +24,15 @@ GCOV_PROFILE			:= n
 KASAN_SANITIZE			:= n
 
 lib-y				:= efi-stub-helper.o
-lib-$(CONFIG_EFI_ARMSTUB)	+= arm-stub.o fdt.o
+
+# include the stub's generic dependencies from lib/ when building for ARM/arm64
+arm-deps := fdt_rw.c fdt_ro.c fdt_wip.c fdt.c fdt_empty_tree.c fdt_sw.c sort.c
+
+$(obj)/lib-%.o: $(srctree)/lib/%.c FORCE
+	$(call if_changed_rule,cc_o_c)
+
+lib-$(CONFIG_EFI_ARMSTUB)	+= arm-stub.o fdt.o string.o \
+				   $(patsubst %.c,lib-%.o,$(arm-deps))
 
 #
 # arm64 puts the stub in the kernel proper, which will unnecessarily retain all
@@ -30,10 +40,27 @@ lib-$(CONFIG_EFI_ARMSTUB)	+= arm-stub.o fdt.o
 # So let's apply the __init annotations at the section level, by prefixing
 # the section names directly. This will ensure that even all the inline string
 # literals are covered.
+# The fact that the stub and the kernel proper are essentially the same binary
+# also means that we need to be extra careful to make sure that the stub does
+# not rely on any absolute symbol references, considering that the virtual
+# kernel mapping that the linker uses is not active yet when the stub is
+# executing. So build all C dependencies of the EFI stub into libstub, and do
+# a verification pass to see if any absolute relocations exist in any of the
+# object files.
 #
-extra-$(CONFIG_ARM64)		:= $(lib-y)
-lib-$(CONFIG_ARM64)		:= $(patsubst %.o,%.init.o,$(lib-y))
+extra-$(CONFIG_EFI_ARMSTUB)	:= $(lib-y)
+lib-$(CONFIG_EFI_ARMSTUB)	:= $(patsubst %.o,%.stub.o,$(lib-y))
 
-OBJCOPYFLAGS := --prefix-alloc-sections=.init
-$(obj)/%.init.o: $(obj)/%.o FORCE
-	$(call if_changed,objcopy)
+STUBCOPY_FLAGS-y		:= -R .debug* -R *ksymtab*
+STUBCOPY_FLAGS-$(CONFIG_ARM64)	+= --prefix-alloc-sections=.init \
+				   --prefix-symbols=__efistub_
+STUBCOPY_RELOC-$(CONFIG_ARM64)	:= R_AARCH64_ABS
+
+$(obj)/%.stub.o: $(obj)/%.o FORCE
+	$(call if_changed,stubcopy)
+
+quiet_cmd_stubcopy = STUBCPY $@
+      cmd_stubcopy = if $(OBJCOPY) $(STUBCOPY_FLAGS-y) $< $@; then	\
+		     $(OBJDUMP) -r $@ | grep $(STUBCOPY_RELOC-y)	\
+		     && (echo >&2 "$@: absolute symbol references not allowed in the EFI stub"; \
+			 rm -f $@; /bin/false); else /bin/false; fi

drivers/firmware/efi/libstub/string.c

@@ -0,0 +1,57 @@
+/*
+ * Taken from:
+ *  linux/lib/string.c
+ *
+ *  Copyright (C) 1991, 1992  Linus Torvalds
+ */
+
+#include <linux/types.h>
+#include <linux/string.h>
+
+#ifndef __HAVE_ARCH_STRSTR
+/**
+ * strstr - Find the first substring in a %NUL terminated string
+ * @s1: The string to be searched
+ * @s2: The string to search for
+ */
+char *strstr(const char *s1, const char *s2)
+{
+	size_t l1, l2;
+
+	l2 = strlen(s2);
+	if (!l2)
+		return (char *)s1;
+	l1 = strlen(s1);
+	while (l1 >= l2) {
+		l1--;
+		if (!memcmp(s1, s2, l2))
+			return (char *)s1;
+		s1++;
+	}
+	return NULL;
+}
+#endif
+
+#ifndef __HAVE_ARCH_STRNCMP
+/**
+ * strncmp - Compare two length-limited strings
+ * @cs: One string
+ * @ct: Another string
+ * @count: The maximum number of bytes to compare
+ */
+int strncmp(const char *cs, const char *ct, size_t count)
+{
+	unsigned char c1, c2;
+
+	while (count) {
+		c1 = *cs++;
+		c2 = *ct++;
+		if (c1 != c2)
+			return c1 < c2 ? -1 : 1;
+		if (!c1)
+			break;
+		count--;
+	}
+	return 0;
+}
+#endif