[PATCH 2/2] arm64/efi: isolate EFI stub from the kernel proper
Ard Biesheuvel
ard.biesheuvel at linaro.org
Thu Sep 24 09:37:16 PDT 2015
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 at linaro.org>
---
arch/arm64/kernel/Makefile | 12 ++++-
arch/arm64/kernel/efi-entry.S | 10 ++--
arch/arm64/kernel/head.S | 14 ++---
arch/arm64/kernel/image.h | 27 ++++++++++
drivers/firmware/efi/libstub/Makefile | 31 +++++++++--
drivers/firmware/efi/libstub/string.c | 55 ++++++++++++++++++++
6 files changed, 132 insertions(+), 17 deletions(-)
diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
index 22dc9bc781be..7b17f6245f1e 100644
--- a/arch/arm64/kernel/Makefile
+++ b/arch/arm64/kernel/Makefile
@@ -20,6 +20,14 @@ arm64-obj-y := debug-monitors.o entry.o irq.o fpsimd.o \
cpufeature.o alternative.o cacheinfo.o \
smp.o smp_spin_table.o topology.o
+stub-obj := efi-stub.o efi-entry.o
+extra-y := $(stub-obj)
+stub-obj := $(patsubst %.o,%.stub.o,$(stub-obj))
+
+OBJCOPYFLAGS := --prefix-symbols=__efistub_
+$(obj)/%.stub.o: $(obj)/%.o FORCE
+ $(call if_changed,objcopy)
+
arm64-obj-$(CONFIG_COMPAT) += sys32.o kuser32.o signal32.o \
sys_compat.o entry32.o \
../../arm/kernel/opcodes.o
@@ -32,7 +40,7 @@ arm64-obj-$(CONFIG_CPU_PM) += sleep.o suspend.o
arm64-obj-$(CONFIG_CPU_IDLE) += cpuidle.o
arm64-obj-$(CONFIG_JUMP_LABEL) += jump_label.o
arm64-obj-$(CONFIG_KGDB) += kgdb.o
-arm64-obj-$(CONFIG_EFI) += efi.o efi-stub.o efi-entry.o
+arm64-obj-$(CONFIG_EFI) += efi.o $(stub-obj)
arm64-obj-$(CONFIG_PCI) += pci.o
arm64-obj-$(CONFIG_ARMV8_DEPRECATED) += armv8_deprecated.o
arm64-obj-$(CONFIG_ACPI) += acpi.o
@@ -40,7 +48,7 @@ arm64-obj-$(CONFIG_ACPI) += acpi.o
obj-y += $(arm64-obj-y) vdso/
obj-m += $(arm64-obj-m)
head-y := head.o
-extra-y := $(head-y) vmlinux.lds
+extra-y += $(head-y) vmlinux.lds
# vDSO - this must be built first to generate the symbol offsets
$(call objectify,$(arm64-obj-y)): $(obj)/vdso/vdso-offsets.h
diff --git a/arch/arm64/kernel/efi-entry.S b/arch/arm64/kernel/efi-entry.S
index 8ce9b0577442..a773db92908b 100644
--- a/arch/arm64/kernel/efi-entry.S
+++ b/arch/arm64/kernel/efi-entry.S
@@ -29,7 +29,7 @@
* we want to be. The kernel image wants to be placed at TEXT_OFFSET
* from start of RAM.
*/
-ENTRY(efi_stub_entry)
+ENTRY(entry)
/*
* Create a stack frame to save FP/LR with extra space
* for image_addr variable passed to efi_entry().
@@ -86,8 +86,8 @@ ENTRY(efi_stub_entry)
* entries for the VA range of the current image, so no maintenance is
* necessary.
*/
- adr x0, efi_stub_entry
- adr x1, efi_stub_entry_end
+ adr x0, entry
+ adr x1, entry_end
sub x1, x1, x0
bl __flush_dcache_area
@@ -120,5 +120,5 @@ efi_load_fail:
ldp x29, x30, [sp], #32
ret
-efi_stub_entry_end:
-ENDPROC(efi_stub_entry)
+entry_end:
+ENDPROC(entry)
diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index a055be6125cf..e8f52943ac65 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -120,8 +120,8 @@ efi_head:
#endif
#ifdef CONFIG_EFI
- .globl stext_offset
- .set stext_offset, stext - efi_head
+ .globl __efistub_stext_offset
+ .set __efistub_stext_offset, stext - efi_head
.align 3
pe_header:
.ascii "PE"
@@ -144,8 +144,8 @@ optional_header:
.long _end - stext // SizeOfCode
.long 0 // SizeOfInitializedData
.long 0 // SizeOfUninitializedData
- .long efi_stub_entry - efi_head // AddressOfEntryPoint
- .long stext_offset // BaseOfCode
+ .long __efistub_entry - efi_head // AddressOfEntryPoint
+ .long __efistub_stext_offset // BaseOfCode
extra_header_fields:
.quad 0 // ImageBase
@@ -162,7 +162,7 @@ extra_header_fields:
.long _end - efi_head // SizeOfImage
// Everything before the kernel image is considered part of the header
- .long stext_offset // SizeOfHeaders
+ .long __efistub_stext_offset // SizeOfHeaders
.long 0 // CheckSum
.short 0xa // Subsystem (EFI application)
.short 0 // DllCharacteristics
@@ -207,9 +207,9 @@ section_table:
.byte 0
.byte 0 // end of 0 padding of section name
.long _end - stext // VirtualSize
- .long stext_offset // VirtualAddress
+ .long __efistub_stext_offset // VirtualAddress
.long _edata - stext // SizeOfRawData
- .long stext_offset // PointerToRawData
+ .long __efistub_stext_offset // PointerToRawData
.long 0 // PointerToRelocations (0 for executables)
.long 0 // PointerToLineNumbers (0 for executables)
diff --git a/arch/arm64/kernel/image.h b/arch/arm64/kernel/image.h
index 8fae0756e175..2f6a426de864 100644
--- a/arch/arm64/kernel/image.h
+++ b/arch/arm64/kernel/image.h
@@ -59,4 +59,31 @@
_kernel_offset_le = DATA_LE64(TEXT_OFFSET); \
_kernel_flags_le = DATA_LE64(__HEAD_FLAGS);
+#ifdef CONFIG_EFI
+
+/*
+ * The EFI stub has its own symbol namespace prefixed by __efistub_, to
+ * isolate it from the kernel proper. The following symbols are legally
+ * accessed by the stub, so provide some aliases to make them accessible.
+ * Only include data symbols here, or text symbols of functions that are
+ * guaranteed to be safe when executed at another offset than they were
+ * linked at. The routines below are all implemented in assembler in a
+ * position independent manner
+ */
+__efistub_memcmp = __pi_memcmp;
+__efistub_memchr = __pi_memchr;
+__efistub_memcpy = __pi_memcpy;
+__efistub_memmove = __pi_memmove;
+__efistub_memset = __pi_memset;
+__efistub_strlen = __pi_strlen;
+__efistub_strncmp = __pi_strncmp;
+__efistub___flush_dcache_area = __pi___flush_dcache_area;
+
+__efistub_linux_banner = linux_banner;
+__efistub__text = _text;
+__efistub__end = _end;
+__efistub__edata = _edata;
+
+#endif
+
#endif /* __ASM_IMAGE_H */
diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
index 816dbe9f4b82..50557f4d12db 100644
--- a/drivers/firmware/efi/libstub/Makefile
+++ b/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 +40,25 @@ 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))
-OBJCOPYFLAGS := --prefix-alloc-sections=.init
+OBJCOPYFLAGS := --prefix-alloc-sections=.init --prefix-symbols=__efistub_ \
+ -R .debug* -R *ksymtab*
+
$(obj)/%.init.o: $(obj)/%.o FORCE
- $(call if_changed,objcopy)
+ $(call if_changed,stubcopy)
+
+quiet_cmd_stubcopy = STUBCPY $@
+ cmd_stubcopy = if $(OBJCOPY) $(OBJCOPYFLAGS) $< $@; then \
+ $(OBJDUMP) -r $@ | grep R_AARCH64_ABS \
+ && (echo >&2 "$@: absolute symbol references not allowed in the EFI stub"; \
+ rm -f $@; /bin/false); else /bin/false; fi
diff --git a/drivers/firmware/efi/libstub/string.c b/drivers/firmware/efi/libstub/string.c
new file mode 100644
index 000000000000..a5e2af8a3018
--- /dev/null
+++ b/drivers/firmware/efi/libstub/string.c
@@ -0,0 +1,55 @@
+/*
+ * Taken from:
+ * linux/lib/string.c
+ *
+ * Copyright (C) 1991, 1992 Linus Torvalds
+ */
+
+#include <linux/types.h>
+#include <linux/string.h>
+
+/**
+ * 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;
+}
+
+#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
--
1.9.1
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