[PATCH 5.19 0002/1157] x86: link vdso and boot with -z noexecstack --no-warn-rwx-segments

Greg Kroah-Hartman
2022-08-15T21:45:19Z

From: Nick Desaulniers <ndesaulniers@google.com>

commit ffcf9c5700e49c0aee42dcba9a12ba21338e8136 upstream.

Users of GNU ld (BFD) from binutils 2.39+ will observe multiple
instances of a new warning when linking kernels in the form:

ld: warning: arch/x86/boot/pmjump.o: missing .note.GNU-stack section implies executable stack
ld: NOTE: This behaviour is deprecated and will be removed in a future version of the linker
ld: warning: arch/x86/boot/compressed/vmlinux has a LOAD segment with RWX permissions

Generally, we would like to avoid the stack being executable. Because
there could be a need for the stack to be executable, assembler sources
have to opt-in to this security feature via explicit creation of the
.note.GNU-stack feature (which compilers create by default) or command
line flag --noexecstack. Or we can simply tell the linker the
production of such sections is irrelevant and to link the stack as
--noexecstack.

LLVM's LLD linker defaults to -z noexecstack, so this flag isn't
strictly necessary when linking with LLD, only BFD, but it doesn't hurt
to be explicit here for all linkers IMO. --no-warn-rwx-segments is
currently BFD specific and only available in the current latest release,
so it's wrapped in an ld-option check.

While the kernel makes extensive usage of ELF sections, it doesn't use
permissions from ELF segments.

Link: https://lore.kernel.org/linux-block/3af4127a-f453-4cf7-f133-a181cce06f73@kernel.dk/
Link: https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=ba951afb99912da01a6e8434126b8fac7aa75107
Link: https://github.com/llvm/llvm-project/issues/57009
Reported-and-tested-by: Jens Axboe <axboe@kernel.dk>
Suggested-by: Fangrui Song <maskray@google.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/boot/Makefile | 2 +-
arch/x86/boot/compressed/Makefile | 4 ++++
arch/x86/entry/vdso/Makefile | 2 +-
3 files changed, 6 insertions(+), 2 deletions(-)

--- a/arch/x86/boot/Makefile
+++ b/arch/x86/boot/Makefile
@@ -103,7 +103,7 @@ $(obj)/zoffset.h: $(obj)/compressed/vmli
AFLAGS_header.o += -I$(objtree)/$(obj)
$(obj)/header.o: $(obj)/zoffset.h

-LDFLAGS_setup.elf := -m elf_i386 -T
+LDFLAGS_setup.elf := -m elf_i386 -z noexecstack -T
$(obj)/setup.elf: $(src)/setup.ld $(SETUP_OBJS) FORCE
$(call if_changed,ld)

--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -69,6 +69,10 @@ LDFLAGS_vmlinux := -pie $(call ld-option
ifdef CONFIG_LD_ORPHAN_WARN
LDFLAGS_vmlinux += --orphan-handling=warn
endif
+LDFLAGS_vmlinux += -z noexecstack
+ifeq ($(CONFIG_LD_IS_BFD),y)
+LDFLAGS_vmlinux += $(call ld-option,--no-warn-rwx-segments)
+endif
LDFLAGS_vmlinux += -T

hostprogs := mkpiggy
--- a/arch/x86/entry/vdso/Makefile
+++ b/arch/x86/entry/vdso/Makefile
@@ -180,7 +180,7 @@ quiet_cmd_vdso = VDSO $@
sh $(srctree)/$(src)/checkundef.sh '$(NM)' '$@'

VDSO_LDFLAGS = -shared --hash-style=both --build-id=sha1 \
- $(call ld-option, --eh-frame-hdr) -Bsymbolic
+ $(call ld-option, --eh-frame-hdr) -Bsymbolic -z noexecstack
GCOV_PROFILE := n

quiet_cmd_vdso_and_check = VDSO $@