linux-kernel - [PATCH 4.9 026/101] x86: link vdso and boot with -z noexecstack --no-warn-rwx-segments

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Message-Id: <20220823080035.574198870@linuxfoundation.org>
Date:   Tue, 23 Aug 2022 10:02:59 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Fangrui Song <maskray@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>
Subject: [PATCH 4.9 026/101] x86: link vdso and boot with -z noexecstack --no-warn-rwx-segments

From: Nick Desaulniers <ndesaulniers@...gle.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@...nel.dk>
Suggested-by: Fangrui Song <maskray@...gle.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.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
@@ -100,7 +100,7 @@ $(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
@@ -54,6 +54,10 @@ else
 LDFLAGS += $(shell $(LD) --help 2>&1 | grep -q "\-z noreloc-overflow" \
 	&& echo "-z noreloc-overflow -pie --no-dynamic-linker")
 endif
+
+LDFLAGS += -z noexecstack
+LDFLAGS += $(call ld-option,--no-warn-rwx-segments)
+
 LDFLAGS_vmlinux := -T
 
 hostprogs-y	:= mkpiggy
--- a/arch/x86/entry/vdso/Makefile
+++ b/arch/x86/entry/vdso/Makefile
@@ -168,7 +168,7 @@ quiet_cmd_vdso = VDSO    $@
 
 VDSO_LDFLAGS = -shared $(call ld-option, --hash-style=both) \
 	$(call ld-option, --build-id) $(call ld-option, --eh-frame-hdr) \
-	-Bsymbolic
+	-Bsymbolic -z noexecstack
 GCOV_PROFILE := n
 
 #