linux-kernel - [PATCH v2 2/2] x86: link vdso and boot with -z noexecstack --no-warn-rwx-segments

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Date:   Wed, 10 Aug 2022 15:24:41 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Masahiro Yamada <masahiroy@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Fangrui Song <maskray@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Nick Clifton <nickc@...hat.com>, axboe@...nel.dk,
        brijesh.singh@....com, hpa@...or.com,
        kirill.shutemov@...ux.intel.com, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, michael.roth@....com, n.schier@....de,
        nathan@...nel.org, sathyanarayanan.kuppuswamy@...ux.intel.com,
        trix@...hat.com, x86@...nel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Andy Lutomirski <luto@...nel.org>
Subject: [PATCH v2 2/2] x86: link vdso and boot with -z noexecstack --no-warn-rwx-segments

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-by: Jens Axboe <axboe@...nel.dk>
Suggested-by: Fangrui Song <maskray@...gle.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
---
 arch/x86/boot/Makefile            | 2 +-
 arch/x86/boot/compressed/Makefile | 4 ++++
 arch/x86/entry/vdso/Makefile      | 2 +-
 3 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/boot/Makefile b/arch/x86/boot/Makefile
index b5aecb524a8a..ffec8bb01ba8 100644
--- a/arch/x86/boot/Makefile
+++ b/arch/x86/boot/Makefile
@@ -103,7 +103,7 @@ $(obj)/zoffset.h: $(obj)/compressed/vmlinux FORCE
 AFLAGS_header.o += -I$(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)
 
diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
index 19e1905dcbf6..35ce1a64068b 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -69,6 +69,10 @@ LDFLAGS_vmlinux := -pie $(call ld-option, --no-dynamic-linker)
 ifdef CONFIG_LD_ORPHAN_WARN
 LDFLAGS_vmlinux += --orphan-handling=warn
 endif
+LDFLAGS_vmlinux += -z noexecstack
+ifeq ($(CONFIG_LD_IS_BFD),y)
+LDFLAGS_vmlinux += $(call ld-option,--no-warn-rwx-segments)
+endif
 LDFLAGS_vmlinux += -T
 
 hostprogs	:= mkpiggy
diff --git a/arch/x86/entry/vdso/Makefile b/arch/x86/entry/vdso/Makefile
index 76cd790ed0bd..12f6c4d714cd 100644
--- 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    $@
-- 
2.37.1.559.g78731f0fdb-goog