linux-kernel - [PATCH 4.9 025/101] Makefile: link with -z noexecstack --no-warn-rwx-segments

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Message-Id: <20220823080035.534613756@linuxfoundation.org>
Date:   Tue, 23 Aug 2022 10:02:58 +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 4.9 025/101] Makefile: link with -z noexecstack --no-warn-rwx-segments

From: Nick Desaulniers <ndesaulniers@...gle.com>

commit 0d362be5b14200b77ecc2127936a5ff82fbffe41 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: vmlinux: 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: 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>
---
 Makefile |    3 +++
 1 file changed, 3 insertions(+)

--- a/Makefile
+++ b/Makefile
@@ -870,6 +870,9 @@ ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATI
 LDFLAGS_vmlinux	+= $(call ld-option, --gc-sections,)
 endif
 
+LDFLAGS	+= -z noexecstack
+LDFLAGS	+= $(call ld-option,--no-warn-rwx-segments)
+
 ifeq ($(CONFIG_STRIP_ASM_SYMS),y)
 LDFLAGS_vmlinux	+= $(call ld-option, -X,)
 endif