On Mon, Jun 29, 2020 at 10:15:47AM +0200, Ard Biesheuvel wrote:

On Mon, 29 Jun 2020 at 08:18, Kees Cook [off-list ref] wrote:

quoted

In preparation for building efi/libstub with -mbranch-protection=none
(EFI does not support branch protection features[1]), add no-op code
to work around a Clang bug that emits an unwanted .note.gnu.property
section for object files without code[2].

[1] https://lore.kernel.org/lkml/CAMj1kXHck12juGi=E=P4hWP_8vQhQ+-x3vBMc3TGeRWdQ-XkxQ@mail.gmail.com (local)
[2] https://bugs.llvm.org/show_bug.cgi?id=46480

Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Will Deacon <will@kernel.org>
Cc: Dave Martin <Dave.Martin@arm.com>
Cc: clang-built-linux@googlegroups.com
Signed-off-by: Kees Cook <redacted>
---
 lib/ctype.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/lib/ctype.c b/lib/ctype.c
index c819fe269eb2..21245ed57d90 100644
--- a/lib/ctype.c
+++ b/lib/ctype.c

@@ -36,3 +36,13 @@ _L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,     /* 224-239 */
 _L,_L,_L,_L,_L,_L,_L,_P,_L,_L,_L,_L,_L,_L,_L,_L};      /* 240-255 */

 EXPORT_SYMBOL(_ctype);
+
+/*
+ * Clang will generate .note.gnu.property sections for object files
+ * without code, even in the presence of -mbranch-protection=none.
+ * To work around this, define an unused static function.
+ * https://bugs.llvm.org/show_bug.cgi?id=46480
+ */
+#ifdef CONFIG_CC_IS_CLANG
+void __maybe_unused __clang_needs_code_here(void) { }
+#endif
--

2.25.1

I take it we don't need this horrible hack if we build the EFI stub
with branch protections and filter out the .note.gnu.property section
explicitly?

Sorry to backpedal, but that is probably a better approach after all,
given that the instructions don't hurt, and we will hopefully be able
to arm them once UEFI (as well as PE/COFF) gets around to describing
this in a way that both the firmware and the OS can consume.

How does this look?


commit 051ef0b75a386c3fe2f216d16246468147a48c5b
Author: Kees Cook [off-list ref]
Date:   Tue Jun 23 18:02:56 2020 -0700

    efi/libstub: Disable -mbranch-protection
    
    In preparation for adding --orphan-handling=warn to more architectures,
    disable -mbranch-protection, as EFI does not yet support it[1].  This was
    noticed due to it producing unwanted .note.gnu.property sections (prefixed
    with .init due to the objcopy build step).
    
    However, we must also work around a bug in Clang where the section is
    still emitted for code-less object files[2], so also remove the section
    during the objcopy.
    
    [1] https://lore.kernel.org/lkml/CAMj1kXHck12juGi=E=P4hWP_8vQhQ+-x3vBMc3TGeRWdQ-XkxQ@mail.gmail.com (local)
    [2] https://bugs.llvm.org/show_bug.cgi?id=46480
    
    Cc: Ard Biesheuvel [off-list ref]
    Cc: Arvind Sankar [off-list ref]
    Cc: Atish Patra [off-list ref]
    Cc: linux-efi@vger.kernel.org
    Signed-off-by: Kees Cook [off-list ref]

diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
index 75daaf20374e..f9f1922f8f28 100644
--- a/drivers/firmware/efi/libstub/Makefile
+++ b/drivers/firmware/efi/libstub/Makefile

@@ -18,7 +18,8 @@ cflags-$(CONFIG_X86)		+= -m$(BITS) -D__KERNEL__ \
 # arm64 uses the full KBUILD_CFLAGS so it's necessary to explicitly
 # disable the stackleak plugin
 cflags-$(CONFIG_ARM64)		:= $(subst $(CC_FLAGS_FTRACE),,$(KBUILD_CFLAGS)) \
-				   -fpie $(DISABLE_STACKLEAK_PLUGIN)
+				   -fpie $(DISABLE_STACKLEAK_PLUGIN) \
+				   $(call cc-option,-mbranch-protection=none)
 cflags-$(CONFIG_ARM)		:= $(subst $(CC_FLAGS_FTRACE),,$(KBUILD_CFLAGS)) \
 				   -fno-builtin -fpic \
 				   $(call cc-option,-mno-single-pic-base)

@@ -66,6 +67,12 @@ lib-$(CONFIG_X86)		+= x86-stub.o
 CFLAGS_arm32-stub.o		:= -DTEXT_OFFSET=$(TEXT_OFFSET)
 CFLAGS_arm64-stub.o		:= -DTEXT_OFFSET=$(TEXT_OFFSET)
 
+# Even when -mbranch-protection=none is set, Clang will generate a
+# .note.gnu.property for code-less object files (like lib/ctype.c),
+# so work around this by explicitly removing the unwanted section.
+# https://bugs.llvm.org/show_bug.cgi?id=46480
+STUBCOPY_FLAGS-y		+= --remove-section=.note.gnu.property
+
 #
 # For x86, bootloaders like systemd-boot or grub-efi do not zero-initialize the
 # .bss section, so the .bss section of the EFI stub needs to be included in the

-- 
Kees Cook

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