linux-kernel - [PATCH] lkdtm: Disable return thunks in rodata.c

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Date:   Mon, 18 Jul 2022 07:50:25 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        kernel test robot <oliver.sang@...el.com>
Subject: [PATCH] lkdtm: Disable return thunks in rodata.c

The following warning was seen:

  WARNING: CPU: 0 PID: 0 at arch/x86/kernel/alternative.c:557 apply_returns (arch/x86/kernel/alternative.c:557 (discriminator 1))
  Modules linked in:
  CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.19.0-rc4-00008-gee88d363d156 #1
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.0-debian-1.16.0-4 04/01/2014
  RIP: 0010:apply_returns (arch/x86/kernel/alternative.c:557 (discriminator 1))
  Code: ff ff 74 cb 48 83 c5 04 49 39 ee 0f 87 81 fe ff ff e9 22 ff ff ff 0f 0b 48 83 c5 04 49 39 ee 0f 87 6d fe ff ff e9 0e ff ff ff <0f> 0b 48 83 c5 04 49 39 ee 0f 87 59 fe ff ff e9 fa fe ff ff 48 89

The warning happened when apply_returns() failed to convert "JMP
__x86_return_thunk" to RET.  It was instead a JMP to nowhere, due to the
thunk relocation not getting resolved.

That rodata.o code is objcopy'd to .rodata, and later memcpy'd, so
relocations don't work (and are apparently silently ignored).

LKDTM is only used for testing, so the naked RET should be fine.  So
just disable return thunks for that file.

While at it, disable objtool and KCSAN for the file.

Fixes: 0b53c374b9ef ("x86/retpoline: Use -mfunction-return")
Reported-by: kernel test robot <oliver.sang@...el.com>
Link: https://lore.kernel.org/lkml/Ys58BxHxoDZ7rfpr@xsang-OptiPlex-9020/
Debugged-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
---
 arch/x86/Makefile           | 1 +
 drivers/misc/lkdtm/Makefile | 9 ++++++---
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 1f40dad30d50..7854685c5f25 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -27,6 +27,7 @@ RETHUNK_CFLAGS		:= -mfunction-return=thunk-extern
 RETPOLINE_CFLAGS	+= $(RETHUNK_CFLAGS)
 endif
 
+export RETHUNK_CFLAGS
 export RETPOLINE_CFLAGS
 export RETPOLINE_VDSO_CFLAGS
 
diff --git a/drivers/misc/lkdtm/Makefile b/drivers/misc/lkdtm/Makefile
index 2e0aa74ac185..95ef971b5e1c 100644
--- a/drivers/misc/lkdtm/Makefile
+++ b/drivers/misc/lkdtm/Makefile
@@ -13,10 +13,13 @@ lkdtm-$(CONFIG_LKDTM)		+= cfi.o
 lkdtm-$(CONFIG_LKDTM)		+= fortify.o
 lkdtm-$(CONFIG_PPC_64S_HASH_MMU)	+= powerpc.o
 
-KASAN_SANITIZE_rodata.o		:= n
 KASAN_SANITIZE_stackleak.o	:= n
-KCOV_INSTRUMENT_rodata.o	:= n
-CFLAGS_REMOVE_rodata.o		+= $(CC_FLAGS_LTO)
+
+KASAN_SANITIZE_rodata.o			:= n
+KCSAN_SANITIZE_rodata.o			:= n
+KCOV_INSTRUMENT_rodata.o		:= n
+OBJECT_FILES_NON_STANDARD_rodata.o	:= y
+CFLAGS_REMOVE_rodata.o			+= $(CC_FLAGS_LTO) $(RETHUNK_CFLAGS)
 
 OBJCOPYFLAGS :=
 OBJCOPYFLAGS_rodata_objcopy.o	:= \
-- 
2.36.1