From: Josh Poimboeuf <jpoimboe@kernel.org>

Date: Mon, 18 Jul 2022 07:50:25 -0700

Subject: lkdtm: Disable return thunks in rodata.c

Git-commit: efc72a665a61fd48c462f5248a9e3dc991398ddd

Patch-mainline: v5.19-rc8

References: bsc#1114648

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@intel.com>

Debugged-by: Peter Zijlstra <peterz@infradead.org>

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Link: https://lore.kernel.org/lkml/Ys58BxHxoDZ7rfpr@xsang-OptiPlex-9020/

Acked-by: Borislav Petkov <bp@suse.de>

---

 arch/x86/Makefile     |    2 ++

 drivers/misc/Makefile |    1 +

 2 files changed, 3 insertions(+)

--- a/arch/x86/Makefile

+++ b/arch/x86/Makefile

@@ -236,6 +236,8 @@ endif

 RETHUNK_CFLAGS		:= -mfunction-return=thunk-extern

 RETPOLINE_CFLAGS	+= $(RETHUNK_CFLAGS)

+export RETHUNK_CFLAGS

+

 # for vdso Makefile to exclude

 export RETPOLINE_CFLAGS

--- a/drivers/misc/Makefile

+++ b/drivers/misc/Makefile

@@ -64,6 +64,7 @@ lkdtm-$(CONFIG_LKDTM)		+= lkdtm_rodata_o

 lkdtm-$(CONFIG_LKDTM)		+= lkdtm_usercopy.o

 KCOV_INSTRUMENT_lkdtm_rodata.o	:= n

+CFLAGS_REMOVE_lkdtm_rodata.o	+= $(RETHUNK_CFLAGS)

 OBJCOPYFLAGS :=

 OBJCOPYFLAGS_lkdtm_rodata_objcopy.o := \