Re: [tip: x86/bugs] x86/retpoline: Ensure default return thunk isn't used at runtime

[Josh Poimboeuf]

On Tue, Oct 17, 2023 at 04:31:09AM +0000, Kaplan, David wrote:
> I think I found the problem, although I'm not sure the best way to fix it.
> 
> When KCSAN is enabled, GCC generates lots of constructor functions named _sub_I_00099_0 which call __tsan_init and then return.  The returns in these are generally annotated normally by objtool and fixed up at runtime.  But objtool runs on vmlinux.o and vmlinux.o does not include a couple of object files that are in vmlinux, like init/version-timestamp.o and .vmlinux.export.o, both of which contain _sub_I_00099_0 functions.  As a result, the returns in these functions are not annotated, and the panic occurs when we call one of them in do_ctors and it uses the default return thunk.
> 
> This difference can be seen by counting the number of these functions in the object files:
> $ objdump -d vmlinux.o|grep -c "<_sub_I_00099_0>:"
> 2601
> $ objdump -d vmlinux|grep -c "<_sub_I_00099_0>:"
> 2603
> 
> If these functions are only run during kernel boot, there is no speculation concern.  My first thought is that these two object files perhaps should be built without -mfunction-return=thunk-extern.  The use of that flag requires objtool to have the intended behavior and objtool isn't seeing these files.
> 
> Perhaps another option would be to not compile these two files with KCSAN, as they are already excluded from KASAN and GCOV it looks like.

I think the latter would be the easy fix, does this make it go away?

diff --git a/init/Makefile b/init/Makefile
index ec557ada3c12..cbac576c57d6 100644
--- a/init/Makefile
+++ b/init/Makefile
@@ -60,4 +60,5 @@ include/generated/utsversion.h: FORCE
 $(obj)/version-timestamp.o: include/generated/utsversion.h
 CFLAGS_version-timestamp.o := -include include/generated/utsversion.h
 KASAN_SANITIZE_version-timestamp.o := n
+KCSAN_SANITIZE_version-timestamp.o := n
 GCOV_PROFILE_version-timestamp.o := n
diff --git a/scripts/Makefile.vmlinux b/scripts/Makefile.vmlinux
index 3cd6ca15f390..c9f3e03124d7 100644
--- a/scripts/Makefile.vmlinux
+++ b/scripts/Makefile.vmlinux
@@ -19,6 +19,7 @@ quiet_cmd_cc_o_c = CC      $@
 
 ifdef CONFIG_MODULES
 KASAN_SANITIZE_.vmlinux.export.o := n
+KCSAN_SANITIZE_.vmlinux.export.o := n
 GCOV_PROFILE_.vmlinux.export.o := n
 targets += .vmlinux.export.o
 vmlinux: .vmlinux.export.o