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x86/retpoline: Make sure there are no unconverted return thunks due to KCSAN

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Enabling CONFIG_KCSAN leads to unconverted, default return thunks to
remain after patching.

As David Kaplan describes in his debugging of the issue, it is caused by
a couple of KCSAN-generated constructors which aren't processed by
objtool:

  "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."

Fix it by disabling KCSAN on version-timestamp.o and .vmlinux.export.o
so the extra functions don't get generated.  KASAN and GCOV are already
disabled for those files.

  [ bp: Massage commit message. ]

Closes: https://lore.kernel.org/lkml/20231016214810.GA3942238@dev-arch.thelio-3990X/
Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: Marco Elver <elver@google.com>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20231017165946.v4i2d4exyqwqq3bx@treble
This commit is contained in:
Josh Poimboeuf 2023-10-17 09:59:46 -07:00 committed by Borislav Petkov (AMD)
parent 321a145137
commit 2d7ce49f58
2 changed files with 2 additions and 0 deletions
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1
init/Makefile
View File

@ -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

1
scripts/Makefile.vmlinux
View File

@ -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