From 54babdc0343fff2f32dfaafaaa9e42c4db278204 Mon Sep 17 00:00:00 2001 From: "Borislav Petkov (AMD)" Date: Tue, 26 Mar 2024 21:25:48 +0100 Subject: [PATCH] kbuild: Disable KCSAN for autogenerated *.mod.c intermediaries When KCSAN and CONSTRUCTORS are enabled, one can trigger the "Unpatched return thunk in use. This should not happen!" catch-all warning. Usually, when objtool runs on the .o objects, it does generate a section .return_sites which contains all offsets in the objects to the return thunks of the functions present there. Those return thunks then get patched at runtime by the alternatives. KCSAN and CONSTRUCTORS add this to the object file's .text.startup section: ------------------- Disassembly of section .text.startup: ... 0000000000000010 <_sub_I_00099_0>: 10: f3 0f 1e fa endbr64 14: e8 00 00 00 00 call 19 <_sub_I_00099_0+0x9> 15: R_X86_64_PLT32 __tsan_init-0x4 19: e9 00 00 00 00 jmp 1e <__UNIQUE_ID___addressable_cryptd_alloc_aead349+0x6> 1a: R_X86_64_PLT32 __x86_return_thunk-0x4 ------------------- which, if it is built as a module goes through the intermediary stage of creating a .mod.c file which, when translated, receives a second constructor: ------------------- Disassembly of section .text.startup: 0000000000000010 <_sub_I_00099_0>: 10: f3 0f 1e fa endbr64 14: e8 00 00 00 00 call 19 <_sub_I_00099_0+0x9> 15: R_X86_64_PLT32 __tsan_init-0x4 19: e9 00 00 00 00 jmp 1e <_sub_I_00099_0+0xe> 1a: R_X86_64_PLT32 __x86_return_thunk-0x4 ... 0000000000000030 <_sub_I_00099_0>: 30: f3 0f 1e fa endbr64 34: e8 00 00 00 00 call 39 <_sub_I_00099_0+0x9> 35: R_X86_64_PLT32 __tsan_init-0x4 39: e9 00 00 00 00 jmp 3e <__ksymtab_cryptd_alloc_ahash+0x2> 3a: R_X86_64_PLT32 __x86_return_thunk-0x4 ------------------- in the .ko file. Objtool has run already so that second constructor's return thunk cannot be added to the .return_sites section and thus the return thunk remains unpatched and the warning rightfully fires. Drop KCSAN flags from the mod.c generation stage as those constructors do not contain data races one would be interested about. Debugged together with David Kaplan and Nikolay Borisov . Reported-by: Paul Menzel Closes: https://lore.kernel.org/r/0851a207-7143-417e-be31-8bf2b3afb57d@molgen.mpg.de Signed-off-by: Borislav Petkov (AMD) Tested-by: Paul Menzel # Dell XPS 13 Reviewed-by: Nikolay Borisov Reviewed-by: Marco Elver Signed-off-by: Masahiro Yamada --- scripts/Makefile.modfinal | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal index 8568d256d6fb..79fcf2731686 100644 --- a/scripts/Makefile.modfinal +++ b/scripts/Makefile.modfinal @@ -23,7 +23,7 @@ modname = $(notdir $(@:.mod.o=)) part-of-module = y quiet_cmd_cc_o_c = CC [M] $@ - cmd_cc_o_c = $(CC) $(filter-out $(CC_FLAGS_CFI) $(CFLAGS_GCOV), $(c_flags)) -c -o $@ $< + cmd_cc_o_c = $(CC) $(filter-out $(CC_FLAGS_CFI) $(CFLAGS_GCOV) $(CFLAGS_KCSAN), $(c_flags)) -c -o $@ $< %.mod.o: %.mod.c FORCE $(call if_changed_dep,cc_o_c)