[PATCH] x86/srso: Fix panic in return thunk during boot

[Josh Poimboeuf]

Enabling CONFIG_KCSAN causes a panic during boot due to an "invalid
opcode" in __x86_return_thunk():

  invalid opcode: 0000 [#1] PREEMPT SMP NOPTI
  CPU: 0 PID: 1 Comm: swapper/0 Not tainted 6.6.0-rc2-00316-g91174087dcc7 #1
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.2-14-g1e1da7a96300-prebuilt.qemu.org 04/01/2014
  RIP: 0010:__x86_return_thunk+0x0/0x10
  Code: e8 01 00 00 00 cc e8 01 00 00 00 cc 48 81 c4 80 00 00 00 65 48 c7 04 25 d0 ac 02 00 ff ff ff ff c3 cc 0f 1f 84 00 00 00 00 00 <0f> 0b cc cc cc cc cc cc cc cc cc cc cc cc cc cc e9 db 8c 8e fe 0f
  RSP: 0018:ffffaef1c0013ed0 EFLAGS: 00010246
  RAX: ffffffffa0e80eb0 RBX: ffffffffa0f05240 RCX: 0001ffffffffffff
  RDX: 0000000000000551 RSI: ffffffffa0dcc64e RDI: ffffffffa0f05238
  RBP: ffff8f93c11708e0 R08: ffffffffa1387280 R09: 0000000000000000
  R10: 0000000000000282 R11: 0001ffffa0f05238 R12: 0000000000000002
  R13: 0000000000000282 R14: 0000000000000001 R15: 0000000000000000
  FS:  0000000000000000(0000) GS:ffff8f93df000000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: ffff8f93d6c01000 CR3: 0000000015c2e000 CR4: 0000000000350ef0

The panic is triggered by the UD2 instruction which gets patched into
__x86_return_thunk() when alternatives are applied.  After that point,
the default return thunk should no longer be used.

As David Kaplan describes, the issue 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.

Fixes: 91174087dcc7 ("x86/retpoline: Ensure default return thunk isn't used at runtime")
Reported-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Closes: https://lore.kernel.org/lkml/20231016214810.GA3942238@dev-arch.thelio-3990X/
Debugged-by: David Kaplan <David.Kaplan@xxxxxxx>
Tested-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Acked-by: Marco Elver <elver@xxxxxxxxxx>
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 init/Makefile            | 1 +
 scripts/Makefile.vmlinux | 1 +
 2 files changed, 2 insertions(+)

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
-- 
2.41.0