[PATCH] x86/cfi: Fix FineIBT hash offset in cfi_get_func_hash()

[Soheil Hassas Yeganeh]

The switch of the FineIBT preamble from "subl $hash, %r10d" to the
shorter "subl $hash, %eax" moved the hash immediate from offset 7 to
offset 5 of the preamble. fineibt_preamble_hash was updated to match,
but the open-coded offset in cfi_get_func_hash() was missed and it
still reads the hash at offset 7.

cfi_get_func_hash() is used by the BPF JIT to give a struct_ops
trampoline the CFI hash of the stub function it stands in for. With
FineIBT the trampoline now gets the upper half of the real hash
followed by the first two bytes of the next instruction, so the first
indirect call from the kernel into a struct_ops program,
tcp_init_congestion_control() calling ->init() of a BPF congestion
control for example, fails the FineIBT check and the kernel dies with
a CFI failure.

Move the FineIBT preamble template and its offset defines above
cfi_get_func_hash() and use fineibt_preamble_hash there, so every
reader of the preamble shares one definition of its layout. The
CFI_FINEIBT arm is only built with CONFIG_FINEIBT, the only
configuration in which cfi_mode can take that value.
cfi_get_func_arity() does not need the same treatment: the __bhi_args
call whose displacement it reads still ends at the function address.

Fixes: 85a2d4a890dc ("x86,ibt: Use UDB instead of 0xEA")
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # 6.18+
Assisted-by: LLM
Signed-off-by: Soheil Hassas Yeganeh <soheil.kdev@xxxxxxxxx>
---
 arch/x86/kernel/alternative.c | 72 +++++++++++++++++++++++--------------------
 1 file changed, 38 insertions(+), 34 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 91b1cdd16569..1b22062cabfe 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -1198,6 +1198,41 @@ static bool cfi_debug __ro_after_init;
 bool cfi_bhi __ro_after_init = false;
 #endif
 
+#ifdef CONFIG_FINEIBT
+/*
+ * <fineibt_preamble_start>:
+ *  0:   f3 0f 1e fa             endbr64
+ *  4:   2d 78 56 34 12          sub    $0x12345678, %eax
+ *  9:   2e 0f 85 03 00 00 00    jne,pn 13 <fineibt_preamble_start+0x13>
+ * 10:   0f 1f 40 d6             nopl   -0x2a(%rax)
+ *
+ * Note that the JNE target is the 0xD6 byte inside the NOPL, this decodes as
+ * UDB on x86_64 and raises #UD.
+ */
+asm(	".pushsection .rodata				\n"
+	"fineibt_preamble_start:			\n"
+	"	endbr64					\n"
+	"	subl	$0x12345678, %eax		\n"
+	"fineibt_preamble_bhi:				\n"
+	"	cs jne.d32 fineibt_preamble_start+0x13	\n"
+	"#fineibt_func:					\n"
+	"	nopl	-42(%rax)			\n"
+	"fineibt_preamble_end:				\n"
+	".popsection\n"
+);
+
+extern u8 fineibt_preamble_start[];
+extern u8 fineibt_preamble_bhi[];
+extern u8 fineibt_preamble_end[];
+
+#define fineibt_preamble_size (fineibt_preamble_end - fineibt_preamble_start)
+#define fineibt_preamble_bhi  (fineibt_preamble_bhi - fineibt_preamble_start)
+#define fineibt_preamble_ud   0x13
+#define fineibt_preamble_hash 5
+
+#define fineibt_prefix_size (fineibt_preamble_size - ENDBR_INSN_SIZE)
+#endif /* CONFIG_FINEIBT */
+
 #ifdef CONFIG_CFI
 u32 cfi_get_func_hash(void *func)
 {
@@ -1205,9 +1240,11 @@ u32 cfi_get_func_hash(void *func)
 
 	func -= cfi_get_offset();
 	switch (cfi_mode) {
+#ifdef CONFIG_FINEIBT
 	case CFI_FINEIBT:
-		func += 7;
+		func += fineibt_preamble_hash;
 		break;
+#endif
 	case CFI_KCFI:
 		func += 1;
 		break;
@@ -1363,39 +1400,6 @@ early_param("cfi", cfi_parse_cmdline);
  * anyway.
  */
 
-/*
- * <fineibt_preamble_start>:
- *  0:   f3 0f 1e fa             endbr64
- *  4:   2d 78 56 34 12          sub    $0x12345678, %eax
- *  9:   2e 0f 85 03 00 00 00    jne,pn 13 <fineibt_preamble_start+0x13>
- * 10:   0f 1f 40 d6             nopl   -0x2a(%rax)
- *
- * Note that the JNE target is the 0xD6 byte inside the NOPL, this decodes as
- * UDB on x86_64 and raises #UD.
- */
-asm(	".pushsection .rodata				\n"
-	"fineibt_preamble_start:			\n"
-	"	endbr64					\n"
-	"	subl	$0x12345678, %eax		\n"
-	"fineibt_preamble_bhi:				\n"
-	"	cs jne.d32 fineibt_preamble_start+0x13	\n"
-	"#fineibt_func:					\n"
-	"	nopl	-42(%rax)			\n"
-	"fineibt_preamble_end:				\n"
-	".popsection\n"
-);
-
-extern u8 fineibt_preamble_start[];
-extern u8 fineibt_preamble_bhi[];
-extern u8 fineibt_preamble_end[];
-
-#define fineibt_preamble_size (fineibt_preamble_end - fineibt_preamble_start)
-#define fineibt_preamble_bhi  (fineibt_preamble_bhi - fineibt_preamble_start)
-#define fineibt_preamble_ud   0x13
-#define fineibt_preamble_hash 5
-
-#define fineibt_prefix_size (fineibt_preamble_size - ENDBR_INSN_SIZE)
-
 /*
  * <fineibt_caller_start>:
  *  0:   b8 78 56 34 12          mov    $0x12345678, %eax

---
base-commit: 08dbfad3f5040f5bdb6c529da20d6d4e81fefd72
change-id: 20260831-b4-x86-cfi-fineibt-func-hash-e160f6eee02c

Best regards,
--  
Soheil Hassas Yeganeh <soheil.kdev@xxxxxxxxx>