x86/cfi: Fix CFI rewrite for odd alignments

Rustam reported his clang builds did not boot properly; turns out his
.config has: CONFIG_DEBUG_FORCE_FUNCTION_ALIGN_64B=y set.

Fix up the FineIBT code to deal with this unusual alignment.

Fixes: 931ab63 ("x86/ibt: Implement FineIBT")
Reported-by: Rustam Kovhaev <rkovhaev@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Rustam Kovhaev <rkovhaev@gmail.com>

Author: Peter Zijlstra

arch/x86/include/asm/cfi.h

@@ -111,6 +111,12 @@ extern bhi_thunk __bhi_args_end[];
 
 struct pt_regs;
 
+#ifdef CONFIG_CALL_PADDING
+#define CFI_OFFSET (CONFIG_FUNCTION_PADDING_CFI+5)
+#else
+#define CFI_OFFSET 5
+#endif
+
 #ifdef CONFIG_CFI
 enum bug_trap_type handle_cfi_failure(struct pt_regs *regs);
 #define __bpfcall
@@ -119,11 +125,9 @@ static inline int cfi_get_offset(void)
 {
 	switch (cfi_mode) {
 	case CFI_FINEIBT:
-		return 16;
+		return /* fineibt_prefix_size */ 16;
 	case CFI_KCFI:
-		if (IS_ENABLED(CONFIG_CALL_PADDING))
-			return 16;
-		return 5;
+		return CFI_OFFSET;
 	default:
 		return 0;
 	}

arch/x86/include/asm/linkage.h

@@ -68,7 +68,7 @@
  * Depending on -fpatchable-function-entry=N,N usage (CONFIG_CALL_PADDING) the
  * CFI symbol layout changes.
  *
- * Without CALL_THUNKS:
+ * Without CALL_PADDING:
  *
  * 	.align	FUNCTION_ALIGNMENT
  * __cfi_##name:
@@ -77,7 +77,7 @@
  * 	.long	__kcfi_typeid_##name
  * name:
  *
- * With CALL_THUNKS:
+ * With CALL_PADDING:
  *
  * 	.align FUNCTION_ALIGNMENT
  * __cfi_##name:

arch/x86/kernel/alternative.c

@@ -1182,7 +1182,7 @@ void __init_or_module noinline apply_seal_endbr(s32 *start, s32 *end)
 
 		poison_endbr(addr);
 		if (IS_ENABLED(CONFIG_FINEIBT))
-			poison_cfi(addr - 16);
+			poison_cfi(addr - CFI_OFFSET);
 	}
 }
 
@@ -1389,6 +1389,8 @@ extern u8 fineibt_preamble_end[];
 #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
@@ -1634,14 +1636,23 @@ static int cfi_rewrite_preamble(s32 *start, s32 *end)
 		 * have determined there are no indirect calls to it and we
 		 * don't need no CFI either.
 		 */
-		if (!is_endbr(addr + 16))
+		if (!is_endbr(addr + CFI_OFFSET))
 			continue;
 
 		hash = decode_preamble_hash(addr, &arity);
 		if (WARN(!hash, "no CFI hash found at: %pS %px %*ph\n",
 			 addr, addr, 5, addr))
 			return -EINVAL;
 
+		/*
+		 * FineIBT relies on being at func-16, so if the preamble is
+		 * actually larger than that, place it the tail end.
+		 *
+		 * NOTE: this is possible with things like DEBUG_CALL_THUNKS
+		 * and DEBUG_FORCE_FUNCTION_ALIGN_64B.
+		 */
+		addr += CFI_OFFSET - fineibt_prefix_size;
+
 		text_poke_early(addr, fineibt_preamble_start, fineibt_preamble_size);
 		WARN_ON(*(u32 *)(addr + fineibt_preamble_hash) != 0x12345678);
 		text_poke_early(addr + fineibt_preamble_hash, &hash, 4);
@@ -1664,10 +1675,10 @@ static void cfi_rewrite_endbr(s32 *start, s32 *end)
 	for (s = start; s < end; s++) {
 		void *addr = (void *)s + *s;
 
-		if (!exact_endbr(addr + 16))
+		if (!exact_endbr(addr + CFI_OFFSET))
 			continue;
 
-		poison_endbr(addr + 16);
+		poison_endbr(addr + CFI_OFFSET);
 	}
 }
 
@@ -1772,7 +1783,8 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
 	if (FINEIBT_WARN(fineibt_preamble_size, 20)			||
 	    FINEIBT_WARN(fineibt_preamble_bhi + fineibt_bhi1_size, 20)	||
 	    FINEIBT_WARN(fineibt_caller_size, 14)			||
-	    FINEIBT_WARN(fineibt_paranoid_size, 20))
+	    FINEIBT_WARN(fineibt_paranoid_size, 20)			||
+	    WARN_ON_ONCE(CFI_OFFSET < fineibt_prefix_size))
 		return;
 
 	if (cfi_mode == CFI_AUTO) {
@@ -1885,6 +1897,11 @@ static void poison_cfi(void *addr)
 	 */
 	switch (cfi_mode) {
 	case CFI_FINEIBT:
+		/*
+		 * FineIBT preamble is at func-16.
+		 */
+		addr += CFI_OFFSET - fineibt_prefix_size;
+
 		/*
 		 * FineIBT prefix should start with an ENDBR.
 		 */
@@ -1923,8 +1940,6 @@ static void poison_cfi(void *addr)
 	}
 }
 
-#define fineibt_prefix_size (fineibt_preamble_size - ENDBR_INSN_SIZE)
-
 /*
  * When regs->ip points to a 0xD6 byte in the FineIBT preamble,
  * return true and fill out target and type.

arch/x86/net/bpf_jit_comp.c

@@ -438,17 +438,8 @@ static void emit_kcfi(u8 **pprog, u32 hash)
 
 	EMIT1_off32(0xb8, hash);			/* movl $hash, %eax	*/
 #ifdef CONFIG_CALL_PADDING
-	EMIT1(0x90);
-	EMIT1(0x90);
-	EMIT1(0x90);
-	EMIT1(0x90);
-	EMIT1(0x90);
-	EMIT1(0x90);
-	EMIT1(0x90);
-	EMIT1(0x90);
-	EMIT1(0x90);
-	EMIT1(0x90);
-	EMIT1(0x90);
+	for (int i = 0; i < CONFIG_FUNCTION_PADDING_CFI; i++)
+		EMIT1(0x90);
 #endif
 	EMIT_ENDBR();