x86/cfi: Extend ENDBR sealing to kCFI

Kees noted that IBT sealing could be extended to kCFI.

Fundamentally it is the list of functions that do not have their
address taken and are thus never called indirectly. It doesn't matter
that objtool uses IBT infrastructure to determine this list, once we
have it it can also be used to clobber kCFI hashes and avoid kCFI
indirect calls.

Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Link: https://lkml.kernel.org/r/20230622144321.494426891%40infradead.org

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 27e0cb4d062a15122179e67ca3c2a7f275de74f1..04b25a27523395f1f1960558cebad41e0aaa9cee 100644 (file)
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -778,6 +778,8 @@ void __init_or_module noinline apply_returns(s32 *start, s32 *end) { }
 
 #ifdef CONFIG_X86_KERNEL_IBT
 
+static void poison_cfi(void *addr);
+
 static void __init_or_module poison_endbr(void *addr, bool warn)
 {
        u32 endbr, poison = gen_endbr_poison();
@@ -802,6 +804,9 @@ static void __init_or_module poison_endbr(void *addr, bool warn)
 
 /*
  * Generated by: objtool --ibt
+ *
+ * Seal the functions for indirect calls by clobbering the ENDBR instructions
+ * and the kCFI hash value.
  */
 void __init_or_module noinline apply_seal_endbr(s32 *start, s32 *end)
 {
@@ -812,7 +817,7 @@ void __init_or_module noinline apply_seal_endbr(s32 *start, s32 *end)
 
                poison_endbr(addr, true);
                if (IS_ENABLED(CONFIG_FINEIBT))
-                       poison_endbr(addr - 16, false);
+                       poison_cfi(addr - 16);
        }
 }
 
@@ -1177,6 +1182,41 @@ err:
        pr_err("Something went horribly wrong trying to rewrite the CFI implementation.\n");
 }
 
+static inline void poison_hash(void *addr)
+{
+       *(u32 *)addr = 0;
+}
+
+static void poison_cfi(void *addr)
+{
+       switch (cfi_mode) {
+       case CFI_FINEIBT:
+               /*
+                * __cfi_\func:
+                *      osp nopl (%rax)
+                *      subl    $0, %r10d
+                *      jz      1f
+                *      ud2
+                * 1:   nop
+                */
+               poison_endbr(addr, false);
+               poison_hash(addr + fineibt_preamble_hash);
+               break;
+
+       case CFI_KCFI:
+               /*
+                * __cfi_\func:
+                *      movl    $0, %eax
+                *      .skip   11, 0x90
+                */
+               poison_hash(addr + 1);
+               break;
+
+       default:
+               break;
+       }
+}
+
 #else
 
 static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
@@ -1184,6 +1224,8 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
 {
 }
 
+static void poison_cfi(void *addr) { }
+
 #endif
 
 void apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,