linux-hardening - [PATCH v4 03/15] linkage: Add DECLARE_ASM_FUNC

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Date:   Thu, 30 Sep 2021 11:05:19 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     x86@...nel.org
Cc:     Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, Sami Tolvanen <samitolvanen@...gle.com>
Subject: [PATCH v4 03/15] linkage: Add DECLARE_ASM_FUNC_SYMBOL

The kernel has several assembly functions, which are not directly
callable from C but need to be referred to from C code. This change adds
the DECLARE_ASM_FUNC_SYMBOL macro, which allows us to declare these
symbols using an opaque type, which makes misuse harder, and avoids the
need to annotate references to the functions for Clang's Control-Flow
Integrity (CFI).

Suggested-by: Andy Lutomirski <luto@...capital.net>
Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
---
 include/linux/linkage.h | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/include/linux/linkage.h b/include/linux/linkage.h
index dbf8506decca..f1eac26b2dd6 100644
--- a/include/linux/linkage.h
+++ b/include/linux/linkage.h
@@ -48,6 +48,19 @@
 #define __PAGE_ALIGNED_DATA	.section ".data..page_aligned", "aw"
 #define __PAGE_ALIGNED_BSS	.section ".bss..page_aligned", "aw"
 
+/*
+ * Declares a function not callable from C using an opaque type. Defined as
+ * an array to allow the address of the symbol to be taken without '&'.
+ */
+#ifndef DECLARE_ASM_FUNC_SYMBOL
+#define DECLARE_ASM_FUNC_SYMBOL(sym) \
+	extern const u8 sym[]
+#endif
+
+#ifndef __ASSEMBLY__
+typedef const u8 *asm_func_ptr;
+#endif
+
 /*
  * This is used by architectures to keep arguments on the stack
  * untouched by the compiler by keeping them live until the end.
-- 
2.33.0.800.g4c38ced690-goog