linux-hardening - [PATCH 05/15] x86: Implement function

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Date:   Fri, 16 Apr 2021 13:38:34 -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,
        clang-built-linux@...glegroups.com,
        Sami Tolvanen <samitolvanen@...gle.com>
Subject: [PATCH 05/15] x86: Implement function_nocfi

With CONFIG_CFI_CLANG, the compiler replaces function addresses in
instrumented C code with jump table addresses. This change implements
the function_nocfi() macro, which returns the actual function address
instead.

Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
---
 arch/x86/include/asm/page.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/arch/x86/include/asm/page.h b/arch/x86/include/asm/page.h
index 7555b48803a8..5499a05c44fc 100644
--- a/arch/x86/include/asm/page.h
+++ b/arch/x86/include/asm/page.h
@@ -71,6 +71,20 @@ static inline void copy_user_page(void *to, void *from, unsigned long vaddr,
 extern bool __virt_addr_valid(unsigned long kaddr);
 #define virt_addr_valid(kaddr)	__virt_addr_valid((unsigned long) (kaddr))
 
+#ifdef CONFIG_CFI_CLANG
+/*
+ * With CONFIG_CFI_CLANG, the compiler replaces function address
+ * references with the address of the function's CFI jump table
+ * entry. The function_nocfi macro always returns the address of the
+ * actual function instead.
+ */
+#define function_nocfi(x) ({						\
+	void *addr;							\
+	asm("leaq " __stringify(x) "(%%rip), %0\n\t" : "=r" (addr));	\
+	addr;								\
+})
+#endif
+
 #endif	/* __ASSEMBLY__ */
 
 #include <asm-generic/memory_model.h>
-- 
2.31.1.368.gbe11c130af-goog