linux-kernel - [PATCH v6 1/2] static_call: use non-function types to refer to the trampolines

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Date:   Fri,  5 Nov 2021 15:59:16 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     linux-arm-kernel@...ts.infradead.org
Cc:     linux-kernel@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Quentin Perret <qperret@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>,
        Will Deacon <will@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: [PATCH v6 1/2] static_call: use non-function types to refer to the trampolines

In order to prevent CFI enabled code from grabbing a jump table entry
that jumps to the trampoline, rather than the trampoline itself, use an
incomplete non-function type for the trampoline, and cast it to the
right type only when invoking it.

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 include/linux/static_call.h       |  4 ++--
 include/linux/static_call_types.h | 11 ++++++++---
 2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/include/linux/static_call.h b/include/linux/static_call.h
index 3e56a9751c06..616607393273 100644
--- a/include/linux/static_call.h
+++ b/include/linux/static_call.h
@@ -151,7 +151,7 @@ extern void arch_static_call_transform(void *site, void *tramp, void *func, bool
 
 #define static_call_update(name, func)					\
 ({									\
-	typeof(&STATIC_CALL_TRAMP(name)) __F = (func);			\
+	typeof(&STATIC_CALL_TYPE(name)) __F = (func);			\
 	__static_call_update(&STATIC_CALL_KEY(name),			\
 			     STATIC_CALL_TRAMP_ADDR(name), __F);	\
 })
@@ -306,7 +306,7 @@ static inline void __static_call_nop(void) { }
 	void *func = READ_ONCE(STATIC_CALL_KEY(name).func);		\
 	if (!func)							\
 		func = &__static_call_nop;				\
-	(typeof(STATIC_CALL_TRAMP(name))*)func;				\
+	(typeof(&STATIC_CALL_TYPE(name)))func;				\
 })
 
 #define static_call_cond(name)	(void)__static_call_cond(name)
diff --git a/include/linux/static_call_types.h b/include/linux/static_call_types.h
index 5a00b8b2cf9f..5e658ef537e4 100644
--- a/include/linux/static_call_types.h
+++ b/include/linux/static_call_types.h
@@ -18,6 +18,9 @@
 #define STATIC_CALL_TRAMP(name)		__PASTE(STATIC_CALL_TRAMP_PREFIX, name)
 #define STATIC_CALL_TRAMP_STR(name)	__stringify(STATIC_CALL_TRAMP(name))
 
+#define STATIC_CALL_TYPE_PREFIX		__SCtype__
+#define STATIC_CALL_TYPE(name)		__PASTE(STATIC_CALL_TYPE_PREFIX, name)
+
 /*
  * Flags in the low bits of static_call_site::key.
  */
@@ -36,11 +39,13 @@ struct static_call_site {
 
 #define DECLARE_STATIC_CALL(name, func)					\
 	extern struct static_call_key STATIC_CALL_KEY(name);		\
-	extern typeof(func) STATIC_CALL_TRAMP(name);
+	extern struct static_call_tramp STATIC_CALL_TRAMP(name);	\
+	extern typeof(func) STATIC_CALL_TYPE(name)
 
 #ifdef CONFIG_HAVE_STATIC_CALL
 
-#define __raw_static_call(name)	(&STATIC_CALL_TRAMP(name))
+#define __raw_static_call(name)						\
+	((typeof(&STATIC_CALL_TYPE(name)))&STATIC_CALL_TRAMP(name))
 
 #ifdef CONFIG_HAVE_STATIC_CALL_INLINE
 
@@ -96,7 +101,7 @@ struct static_call_key {
 };
 
 #define static_call(name)						\
-	((typeof(STATIC_CALL_TRAMP(name))*)(STATIC_CALL_KEY(name).func))
+	((typeof(&STATIC_CALL_TYPE(name)))(STATIC_CALL_KEY(name).func))
 
 #endif /* CONFIG_HAVE_STATIC_CALL */
 
-- 
2.30.2