[PATCH v6 02/18] cfi: add __cficanonical

[Sami Tolvanen]

With CONFIG_CFI_CLANG, the compiler replaces a function address taken
in C code with the address of a local jump table entry, which passes
runtime indirect call checks. However, the compiler won't replace
addresses taken in assembly code, which will result in a CFI failure
if we later jump to such an address in instrumented C code. The code
generated for the non-canonical jump table looks this:

  <noncanonical.cfi_jt>: /* In C, &noncanonical points here */
	jmp noncanonical
  ...
  <noncanonical>:        /* function body */
	...

This change adds the __cficanonical attribute, which tells the
compiler to use a canonical jump table for the function instead. This
means the compiler will rename the actual function to <function>.cfi
and points the original symbol to the jump table entry instead:

  <canonical>:           /* jump table entry */
	jmp canonical.cfi
  ...
  <canonical.cfi>:       /* function body */
	...

As a result, the address taken in assembly, or other non-instrumented
code always points to the jump table and therefore, can be used for
indirect calls in instrumented code without tripping CFI checks.

Signed-off-by: Sami Tolvanen <samitolvanen at google.com>
Reviewed-by: Kees Cook <keescook at chromium.org>
Acked-by: Bjorn Helgaas <bhelgaas at google.com>   # pci.h
Tested-by: Nathan Chancellor <nathan at kernel.org>
---
 include/linux/compiler-clang.h | 1 +
 include/linux/compiler_types.h | 4 ++++
 include/linux/init.h           | 4 ++--
 include/linux/pci.h            | 4 ++--
 4 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
index 6de9d0c9377e..adbe76b203e2 100644
--- a/include/linux/compiler-clang.h
+++ b/include/linux/compiler-clang.h
@@ -63,3 +63,4 @@
 #endif
 
 #define __nocfi		__attribute__((__no_sanitize__("cfi")))
+#define __cficanonical	__attribute__((__cfi_canonical_jump_table__))
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 796935a37e37..d29bda7f6ebd 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -246,6 +246,10 @@ struct ftrace_likely_data {
 # define __nocfi
 #endif
 
+#ifndef __cficanonical
+# define __cficanonical
+#endif
+
 #ifndef asm_volatile_goto
 #define asm_volatile_goto(x...) asm goto(x)
 #endif
diff --git a/include/linux/init.h b/include/linux/init.h
index b3ea15348fbd..045ad1650ed1 100644
--- a/include/linux/init.h
+++ b/include/linux/init.h
@@ -220,8 +220,8 @@ extern bool initcall_debug;
 	__initcall_name(initstub, __iid, id)
 
 #define __define_initcall_stub(__stub, fn)			\
-	int __init __stub(void);				\
-	int __init __stub(void)					\
+	int __init __cficanonical __stub(void);			\
+	int __init __cficanonical __stub(void)			\
 	{ 							\
 		return fn();					\
 	}							\
diff --git a/include/linux/pci.h b/include/linux/pci.h
index 86c799c97b77..39684b72db91 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -1944,8 +1944,8 @@ enum pci_fixup_pass {
 #ifdef CONFIG_LTO_CLANG
 #define __DECLARE_PCI_FIXUP_SECTION(sec, name, vendor, device, class,	\
 				  class_shift, hook, stub)		\
-	void stub(struct pci_dev *dev);					\
-	void stub(struct pci_dev *dev)					\
+	void __cficanonical stub(struct pci_dev *dev);			\
+	void __cficanonical stub(struct pci_dev *dev)			\
 	{ 								\
 		hook(dev); 						\
 	}								\
-- 
2.31.1.295.g9ea45b61b8-goog