[PATCH 02/17] cfi: add __cficanonical

Sami Tolvanen
2021-03-12T00:50:18Z

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@google.com>
---
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 1ff22bdad992..c275f23ce023 100644
--- a/include/linux/compiler-clang.h
+++ b/include/linux/compiler-clang.h
@@ -57,3 +57,4 @@
#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); \
} \
--
2.31.0.rc2.261.g7f71774620-goog