mm: add generic function_nocfi macro

With CONFIG_CFI_CLANG, the compiler replaces function addresses
in instrumented C code with jump table addresses. This means that
__pa_symbol(function) returns the physical address of the jump table
entry instead of the actual function, which may not work as the jump
table code will immediately jump to a virtual address that may not be
mapped.

To avoid this address space confusion, this change adds a generic
definition for function_nocfi(), which architectures that support CFI
can override. The typical implementation of would use inline assembly
to take the function address, which avoids compiler instrumentation.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210408182843.1754385-4-samitolvanen@google.com

1 files changed, 10 insertions, 0 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8ba434287387..22cce9c7dd05 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -125,6 +125,16 @@ extern int mmap_rnd_compat_bits __read_mostly;
 #endif
 
 /*
+ * With CONFIG_CFI_CLANG, the compiler replaces function addresses in
+ * instrumented C code with jump table addresses. Architectures that
+ * support CFI can define this macro to return the actual function address
+ * when needed.
+ */
+#ifndef function_nocfi
+#define function_nocfi(x) (x)
+#endif
+
+/*
  * To prevent common memory management code establishing
  * a zero page mapping on a read fault.
  * This macro should be defined within <asm/pgtable.h>.