[v5,03/18] mm: add generic function_nocfi macro

Commit Message

Sami Tolvanen April 1, 2021, 11:32 p.m. UTC

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 <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
---
 include/linux/mm.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

Comments

Christoph Hellwig April 2, 2021, 6:37 a.m. UTC | #1

Thanks, this looks much better than the earlier naming:

Acked-by: Christoph Hellwig <[e-mail]>

Mark Rutland April 6, 2021, 11:27 a.m. UTC | #2

On Thu, Apr 01, 2021 at 04:32:01PM -0700, Sami Tolvanen wrote:
> 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 <[e-mail]>
> Reviewed-by: Kees Cook <[e-mail]>

FWIW:

Acked-by: Mark Rutland <[e-mail]>

Mark.

> ---
>  include/linux/mm.h | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> 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
> @@ -124,6 +124,16 @@ extern int mmap_rnd_compat_bits __read_mostly;
>  #define lm_alias(x)	__va(__pa_symbol(x))
>  #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.
> -- 
> 2.31.0.208.g409f899ff0-goog
>

Patch

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
@@ -124,6 +124,16 @@  extern int mmap_rnd_compat_bits __read_mostly;
 #define lm_alias(x)	__va(__pa_symbol(x))
 #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.