linux-hardening - Re: [PATCH v5 03/18] mm: add generic function

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Date:   Tue, 6 Apr 2021 12:27:06 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Will Deacon <will@...nel.org>, Jessica Yu <jeyu@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Tejun Heo <tj@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Catalin Marinas <catalin.marinas@....com>, bpf@...r.kernel.org,
        linux-hardening@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kbuild@...r.kernel.org,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH v5 03/18] mm: add generic function_nocfi macro

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 <samitolvanen@...gle.com>
> Reviewed-by: Kees Cook <keescook@...omium.org>

FWIW:

Acked-by: Mark Rutland <mark.rutland@....com>

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
>