linux-hardening - Re: [RFC PATCH v2 07/21] cfi: Add type helper macros

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Date:   Sat, 14 May 2022 14:49:36 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     linux-kernel@...r.kernel.org, Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Joao Moreira <joao@...rdrivepizza.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-hardening@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, llvm@...ts.linux.dev
Subject: Re: [RFC PATCH v2 07/21] cfi: Add type helper macros

On Fri, May 13, 2022 at 01:21:45PM -0700, Sami Tolvanen wrote:
> With CONFIG_CFI_CLANG, assembly functions called indirectly
> from C code must be annotated with type identifiers to pass CFI
> checking. The compiler emits a __kcfi_typeid_<function> symbol for
> each address-taken function declaration in C, which contains the
> expected type identifier. Add typed versions of SYM_FUNC_START and
> SYM_FUNC_START_ALIAS, which emit the type identifier before the
> function.
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>

And the reason to not make this change universally (i.e. directly in
SYM_FUNC_START) is to minimize how many of these symbol annotations get
emitted? (And to more directly indicate which asm is called indirectly?)

What happens if an asm function is called indirectly and it doesn't have
this annotation? (Is this case detectable at compile-time?)

Regardless:

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

-- 
Kees Cook