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Date:   Sat, 14 May 2022 20:19:47 -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 20/21] x86: Add support for CONFIG_CFI_CLANG

On Fri, May 13, 2022 at 01:21:58PM -0700, Sami Tolvanen wrote:
> With CONFIG_CFI_CLANG, the compiler injects a type preamble
> immediately before each function and a check to validate the target
> function type before indirect calls:
> 
>   ; type preamble
>   __cfi_function:
>     int3
>     int3
>     mov <id>, %eax
>     int3
>     int3
>   function:
>     ...
>   ; indirect call check
>     cmpl    <id>, -6(%r11)
>     je      .Ltmp1
>     ud2
>   .Ltmp1:
>     call    __x86_indirect_thunk_r11
> 
> Define the __CFI_TYPE helper macro for manual type annotations in
> assembly code, add error handling for the CFI ud2 traps, and allow
> CONFIG_CFI_CLANG to be selected on x86_64.
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>

Looks good testing with LKDTM...

$ echo CFI_FORWARD_PROTO | cat >/sys/kernel/debug/provoke-crash/DIRECT
[  144.084017] lkdtm: Performing direct entry CFI_FORWARD_PROTO
[  144.086138] lkdtm: Calling matched prototype ...
[  144.087833] lkdtm: Calling mismatched prototype ...
[  144.089777] CFI failure at lkdtm_CFI_FORWARD_PROTO+0x34/0x67 [lkdtm] (target: lkdtm_increment_int+0x0/0x7 [lkdtm]; expected type: 0x7e0c52a5)


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

-- 
Kees Cook

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