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Date:   Mon, 26 Sep 2022 13:16:04 -0700
From:   "H.J. Lu" <hjl.tools@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>, 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: [PATCH v5 00/22] KCFI support

On Mon, Sep 26, 2022 at 5:39 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Thu, Sep 08, 2022 at 02:54:42PM -0700, Sami Tolvanen wrote:
> > KCFI is a forward-edge control-flow integrity scheme in the upcoming
> > Clang 16 release, which is more suitable for kernel use than the
> > existing CFI scheme used by CONFIG_CFI_CLANG. KCFI doesn't require
> > LTO, doesn't alter function references to point to a jump table, and
> > won't break function address equality.
> >
> > This series replaces the current arm64 CFI implementation with KCFI
> > and adds support for x86_64.
> >
> > KCFI requires assembly functions that are indirectly called from C
> > code to be annotated with type identifiers. As type information is
> > only available in C, the compiler emits expected type identifiers
> > into the symbol table, so they can be referenced from assembly
> > without having to hardcode type hashes. Patch 6 adds helper macros
> > for annotating functions, and patches 9 and 20 add annotations.
> >
> > In case of a type mismatch, KCFI always traps. To support error
> > handling, the compiler generates a .kcfi_traps section for x86_64,
> > which contains the locations of each trap, and for arm64, encodes
> > the necessary register information to the ESR. Patches 10 and 22 add
> > arch-specific error handlers.
> >
> > To test this series, you'll need a ToT Clang toolchain. The series
> > is also available pn GitHub:
> >
> >   https://github.com/samitolvanen/linux/commits/kcfi-v5
>
> As mentioned at plumbers, my only concern is somewhat excessive use of
> CFI_CLANG as oposed to something more compiler neutral. But I suppose
> that's something we can cleanup/fix when GCC grows support for this.
>
> Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Tested-by: Peter Zijlstra (Intel) <peterz@...radead.org>
>
> (as in I ran kCFI + call-depth-tracking + FineIBT on a bunch of
> hardware)
>
> HJL, can you look at adding kCFI support to GCC ?

Adding -fsanitize=kcfi to GCC is a big effort.  Please open a
GCC bug:

https://gcc.gnu.org/bugzilla/enter_bug.cgi?product=gcc

-- 
H.J.

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