linux-hardening - Re: [PATCH v5 00/15] x86: Add support for Clang CFI

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Message-ID: <20211026201622.GG174703@worktop.programming.kicks-ass.net>
Date:   Tue, 26 Oct 2021 22:16:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     x86@...nel.org, Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, ardb@...nel.org, mark.rutland@....com
Subject: Re: [PATCH v5 00/15] x86: Add support for Clang CFI

On Wed, Oct 13, 2021 at 11:16:43AM -0700, Sami Tolvanen wrote:
> This series adds support for Clang's Control-Flow Integrity (CFI)
> checking to x86_64. With CFI, the compiler injects a runtime
> check before each indirect function call to ensure the target is
> a valid function with the correct static type. This restricts
> possible call targets and makes it more difficult for an attacker
> to exploit bugs that allow the modification of stored function
> pointers. For more details, see:
> 
>   https://clang.llvm.org/docs/ControlFlowIntegrity.html

So, if I understand this right, the compiler emits, for every function
two things: 1) the actual funcion and 2) a jump-table entry.

Then, every time the address of a function is taken, 2) is given instead
of the expected 1), right?

But how does this work with things like static_call(), which we give a
function address (now a jump-table entry) and use that to write direct
call instructions?

Should not this jump-table thingy get converted to an actual function
address somewhere around arch_static_call_transform() ? This also seems
relevant for arm64 (which already has CLANG_CFI supported) given:

  https://lkml.kernel.org/r/20211025122102.46089-3-frederic@kernel.org

Or am I still not understanding this CFI thing?