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Date: Tue, 26 Oct 2021 22:16:22 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Sami Tolvanen <samitolvanen@google.com>
Cc: x86@kernel.org, Kees Cook <keescook@chromium.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Sedat Dilek <sedat.dilek@gmail.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	linux-hardening@vger.kernel.org, linux-kernel@vger.kernel.org,
	llvm@lists.linux.dev, ardb@kernel.org, mark.rutland@arm.com
Subject: Re: [PATCH v5 00/15] x86: Add support for Clang CFI
Message-ID: <20211026201622.GG174703@worktop.programming.kicks-ass.net>
References: <20211013181658.1020262-1-samitolvanen@google.com>
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In-Reply-To: <20211013181658.1020262-1-samitolvanen@google.com>

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?