linux-hardening - Re: [PATCH v4 00/15] x86: Add support for Clang CFI

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Date:   Tue, 5 Oct 2021 13:36:55 -0700
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     x86@...nel.org, Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH v4 00/15] x86: Add support for Clang CFI

On Thu, Sep 30, 2021 at 11:05:16AM -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
> 
> Note that v4 is based on tip/master. The first two patches contain
> objtool support for CFI, the remaining patches change function
> declarations to use opaque types, fix type mismatch issues that
> confuse the compiler, and disable CFI where it can't be used.
> 
> You can also pull this series from
> 
>   https://github.com/samitolvanen/linux.git x86-cfi-v4

Does this work for indirect calls made from alternatives?

I'm also wondering whether this works on CONFIG_RETPOLINE systems which
disable retpolines at runtime, combined with Peter's patch to use
objtool to replace retpoline thunk calls with indirect branches:

  9bc0bb50727c ("objtool/x86: Rewrite retpoline thunk calls")

Since presumably objtool runs after the CFI stuff is inserted.

-- 
Josh