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

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Date:   Tue, 5 Oct 2021 14:52:46 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     X86 ML <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,
        LKML <linux-kernel@...r.kernel.org>, llvm@...ts.linux.dev
Subject: Re: [PATCH v4 00/15] x86: Add support for Clang CFI

On Tue, Oct 5, 2021 at 1:37 PM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>
> 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?

It works in the sense that indirect calls made from alternatives won't
trip CFI. The compiler doesn't instrument inline assembly.

> 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.

The indirect call checking is before the retpoline thunk call, so
replacing the call with an indirect call isn't a problem.

Sami