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Date:   Tue, 5 Oct 2021 13:29:02 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     X86 ML <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>,
        linux-hardening@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>, llvm@...ts.linux.dev
Subject: Re: [PATCH v4 04/15] cfi: Add DEFINE_CFI_IMMEDIATE_RETURN_STUB

On Mon, Oct 4, 2021 at 11:59 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Mon, Oct 04, 2021 at 12:10:46PM -0700, Sami Tolvanen wrote:
> > On Mon, Oct 4, 2021 at 6:50 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> > > Why DEFINE_CFI_IMMEDIATE_RETURN_STUB() vs __no_cfi attribute that we can
> > > stick on the relvant functions?
> >
> > To avoid accidentally creating useful gadgets for attackers. For
> > example, while excluding an empty stub isn't necessarily ideal,
> > allowing calls to a function that always returns zero would be worse.
>
> I was afraid you'd say something like that...
>
> > > Because I've got at least one more variant for you :-) See
> > > kernel/static_call.c:__static_call_return0
> >
> > Does __static_call_return0 ever get called indirectly on architectures
> > that support static calls? If it's always patched into a direct call,
> > the type mismatch isn't an issue.
>
> For x86_64 it should indeed never get called, however if you plan on
> supporting i386 then you need the annotation. Also, it might get called
> on arm64 which is about to grow basic HAVE_STATIC_CALL support.

Good point. I read through the latest arm64 static call proposal and
while it can fall back to an indirect call, it doesn't look like that
would cause issues with CFI.

> (and just in case you care about CFI on PPC32, they too grew basic
> static_call support)

We are currently targeting only x86_64 and arm64, but I'll keep that
in mind in case we want to add more platforms.

Sami

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