linux-hardening - Re: [PATCH v4 04/15] cfi: Add DEFINE_CFI_IMMEDIATE_RETURN

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Date:   Tue, 5 Oct 2021 08:59:23 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
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 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.

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