Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Sat, 30 Oct 2021 20:02:49 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     Sami Tolvanen <samitolvanen@google.com>,
        Mark Rutland <mark.rutland@arm.com>, X86 ML <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 Mailing List <linux-kernel@vger.kernel.org>,
        llvm@lists.linux.dev
Subject: Re: [PATCH] static_call,x86: Robustify trampoline patching
Message-ID: <20211030180249.GU174703@worktop.programming.kicks-ass.net>
References: <CAMj1kXEx10gC8eH7rV-GbZZj2M3uDue6HYsKb+A5J01zOxm_FA@mail.gmail.com>
 <20211027124852.GK174703@worktop.programming.kicks-ass.net>
 <YXlOd1lyKZKAcJfA@hirez.programming.kicks-ass.net>
 <CAMj1kXHKh7wv6JqusVnoiQDMm7ApFq2ujzbfWmM9AzLKFehhAA@mail.gmail.com>
 <YXlcMluaysPBF92J@hirez.programming.kicks-ass.net>
 <CAMj1kXECTdDLVMk2JduU5mV2TR0Cv=hZ9QOpYRsRM1jfvvNikw@mail.gmail.com>
 <CABCJKufpS4jJxHqk8=bd1JCNbKfmLDKBbjbhjrar2+YQJFiprg@mail.gmail.com>
 <20211029200324.GR174703@worktop.programming.kicks-ass.net>
 <20211030074758.GT174703@worktop.programming.kicks-ass.net>
 <CAMj1kXEJd5=3A_6Jhd4UU-TBGarnHo5+U76Zxxt7SzXsWp4CcA@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAMj1kXEJd5=3A_6Jhd4UU-TBGarnHo5+U76Zxxt7SzXsWp4CcA@mail.gmail.com>
Precedence: bulk

On Sat, Oct 30, 2021 at 07:19:53PM +0200, Ard Biesheuvel wrote:
> I just realized that arm64 has the exact same problem, which is not
> being addressed by my v5 of the static call support patch.

Yeah, it would.

> As it turns out, the v11 Clang that I have been testing with is broken
> wrt BTI landing pads, and omits them from the jump table entries.
> Clang 12+ adds them properly, which means that both the jump table
> entry and the static call trampoline may start with BTI C + direct
> branch, and we also need additional checks to disambiguate.

I'm not sure, why would the static_call trampoline need a BTI C ? The
whole point of static_call() is to be a direct call, we should never
have an indirect call to the trampoline, that would defeat the whole
purpose.