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:   Wed, 3 Nov 2021 09:35:59 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Kees Cook <keescook@chromium.org>,
        Ard Biesheuvel <ardb@kernel.org>,
        Sami Tolvanen <samitolvanen@google.com>,
        Mark Rutland <mark.rutland@arm.com>,
        the arch/x86 maintainers <x86@kernel.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: <20211103083559.GB174703@worktop.programming.kicks-ass.net>
References: <CAMj1kXG+MuGaG3BHk8pnE1MKVmRf5E+nRNoFMHxOA1y84eGikg@mail.gmail.com>
 <YX8AQJqyB+H3PF1d@hirez.programming.kicks-ass.net>
 <CAMj1kXF3n-oQ1WP8=asb60K6UjSYOtz5RVhrcoCoNq3v7mZdQg@mail.gmail.com>
 <20211101090155.GW174703@worktop.programming.kicks-ass.net>
 <CAMj1kXGhRmdM3om289Q2-s1Pzfob3D2iSDMorzggfhSk1oj53A@mail.gmail.com>
 <YYE1yPClPMHvyvIt@hirez.programming.kicks-ass.net>
 <202111021040.6570189A5@keescook>
 <90a14299-ce56-41d5-9df9-f625aae1ac70@www.fastmail.com>
 <202111021603.EDE5780FE@keescook>
 <bbde172b-2dae-fdc0-fbb5-edec6752c740@kernel.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <bbde172b-2dae-fdc0-fbb5-edec6752c740@kernel.org>
Precedence: bulk

On Tue, Nov 02, 2021 at 05:20:05PM -0700, Andy Lutomirski wrote:
> I think that's a big mistake -- any sane ENDBR-using scheme would
> really prefer that ENDBR to be right next to the actual function body,
> and really any scheme would benefit due to better cache locality. 

Agreed, IBT/BTI want the landing pad in front of the actual function.

> But, more importantly, IMO any sane ENDBR-using scheme wants to
> generate the indirect stub as part of code gen for the actual
> function.

Sorta, I really want to be able to not have a landing pad for functions
whose address is never taken. At that point it doesn't matter if it gets
generated along with the function and then stripped/poisoned later, or
generated later.

As such, the landing pad should not be part of the function proper,
direct calls should never observe it.

Less landing pads is more better.