Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
Date:   Tue, 19 Jul 2022 10:58:30 -0700
From:   Joao Moreira <joao@overdrivepizza.com>
To:     Sami Tolvanen <samitolvanen@google.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Josh Poimboeuf <jpoimboe@kernel.org>,
        "Cooper, Andrew" <andrew.cooper3@citrix.com>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Johannes Wikner <kwikner@ethz.ch>,
        Alyssa Milburn <alyssa.milburn@linux.intel.com>,
        Jann Horn <jannh@google.com>, "H.J. Lu" <hjl.tools@gmail.com>,
        "Moreira, Joao" <joao.moreira@intel.com>,
        "Nuzman, Joseph" <joseph.nuzman@intel.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        "Gross, Jurgen" <jgross@suse.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Peter Collingbourne <pcc@google.com>
Subject: Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation
In-Reply-To: <YtboDUndGtMVGRCU@google.com>
References: <87o7xmup5t.ffs@tglx>
 <YtXOMPpmx8TcFtOX@worktop.programming.kicks-ass.net> <87lesqukm5.ffs@tglx>
 <2f7f899cb75b79b08b0662ff4d2cb877@overdrivepizza.com>
 <CABCJKudvSv9bAOrDLHki5XPYNJK6=PS-x8v=E08es8w4LJpxBw@mail.gmail.com>
 <87fsiyuhyz.ffs@tglx>
 <CAHk-=wjEDJ4+xg0CWR7CaCKnO6Nhzn+vjJy7CjaVmf9R+g_3ag@mail.gmail.com>
 <CAHk-=wj6U3UamfLLV+rPu1WmKG_w3p0Bg=YbQcG1DxHpmP40Ag@mail.gmail.com>
 <CAHk-=wiYHXeWnF8Ea5xb735ehJ8FbjTT6UCvHYjX=Ooc7Z5sOw@mail.gmail.com>
 <a6d75b81cdba4a244b142e2f8bb65d71@overdrivepizza.com>
 <YtboDUndGtMVGRCU@google.com>
Message-ID: <656a965d6241d3a697180cc4d05ada2b@overdrivepizza.com>
Content-Type: text/plain; charset=US-ASCII;
 format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

> Clang always uses r11 for the indirect call with retpolines, so we'd
> need to use another register. Nevertheless, splitting the constant into
> two instructions would solve the call target gadget issue.

Yeah, it clicked later yesterday. But, FWIIW, R10 is also considered a 
scratch register, although used for passing static chain pointers which 
I think is not a thing in kernel context. Last case scenario we can 
always do liveness analysis and I doubt we'll have a significant (if 
any) number of spills.

If we are comparing through registers, I would suggest using a sub 
instruction instead of a cmp, as this will destroy the contents of the 
register and prevent it from being re-used on further unprotected 
indirect branches, if any exists.