linux-kernel - Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation

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Date:   Tue, 19 Jul 2022 10:58:30 -0700
From:   Joao Moreira <joao@...rdrivepizza.com>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        "Cooper, Andrew" <andrew.cooper3@...rix.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Johannes Wikner <kwikner@...z.ch>,
        Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
        Jann Horn <jannh@...gle.com>, "H.J. Lu" <hjl.tools@...il.com>,
        "Moreira, Joao" <joao.moreira@...el.com>,
        "Nuzman, Joseph" <joseph.nuzman@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        "Gross, Jurgen" <jgross@...e.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Peter Collingbourne <pcc@...gle.com>
Subject: Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation

> 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.