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From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Mon, 18 Jul 2022 15:48:04 -0700
Message-ID: <CABCJKufACEoUz=SVakAFVKhhwFb6biaPdiC_1cA=NRAUiak5hQ@mail.gmail.com>
Subject: Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.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>,
        Andrew Cooper <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>,
        Joao Moreira <joao.moreira@intel.com>,
        Joseph Nuzman <joseph.nuzman@intel.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Juergen Gross <jgross@suse.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>
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Precedence: bulk

On Mon, Jul 18, 2022 at 2:18 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Mon, Jul 18, 2022 at 10:44:14PM +0200, Thomas Gleixner wrote:
> > And we need input from the Clang folks because their CFI work also puts
> > stuff in front of the function entry, which nicely collides.
>
> Right, I need to go look at the latest kCFI patches, that sorta got
> side-tracked for working on all the retbleed muck :/
>
> Basically kCFI wants to preface every (indirect callable) function with:
>
> __cfi_\func:
>         int3
>         movl $0x12345678, %rax
>         int3
>         int3
> \func:

Yes, and in order to avoid scattering the code with call target
gadgets, the preamble should remain immediately before the function.

> Ofc, we can still put the whole:
>
>         sarq    $5, PER_CPU_VAR(__x86_call_depth);
>         jmp     \func_direct
>
> thing in front of that.

Sure, that would work.

> But it does somewhat destroy the version I had that only needs the
> 10 bytes padding for the sarq.

There's also the question of how function alignment should work in the
KCFI case. Currently, the __cfi_ preamble is 16-byte aligned, which
obviously means the function itself isn't.

Sami