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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

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

On Mon, Jul 18, 2022 at 2:18 PM Peter Zijlstra <peterz@...radead.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