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

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Message-ID: <87fsiyuhyz.ffs@tglx>
Date:   Tue, 19 Jul 2022 01:19:32 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sami Tolvanen <samitolvanen@...gle.com>,
        Joao Moreira <joao@...rdrivepizza.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "Torvalds, Linus" <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>,
        "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

On Mon, Jul 18 2022 at 15:55, Sami Tolvanen wrote:
> On Mon, Jul 18, 2022 at 3:47 PM Joao Moreira <joao@...rdrivepizza.com> wrote:
>> Thus the hash will be 6 bytes before the function entry point. Then we
>> can get the compiler to emit a padding area before the __cfi_\func
>> snippet and, during boot, if the CPU needs the call depth tracking
>> mitigation, we:
>> - move the __cfi_func into the padding area
>> - patch the call depth tracking snippet ahead of it (overwriting the old
>> __cfi_\func:)
>> - fix the cmpl offset in the caller
>>
>> func_whatever:
>>         ...
>>         cmpl $0x\hash, -FIXED_OFFSET(%rax)
>>         je 1f
>>         ud2
>> 1:
>>         call *%rax
>>         ...
>
> The problem with this is that the cmpl instruction contains the full
> type hash, which means that any instruction that's FIXED_OFFSET from
> the cmpl is a valid indirect call target as far as KCFI is concerned.
> -6 was chosen specifically to make the ud2 the only possible target.

But that's an implementation detail, right? Whatever we put in between
will still be a fixed offset, no? It's a different offset, but that's
what patching can deal with.

Thanks,

        tglx