linux-kernel - Re: Virt Call depth tracking mitigation

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Message-ID: <877d48vm3r.ffs@tglx>
Date:   Tue, 19 Jul 2022 23:17:12 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andrew Cooper <Andrew.Cooper3@...rix.com>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     "x86@...nel.org" <x86@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        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>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "kys@...rosoft.com" <kys@...rosoft.com>,
        "haiyangz@...rosoft.com" <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>,
        "decui@...rosoft.com" <decui@...rosoft.com>,
        Michael Kelley <mikelley@...rosoft.com>
Subject: Re: Virt Call depth tracking mitigation

On Tue, Jul 19 2022 at 16:23, Andrew Cooper wrote:
> On 19/07/2022 15:13, Thomas Gleixner wrote:
>> Well, that makes the guest side "safe", but isn't a deep hypercall > 16
>> already underflowing in the hypervisor code before it returns to the
>> guest?
>
> Yeah, but that's the hypervisor's problem to deal with, in whatever
> manner it sees fit.
>
> And if the hypervisor is using IBeeRS then the first ret in guest
> context will underflow.

I have a look tomorrow.

>>> That said, after a return from the hypervisor, Linux has no idea what
>>> state the RSB is in, so the only safe course of action is to re-stuff.
>> Indeed.
>>
>> Another proof for my claim that virt creates more problems than it
>> solves.
>
> So how did you like debugging the gsbase crash on native hardware. :)

First of all I said it's creating more problems than it solves, which
means it solves some problems.

But more important, I'm not a wimp.

Thanks,

        tglx