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Date:   Tue, 25 Oct 2022 17:17:57 -0700
From:   Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
To:     Johannes Berg <johannes@sipsolutions.net>
Cc:     Peter Zijlstra <peterz@infradead.org>, scott.d.constable@intel.com,
        daniel.sneddon@linux.intel.com, Jakub Kicinski <kuba@kernel.org>,
        dave.hansen@intel.com, Paolo Abeni <pabeni@redhat.com>,
        antonio.gomez.iglesias@linux.intel.com,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        linux-wireless@vger.kernel.org, linux-kernel@vger.kernel.org,
        x86@kernel.org, gregkh@linuxfoundation.org, netdev@vger.kernel.org
Subject: Re: [RFC PATCH 0/2] Branch Target Injection (BTI) gadget in minstrel
Message-ID: <20221026001757.gyjzcwe5wznu6drj@desk>
References: <cover.1666651511.git.pawan.kumar.gupta@linux.intel.com>
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 <c27de92c10d05891bc804fe0b955c7428ec534dd.camel@sipsolutions.net>
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On Tue, Oct 25, 2022 at 09:56:21PM +0200, Johannes Berg wrote:
>On Tue, 2022-10-25 at 12:38 -0700, Pawan Gupta wrote:
>>
>> > And how is sprinking random LFENCEs around better than running with
>> > spectre_v2=eibrs,retpoline which is the current recommended mitigation
>> > against all this IIRC (or even eibrs,lfence for lesser values of
>> > paranoia).
>>
>> Its a trade-off between performance and spot fixing (hopefully handful
>> of) gadgets. Even the gadget in question here is not demonstrated to be
>> exploitable. If this scenario changes, polluting the kernel all over is
>> definitely not the right approach.
>>
>Btw, now I'm wondering - you were detecting these with the compiler
>based something, could there be a compiler pass to insert appropriate
>things, perhaps as a gcc plugin or something?

I hear it could be a lot of work for gcc. I am not sure if its worth
especially when we can't establish the exploitability of these gadgets.
There are some other challenges like, hot-path sites would prefer to
mask the indexes instead of using a speculation barrier for performance
reasons. I assume adding this intelligence to compilers would be
extremely hard. Also hardware controls and features in newer processors
will make the software mitigations redundant.