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Date:   Tue, 26 May 2020 09:57:36 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     Andi Kleen <andi@firstfloor.org>, x86@kernel.org,
        keescook@chromium.org, linux-kernel@vger.kernel.org,
        sashal@kernel.org, Andi Kleen <ak@linux.intel.com>,
        stable@vger.kernel.org
Subject: Re: [PATCH v1] x86: Pin cr4 FSGSBASE
Message-ID: <20200526075736.GH317569@hirez.programming.kicks-ass.net>
References: <20200526052848.605423-1-andi@firstfloor.org>
 <20200526065618.GC2580410@kroah.com>
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On Tue, May 26, 2020 at 08:56:18AM +0200, Greg KH wrote:
> On Mon, May 25, 2020 at 10:28:48PM -0700, Andi Kleen wrote:
> > From: Andi Kleen <ak@linux.intel.com>
> > 
> > Since there seem to be kernel modules floating around that set
> > FSGSBASE incorrectly, prevent this in the CR4 pinning. Currently
> > CR4 pinning just checks that bits are set, this also checks
> > that the FSGSBASE bit is not set, and if it is clears it again.
> 
> So we are trying to "protect" ourselves from broken out-of-tree kernel
> modules now?  Why stop with this type of check, why not just forbid them
> entirely if we don't trust them?  :)

Oh, I have a bunch of patches pending for that :-)

It will basically decode the module text and refuse to load the module
for most CPL0 instruction.