linux-kernel - Re: [PATCH v1] x86: Pin cr4 FSGSBASE

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Date:   Tue, 26 May 2020 12:16:39 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andi Kleen <andi@...stfloor.org>, x86@...nel.org,
        keescook@...omium.org, linux-kernel@...r.kernel.org,
        sashal@...nel.org, Andi Kleen <ak@...ux.intel.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH v1] x86: Pin cr4 FSGSBASE

On Tue, May 26, 2020 at 11:17:45AM +0200, Peter Zijlstra wrote:
> On Tue, May 26, 2020 at 10:17:52AM +0200, Greg KH wrote:
> > On Tue, May 26, 2020 at 09:57:36AM +0200, Peter Zijlstra wrote:
> > > 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@...ux.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 :-)
> > 
> > Ah, I thought I had seen something like that go by a while ago.
> > 
> > It's sad that we have to write a "don't do stupid things" checker for
> > kernel modules now :(
> 
> Because people... they get stuff from the interweb and run it :/ The
> days that admins actually knew what they're doing is long long gone.

{sigh}

> > > It will basically decode the module text and refuse to load the module
> > > for most CPL0 instruction.
> > 
> > Ok, so why would Andi's patch even be needed then?  Andi, why post this?
> 
> Andi's patch cures a particularly bad module that floats around that
> people use, probably without being aware that it's an insta-root hole.

Ok, fair enough, thanks for the context.

greg k-h