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

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Message-ID: <20200526091745.GC325280@hirez.programming.kicks-ass.net>
Date:   Tue, 26 May 2020 11:17:45 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Greg KH <gregkh@...uxfoundation.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 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.

> > 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.

My patches will be a while (too many things in the fire :/) and will
certainly not be for stable.