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

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Date:   Wed, 27 May 2020 12:58:19 +0200
From:   Wojtek Porczyk <woju@...isiblethingslab.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Andi Kleen <ak@...ux.intel.com>, Andi Kleen <andi@...stfloor.org>,
        x86@...nel.org, keescook@...omium.org,
        linux-kernel@...r.kernel.org, sashal@...nel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH v1] x86: Pin cr4 FSGSBASE

On Wed, May 27, 2020 at 09:07:55AM +0200, Greg KH wrote:
> On Tue, May 26, 2020 at 07:24:03PM +0200, Wojtek Porczyk wrote:
> > On Tue, May 26, 2020 at 06:32:35PM +0200, Greg KH wrote:
> > > On Tue, May 26, 2020 at 08:48:35AM -0700, Andi Kleen 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?  
> > > > 
> > > > Well it's a specific case where we know they're opening a root hole
> > > > unintentionally. This is just an pragmatic attempt to protect the users in the 
> > > > short term.
> > > 
> > > Can't you just go and fix those out-of-tree kernel modules instead?
> > > What's keeping you all from just doing that instead of trying to force
> > > the kernel to play traffic cop?
> > 
> > We'd very much welcome any help really, but we're under impression that this
> > couldn't be done correctly in a module, so this hack occured.
> 
> Really?  How is this hack anything other than a "prevent a kernel module
> from doing something foolish" change?

By "this hack" I meant our module [1], which just enables FSGSBASE bit without
doing everything else that Sasha's patchset does, and that "everything else"
is there to prevent a gaping root hoole.

Original author wanted module for the reason snipped below, but Sasha's
patchset can't be packaged into module. I'll be happy to be corrected on
this point, I personally have next to no kernel programming experience.

This kernel change I think is correct, because if kernel assumes some
invariants, it's a good idea to actually enforce them, isn't it? So we don't
have anything against this kernel change. We'll have to live with it, with our
hand forced.

> Why can't you just change the kernel module's code to not do this?  What
> prevents that from happening right now which would prevent the need to
> change a core api from being abused in such a way?
[snip]
> I'm sorry, but I still do not understand.  Your kernel module calls the
> core with this bit being set, and this new kernel patch is there to
> prevent the bit from being set and will WARN_ON() if it happens.  Why
> can't you just change your module code to not set the bit?

Because we need userspace access to wrfsbase, which this bit enables:

https://github.com/oscarlab/graphene/blob/58c53ad747579225bf29e3506d883586ff4b8eee/Pal/src/host/Linux-SGX/sgx_api.h#L94-L98
https://github.com/oscarlab/graphene/blob/0dd84ddf943d256e5494f07cb41b1d0ece847c1a/Pal/src/host/Linux-SGX/enclave_entry.S#L675
https://github.com/oscarlab/graphene/blob/e4e16aa10e3c2221170aee7da66370507cc52428/Pal/src/host/Linux-SGX/db_misc.c#L69

> Do you have a pointer to the kernel module code that does this operation
> which this core kernel change will try to prevent from happening?

Sure: https://github.com/oscarlab/graphene-sgx-driver/blob/a73de5fed96fc330fc0417d262ba5e87fea128c2/gsgx.c#L32-L39

The whole module is 86 sloc, and the sole purpose is to set this bit on load
and clear on unload.

[1] ^


-- 
pozdrawiam / best regards
Wojtek Porczyk
Graphene / Invisible Things Lab
 
 I do not fear computers,
 I fear lack of them.
    -- Isaac Asimov

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