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Date:   Wed, 3 Jun 2020 17:21:08 +0200
From:   Joerg Roedel <joro@8bytes.org>
To:     Borislav Petkov <bp@alien8.de>
Cc:     x86@kernel.org, hpa@zytor.com, Andy Lutomirski <luto@kernel.org>,
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Subject: Re: [PATCH v3 35/75] x86/head/64: Build k/head64.c with
 -fno-stack-protector
Message-ID: <20200603152108.GD23071@8bytes.org>
References: <20200428151725.31091-1-joro@8bytes.org>
 <20200428151725.31091-36-joro@8bytes.org>
 <20200519091526.GB444@zn.tnic>
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On Tue, May 19, 2020 at 11:15:26AM +0200, Borislav Petkov wrote:
> On Tue, Apr 28, 2020 at 05:16:45PM +0200, Joerg Roedel wrote:
> > From: Joerg Roedel <jroedel@suse.de>
> > 
> > The code inserted by the stack protector does not work in the early
> > boot environment because it uses the GS segment, at least with memory
> > encryption enabled.
> 
> Can you elaborate on why is that a problem?
> 
> The stack cookie is not generated that early yet so it should be
> comparing %gs:40 to 0.

Yes, and when GS_BASE is 0 it will dereference NULL pointer, which
generates a page-fault before the kernel is able to handle it.


	Joerg