linux-kernel - Re: /dev/mem and secure boot

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Date:   Mon, 9 Sep 2019 15:09:57 +0200
From:   Jean Delvare <jdelvare@...e.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Arnd Bergmann <arnd@...db.de>, Thomas <trenn@...e.de>,
        LKML <linux-kernel@...r.kernel.org>,
        David Howells <dhowells@...hat.com>
Subject: Re: /dev/mem and secure boot

Hi Greg,

On Fri, 6 Sep 2019 14:15:10 +0200, Greg Kroah-Hartman wrote:
> On Fri, Sep 06, 2019 at 01:02:21PM +0200, Jean Delvare wrote:
> > I've been bitten recently by mcelog not working on machines started in
> > secure boot mode. mcelog tries to read DMI information from /dev/mem
> > and fails to open it.  
> 
> What do you mean by "secure boot"?  Is this matthew's patchset that
> restricts /dev/mem/ or something else?

I mean that early in the kernel log is:

Secure boot enabled and kernel locked down

Digging it up, I found that this comes from a patch in our SLES kernel,
that's not upstream (yet). It is from a patch set by David Howells
(Cc'd) posted in April 2017:

https://patchwork.kernel.org/patch/9665591/
https://patchwork.kernel.org/patch/9665015/

I wrongly assumed it had been merged upstream meanwhile but I was
wrong. David, any reason why this didn't happen? Out of curiosity, are
these patches in RHEL kernels?

> > This made me wonder: if not even root can read /dev/mem (nor, I
> > suppose, /dev/kmem and /dev/port) in secure boot mode, why are we
> > creating these device nodes at all in the first place? Can't we detect
> > that we are in secure boot mode and skip that step, and reap the rewards
> > (faster boot, lower memory footprint and less confusion)?  
> 
> Sure, feel free to not register it at all if the mode is enabled.

Now I feel sorry that I asked my question upstream when there's nothing
to be done there. I'll go bother SUSE kernel folks instead, sorry for
the noise. And thanks for the advice.

-- 
Jean Delvare
SUSE L3 Support