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

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Message-ID: <20190906170737.1a00178c@endymion>
Date:   Fri, 6 Sep 2019 17:07:37 +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>
Subject: Re: /dev/mem and secure boot

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

> > 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.
> 
> thanks,
> 
> greg k-h


-- 
Jean Delvare
SUSE L3 Support