linux-kernel - RE: [PATCH v4 13/14] platform/x86: dell-smbios-wmi: introduce userspace interface

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Date:   Thu, 5 Oct 2017 14:22:37 +0000
From:   <Mario.Limonciello@...l.com>
To:     <gnomes@...rguk.ukuu.org.uk>
CC:     <dvhart@...radead.org>, <andy.shevchenko@...il.com>,
        <linux-kernel@...r.kernel.org>,
        <platform-driver-x86@...r.kernel.org>, <luto@...nel.org>,
        <quasisec@...gle.com>, <pali.rohar@...il.com>, <rjw@...ysocki.net>,
        <mjg59@...gle.com>, <hch@....de>, <greg@...ah.com>
Subject: RE: [PATCH v4 13/14] platform/x86: dell-smbios-wmi: introduce
 userspace interface

> -----Original Message-----
> From: Alan Cox [mailto:gnomes@...rguk.ukuu.org.uk]
> Sent: Thursday, October 5, 2017 8:59 AM
> To: Limonciello, Mario <Mario_Limonciello@...l.com>
> Cc: dvhart@...radead.org; Andy Shevchenko <andy.shevchenko@...il.com>;
> LKML <linux-kernel@...r.kernel.org>; platform-driver-x86@...r.kernel.org;
> Andy Lutomirski <luto@...nel.org>; quasisec@...gle.com;
> pali.rohar@...il.com; rjw@...ysocki.net; mjg59@...gle.com; hch@....de; Greg
> KH <greg@...ah.com>
> Subject: Re: [PATCH v4 13/14] platform/x86: dell-smbios-wmi: introduce
> userspace interface
> 
> On Wed,  4 Oct 2017 17:48:39 -0500
> Mario Limonciello <mario.limonciello@...l.com> wrote:
> 
> > This userspace character device will be used to perform SMBIOS calls
> > from any applications.
> >
> > It provides an ioctl that will allow passing the 32k WMI calling
> > interface buffer between userspace and kernel space.
> 
> What is your security model for firing 32K of random crap at the BIOS ?

Adding new class and select methods requires a review with the security
team.  They will do STRIDE analysis and threat modeling.

> Do you fuzz test the BIOS interface ?

Yes there has been internal fuzz testing classes and selects used in the 
ACPI interface in the past.  I can't comment on how regularly that is done.
I do think it's interesting is to use the interface in Linux for further fuzz
testing though.

> 
> How do we know that between now and the end of the universe every call is
> safe to execute as any random user without upsetting other users on the
> same PC ?

Any random user shouldn't be executing the ioctl.
Only root should be executing any of these calls.

If there is a particular call that is deemed too dangerous to have available the ioctl
call can be filtered by the kernel module.

> 
> Right now this patch is scary. U've fuzzed tested BIOS firmware in thepast
> and it almost universally ended up in reaching for the power cord because
> PC firmware is usually closed and usually crap.
>

Can you please share more context?

> In addition you are assuming that every function you ever provide via
> that ioctl has the same securiy model. So if one of them should only be
> usable by the user logged in on the console the system would have to
> enforce that for all. If you have two conflicting security policies we'd
> have to make it root only and owned by a daemon. If a BIOS turns out to
> have a hole then we have to make it CAP_SYS_RAWIO.
> 
> /dev/dorandomvendorspecificshit is not an API and not a security policy.
> 
> Alan

All functionality offered through this interface has the same security model.
This should only be made available to root, but I don't see the need for a
daemon to further sit between calls.  The two applications that I see using
this in the short term (fwupd and fwupdate) both run as root and would
directly use this interface.