linux-kernel - Re: [PATCH] Lock down drivers that can have io ports, io mem, irqs and dma changed

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Date:   Mon, 28 Nov 2016 18:23:43 -0600
From:   Corey Minyard <minyard@....org>
To:     David Howells <dhowells@...hat.com>
Cc:     One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
        keyrings@...r.kernel.org, matthew.garrett@...ula.com,
        linux-security-module@...r.kernel.org, linux-efi@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Lock down drivers that can have io ports, io mem, irqs
 and dma changed

On 11/28/2016 06:11 PM, David Howells wrote:
> Corey Minyard <minyard@....org> wrote:
>
>> This would prevent any IPMI interface from working if any address was given
>> on the kernel command line. I'm not sure what the best policy is, but that
>> sounds like a possible DOS to me.
> Okay, reasonable point.
>
>> Can you put this check in hardcode_find_bmc()?  Thats the only place where
>> the hardcoded addresses are used, and a check there won't affect anything
>> else.
> I could do that.  I presume you'd want hardcode_find_bmc() to return 1 in that
> case without doing anything else.  Another possibility is to give a warning
> and then clear ports[], addrs[] and irqs[].
>

Just returning -EPERM from that routine is fine, without doing anything
else.  You can basically just move your check to the top of that
routine.

>> Also, the error message sounds a little vague to me.  If I was a sysadmin
>> and got this, I wouldn't be sure what was going on.  Maybe something like:
>> The kernel is locked down, but hard-coded device addresses were given on
>> the driver command line.  Ignoring these, but this is a possible security
>> issue.
>>
>> That's fairly wordy, but it gets the point across.  You could also move the
>> pr_err() into kernel_is_locked_down() and pass in the prefix, since there is
>> basically the same pr_err() after every check.
> I don't think your suggested summary quite gets it right.  A lot of drivers,
> sound drivers, for example, that aren't really critical can simply be
> disabled - and some have to be disabled because there's no other way to
> configure them.

Yeah.  My main issue was that the sysadmin would see this and not
have any idea what was going on.

>
> It would have to be more like pr_err("Hard-coded device addresses, irqs and
> dma channels are not permitted when the kernel is locked down."), possibly
> with the addition of either "The driver has been disabled" or "These settings
> have been ignored".

That sounds better than what I had.

-corey