linux-kernel - Re: [RFC] Kdump with signed images

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Date:	Wed, 24 Oct 2012 23:44:32 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Mimi Zohar <zohar@...ux.vnet.ibm.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Matthew Garrett <mjg@...hat.com>,
	Khalid Aziz <khalid@...ehiking.org>, kexec@...ts.infradead.org,
	horms@...ge.net.au, Dave Young <dyoung@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	linux kernel mailing list <linux-kernel@...r.kernel.org>,
	"Kasatkin, Dmitry" <dmitry.kasatkin@...el.com>
Subject: Re: [RFC] Kdump with signed images

On Wed, Oct 24, 2012 at 10:43 PM, Mimi Zohar <zohar@...ux.vnet.ibm.com> wrote:
> On Wed, 2012-10-24 at 13:19 -0400, Vivek Goyal wrote:
>> On Tue, Oct 23, 2012 at 09:44:59AM -0700, Eric W. Biederman wrote:
>> > Matthew Garrett <mjg@...hat.com> writes:
>> >
>> > > On Tue, Oct 23, 2012 at 10:59:20AM -0400, Vivek Goyal wrote:
>> > >
>> > >> But what about creation of a new program which can call kexec_load()
>> > >> and execute an unsigned kernel. Doesn't look like that will be
>> > >> prevented using IMA.
>
> Like the existing kernel modules, kexec_load() is not file descriptor
> based.  There isn't an LSM or IMA-appraisal hook here.
>
>> > > Right. Trusting userspace would require a new system call that passes in
>> > > a signature of the userspace binary, and the kernel would then have to
>> > > verify the ELF object in memory in order to ensure that it
>> > > matches the signature. Verifying that the copy on the filesystem is
>> > > unmodified isn't adequate - an attacker could simply have paused the
>> > > process and injected code.
>
> I haven't looked at kexec_load() in detail, but like kernel modules, I
> think the better solution would be to pass a file descriptor, especially
> if you're discussing a new system call.  (cc'ing Kees.)

Yeah, it looks like kexec_load could use a nearly identical new
syscall that uses an fd, just like init_module is getting.

Another area, kind of related, is firmware loading. The interface for
that is a bit weird, if the documentation is up to date:

        echo 1 > /sys/$DEVPATH/loading
        cat $HOTPLUG_FW_DIR/$FIRMWARE > /sysfs/$DEVPATH/data
        echo 0 > /sys/$DEVPATH/loading

It looks like there's a filp on the reader:

static ssize_t firmware_data_read(struct file *filp, struct kobject *kobj,
                                  struct bin_attribute *bin_attr,
                                  char *buffer, loff_t offset, size_t count)

But it's not clear to me yet if we'll actually get the firmware file,
or if we'll get a random pipe we can't evaluate. Has anyone looked at
handling "signed" firmware loading yet?

-Kees

-- 
Kees Cook
Chrome OS Security
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