Re: [RFC] Kdump with signed images

Mimi Zohar
2012-10-25T07:04:00Z

On Wed, 2012-10-24 at 23:44 -0700, Kees Cook wrote:
> On Wed, Oct 24, 2012 at 10:43 PM, Mimi Zohar <zohar@linux.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@redhat.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

Only looked at it enough to mention at LSS, that it's needed.

Mimi