Re: kexec_load(2) bypasses signature verification

Vivek Goyal
2015-06-17T01:47:47Z

On Tue, Jun 16, 2015 at 08:32:37PM -0500, Eric W. Biederman wrote:
> Vivek Goyal <vgoyal@redhat.com> writes:
>
> > On Tue, Jun 16, 2015 at 02:38:31PM -0500, Eric W. Biederman wrote:
> >>
> >> Adding Vivek as he is the one who implemented kexec_file_load.
> >> I was hoping he would respond to this thread, and it looks like he
> >> simply has not ever been Cc'd.
> >>
> >> Theodore Ts'o <tytso@mit.edu> writes:
> >>
> >> > On Mon, Jun 15, 2015 at 09:37:05AM -0400, Josh Boyer wrote:
> >> >> The bits that actually read Secure Boot state out of the UEFI
> >> >> variables, and apply protections to the machine to avoid compromise
> >> >> under the SB threat model. Things like disabling the old kexec...
> >> >
> >> > I don't have any real interest in using Secure Boot, but I *am*
> >> > interested in using CONFIG_KEXEC_VERIFY_SIG[1]. So perhaps we need to
> >> > have something similar to what we have with signed modules in terms of
> >> > CONFIG_MODULE_SIG_FORCE and module/sig_enforce, but for
> >> > KEXEC_VERIFY_SIG. This would mean creating a separate flag
> >> > independent of the one Linus suggested for Secure Boot, but since we
> >> > have one for signed modules, we do have precedent for this sort of
> >> > thing.
> >>
> >> My overall request with respect to kexec has been that we implement
> >> things that make sense outside of the bizarre threat model of the Linux
> >> folks who were talking about secure boot.
> >>
> >> nI have not navigated the labyrinth of config options but having a way to
> >> only boot signed things with kexec seems a completely sensible way to
> >> operate in the context of signed images.
> >>
> >> I don't know how much that will help given that actors with sufficient
> >> resources have demonstrated the ability to steal private keys, but
> >> assuming binary signing is an effective technique (or why else do it)
> >> then having an option to limit kexec to only loading signed images seems
> >> sensible.
> >
> > I went through the mail chain on web and here are my thoughts.
> >
> > - So yes, upstream does not have the logic which automatically disables
> > the old syscall (kexec_load()) on secureboot systems. Distributions
> > carry those patches.
> >
> > - This KEXEC_VERIFY_SIG option only cotrols the behavior for
> > kexec_file_load() syscall and is not meant to directly affect any
> > behavior of old syscall (kexec_load()). I think I should have named
> > it KEXEC_FILE_VERIFY_SIG. Though help text makes it clear.
> > "Verify kernel signature during kexec_file_load() syscall".
> >
> > - I think disabling old system call if KEXEC_VERIFY_SIG() is set
> > will break existing setup which use old system call by default, except
> > the case of secureboot system. And old syscall path is well tested
> > and new syscall might not be in a position to support all the corner
> > cases, atleast as of now.
> >
> > Ted,
> >
> > So looks like you are looking for a system/option where you just want to
> > always make use of kexec_file_load() and disable kexec_load(). This sounds
> > like you want a kernel where kexec_load() is compiled out and you want
> > only kexec_file_load() in.
> >
> > Right now one can't do that becase kexec_file_load() depends on
> > CONFIG_KEXEC option.
> >
> > I am wondering that how about making CONFIG_KEXEC_FILE_LOAD independent
> > of CONFIG_KEXEC. That way one can set CONFIG_KEXEC_VERIFY_SIG=y, and
> > only signed kernel can be kexeced on that system.
> >
> > This should gel well with long term strategy of deprecating kexec_load()
> > at some point of time when kexec_file_load() is ready to completely
> > replace it.
>
> Interesting.
>
> I suspect that what we want is to have CONFIG_KEXEC for the core
> and additional CONFIG_KEXEC_LOAD option that covers that kexec_load call.
>
> That should make it trivially easy to disable the kexec_load system call
> in cases where people care.

Or, we could create another option CONFIG_KEXEC_CORE/CONFIG_KEXEC_COMMON
which will be automatically selected when either CONFIG_KEXEC or
CONIG_KEXEC_FILE are selected.

All common code can go under this option and rest can go under respective
config options.

That way, those who have CONFIG_KEXEC=y in old config files will not be
broken. They don't have to learn about new options at all.

Thanks
Vivek