Re: kexec_load(2) bypasses signature verification

Dave Young
2015-06-18T01:16:49Z

On 06/16/15 at 09:47pm, Vivek Goyal wrote:
> 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.

Or simply add a new config option KEXEC_VERIFY_SIG_FORCE, so we can return
error in kexec_load and print some error message.

Thanks
Dave