Re: kexec_load(2) bypasses signature verification

From: Eric W. Biederman
Date: Tue Jun 16 2015 - 21:38:20 EST

Vivek Goyal <vgoyal@xxxxxxxxxx> 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@xxxxxxx> 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.


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.

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