linux-kernel - Re: [PATCH 07/27] kexec_file: Disable at runtime if securelevel has been set

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Date:   Sat, 28 Oct 2017 16:34:46 +0800
From:   joeyli <jlee@...e.com>
To:     Mimi Zohar <zohar@...ux.vnet.ibm.com>
Cc:     David Howells <dhowells@...hat.com>,
        linux-security-module@...r.kernel.org, gnomes@...rguk.ukuu.org.uk,
        linux-efi@...r.kernel.org, gregkh@...uxfoundation.org,
        linux-kernel@...r.kernel.org, jforbes@...hat.com,
        Matthew Garrett <mjg59@...gle.com>
Subject: Re: [PATCH 07/27] kexec_file: Disable at runtime if securelevel has
 been set

On Fri, Oct 27, 2017 at 03:32:26PM -0400, Mimi Zohar wrote:
> On Thu, 2017-10-26 at 10:17 -0400, Mimi Zohar wrote:
> > On Thu, 2017-10-26 at 15:42 +0800, joeyli wrote:
> > > Hi Mimi,
> > > 
> > > Thank you for reviewing.
> > > 
> > > On Mon, Oct 23, 2017 at 11:54:43AM -0400, Mimi Zohar wrote:
> > > > On Thu, 2017-10-19 at 15:51 +0100, David Howells wrote:
> > > > > From: Chun-Yi Lee <joeyli.kernel@...il.com>
> > > > > 
> > > > > When KEXEC_VERIFY_SIG is not enabled, kernel should not loads image
> > > > > through kexec_file systemcall if securelevel has been set.
> > > > 
> > > > The patch title and description needs to be updated to refer to
> > > > lockdown, not securelevel.
> > > > 
> > > > As previously mentioned the last time these patches were posted, this
> > > > leaves out testing to see if the integrity subsystem is enabled.
> > > > 
> > > > Commit 503ceaef8e2e "ima: define a set of appraisal rules requiring
> > > > file signatures" was upstreamed.  An additional patch could force
> > > > these rules to be added to the custom policy, if lockdown is enabled.
> > > >  This and other patches in this series could then check to see if
> > > > is_ima_appraise_enabled() is true.
> > > > 
> > > > Mimi
> > > >
> > > 
> > > I have updated the patch title and description, and I also added
> > > is_ima_appraise_enabled() as the following. Is it good to you?
> > 
> > Yes, that works.  Thanks!  Remember is_ima_appraise_enabled() is
> > dependent on the "ima: require secure_boot rules in lockdown mode"
> > patch - http://kernsec.org/pipermail/linux-security-module-archive/201
> > 7-October/003910.html.
> > 
> > The IMA "secure_boot" policy can be specified on the boot command line
> > as ima_policy="secure_boot".  It requires kernel modules, firmware,
> > kexec kernel image and the IMA custom policy to be signed.  In
> > lockdown mode, these rules are enabled by default and added to the
> > custom policy.
> > 
> > > On the other hand, I am not good on IMA. I have traced the code path
> > > in kimage_file_prepare_segments(). Looks that the READING_KEXEC_IMAGE
> > > doesn't show in selinux_kernel_read_file(). Where is the exact code
> > > in IMA for checking the signature when loading crash kernel file?
> > 
> > kernel_read_file_from_fd() calls the security_kernel_read_file() and
> > security_kernel_post_read_file() hooks, which call ima_read_file() and
> > ima_post_read_file() respectively.
> 
> Hm, with "lockdown" enabled on the boot command line, I'm now able to
> do the kexec load, but not the unload.  :/   After the kexec load with

I have tried on Qemu with OVMF, I can load and unload second kernel by
kexec tool (on openSUSE is in kexec-tools RPM):  

# kexec -u -s

I add -s for using kexec-load-file, and I signed kernel by pesign.

> the "--reuse-cmdline" option, the system reboots, but isn't in
> "lockdown" mode.
>

Either enabling secure boot in EFI firmware or using _lockdown_ kernel
parameter, the second kernel can be locked down on my OVMF VM.

I used following commands:

# kexec -s -l /boot/vmlinuz-4.14.0-rc2-default+ --append="$(cat /proc/cmdline)" --initrd=/boot/initrd-4.14.0-rc2-default+
# umount -a; mount -o remount,ro /
# kexec -e

The kernel source is from David's linux-fs git with lockdown-20171026 tag.
The kernel is also signed by pesign.

Regards
Joey Lee