Re: [PATCH] ima: Fix undefined arch_ima_get_secureboot() and co

[Mimi Zohar <zohar@linux.ibm.com>]

On Thu, 2021-12-16 at 12:32 +0800, joeyli wrote:
> On Wed, Dec 15, 2021 at 01:16:48PM -0500, Mimi Zohar wrote:
> > [Cc'ing Eric Snowberg, Jarkko]
> > 
> > Hi Joey,
> > 
> > On Thu, 2021-12-16 at 00:03 +0800, joeyli wrote:
> > > Hi Takashi, Mimi,
> > > 
> > > On Tue, Dec 14, 2021 at 04:58:47PM +0100, Takashi Iwai wrote:
> > > > On Tue, 14 Dec 2021 16:31:21 +0100,
> > > > Mimi Zohar wrote:
> > > > > 
> > > > > Hi Takashi,
> > > > > 
> > > > > On Mon, 2021-12-13 at 17:11 +0100, Takashi Iwai wrote:
> > > > > > Currently arch_ima_get_secureboot() and arch_get_ima_policy() are
> > > > > > defined only when CONFIG_IMA is set, and this makes the code calling
> > > > > > those functions without CONFIG_IMA failing.  Although there is no such
> > > > > > in-tree users, but the out-of-tree users already hit it.
> > > > > > 
> > > > > > Move the declaration and the dummy definition of those functions
> > > > > > outside ifdef-CONFIG_IMA block for fixing the undefined symbols.
> > > > > > 
> > > > > > Signed-off-by: Takashi Iwai <tiwai@suse.de>
> > > > > 
> > > > > Before lockdown was upstreamed, we made sure that IMA and lockdown
> > > > > could co-exist.  This patch makes the stub functions available even
> > > > > when IMA is not configured.  Do the remaining downstream patches
> > > > > require IMA to be disabled or can IMA co-exist?
> > > > 
> > > > I guess Joey (Cc'ed) can explain this better.  AFAIK, currently it's
> > > > used in a part of MODSIGN stuff in SUSE kernels, and it's calling
> > > > unconditionally this function for checking whether the system is with
> > > > the Secure Boot or not.
> > > >
> > > 
> > > Actually in downstream code, I used arch_ima_get_secureboot() in
> > > load_uefi_certs() to prevent the mok be loaded when secure boot is
> > > disabled. Because the security of MOK relies on secure boot.
> > > 
> > > The downstream code likes this:
> > > 
> > > /* the MOK and MOKx can not be trusted when secure boot is disabled */
> > > -      if (!efi_enabled(EFI_SECURE_BOOT))
> > > +      if (!arch_ima_get_secureboot())
> > > 		return 0;
> > > 
> > > The old EFI_SECURE_BOOT bit can only be available on x86_64, so I switch
> > > the code to to arch_ima_get_secureboot() for cross-architectures and sync
> > > with upstream api.
> > > 
> > > The load_uefi.c depends on CONFIG_INTEGRITY but not CONFIG_IMA. So
> > > load_uefi.c still be built when CONFIG_INTEGRITY=y and CONFIG_IMA=n.
> > > Then "implicit declaration of function 'arch_ima_get_secureboot'" is
> > > happened.
> > 
> > The existing upstream code doesn't require secureboot to load the
> > MOK/MOKx keys.  Why is your change being made downstream?
> >
> Because the security of MOK relies on secure boot. When secure boot is
> disabled, EFI firmware will not verify binary code. So arbitrary efi
> binary code can modify MOK when rebooting.
> 
> When user disabled secure boot, a hacker can just replace shim.efi then
> reboot for enrolling new MOK without any interactive. Or hacker can just
> replace shim.efi and wait user to reboot their machine. A hacker's MOK can
> also be enrolled by hacked shim. User can't perceive. 
>  
> > Are you aware of Eric Snowberg's "Enroll kernel keys thru MOK" patch
> > set?  When INTEGRITY_MACHINE_KEYRING is enabled and new UEFI variables
> > are enabled,  instead of loading the MOK keys onto the .platform
> > keyring, they're loaded onto a new keyring name ".machine", which is
> > linked to the secondary keyring.
> > 
> > Eric's patchset doesn't add a new check either to make sure secure boot
> > is enabled before loading the MOK/MOKx keys.
> >
> 
> Kernel doesn't know how was a MOK enrolled. Kernel can only detect the
> state of secure boot. If kernel doesn't want to check the state of secure
> boot before loading MOK, then user should understands that they can not disable
> secure boot when using MOK. 

Thanks, Joey, for the detailed explained.  I was agreeing with you that
MOK/MOKx keys should only be loaded when secure boot is enabled.  A
better way for me to have phrased the questions would have been, why
are you making this change downstream and not upstream?

thanks,

Mimi