linux-kernel - Re: [PATCH] ima: Fix undefined arch_ima_get

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Date:   Sat, 18 Dec 2021 10:27:20 +0800
From:   joeyli <jlee@...e.com>
To:     Mimi Zohar <zohar@...ux.ibm.com>
Cc:     Takashi Iwai <tiwai@...e.de>,
        Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
        Eric Snowberg <eric.snowberg@...cle.com>,
        Jarkko Sakkinen <jarkko@...nel.org>
Subject: Re: [PATCH] ima: Fix undefined arch_ima_get_secureboot() and co

On Thu, Dec 16, 2021 at 08:22:56AM -0500, Mimi Zohar wrote:
> 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@...e.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?
>

I just sent patch. The patch may changes behavior of kernel functions who
used MOK/MOKx. We can discuss in that patch.

Thanks
Joey Lee