Re: IMA appraisal against xz-compressed modules

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

On Wed, 2017-10-18 at 17:49 -0200, Bruno E. O. Meneguele wrote:
> On 14-10, Mimi Zohar wrote:
> > On Thu, 2017-10-12 at 10:55 -0400, Bruno E. O. Meneguele wrote:
> > > Hi, 
> > > 
> > > recently, while playing around with IMA modules check support, I notice
> > > that when the kernel was compiled/installed with XZ-compressed modules
> > > the IMA kernel infra returns -EACCESS on modules initialization. Let me
> > > detail a bit more:
> > > 
> > > I created the policy file (/etc/ima/ima-policy) with
> > > 
> > > measure func=MODULE_CHECK uid=0
> > > (... and more, policy file is attached)
> > > 
> > > then rebooted the kernel (that was built with XZ-compressed modules) and
> > > a bunch of modules didn't load, e.g.:
> > > 
> > > without ima-policy:
> > > # lsmod | wc -l
> > > 32
> > > 
> > > with it:
> > > # lsmod | wc -l
> > > 14
> > > 
> > > these 14 modules were all loaded during initram booting phase, but if I
> > > rmmod some of them and try to modprobe (strace output):
> > > 
> > > init_module(0x55b9bcc9bba0, 17763, "") = -1 EACCES (Permission denied)
> > > 
> > > The point is that there is no violation, because the error occurs right
> > > after kmod calls init_module() and the call follows to ima_read_file()
> > > (kernel tree: security/integrity/ima/ima_main.c) which returns -EACCES,
> > > since there is no 'file' structure available (init_module uses memory
> > > region only and not file descriptor).
> > 
> > IMA hashes/signatures are stored as xattrs, which requires a file
> > descriptor.  IMA only supports the new kernel module syscall, which
> > provides the file descriptor.
> > 
> 
> Patches from Thiago Bauerman
> (http://www.spinics.net/lists/linux-integrity/msg00243.html) could help
> to solve this, don't they?

True.  Initially we're introducing appended signature support for
kernel images.  Afterwards, perhaps we'll be able to use it to close
other appraisal gaps (e.g bpf).

> > > I notice this behavior using Fedora 26 (using SELinux as sec framework)
> > > and up-to-date kernel, the question is: should IMA kernel mechanism
> > > support memory regions integrity measurements, maybe following the steps
> > > that MODULE_SIGNATURE takes (that check for module signature through its
> > > mmap region), allowing compressed modules to be loaded? Or kernels built
> > > with XZ/GZ-compressed modules was never meant to be supported? Is it a
> > > bug or a possible enhancement?
> > 
> > If the IMA policy requires kernel modules to be signed, an appended
> > signature is permitted as long as the kernel is configured with
> > CONFIG_MODULE_SIG_FORCE enabled.
> > 
> 
> Right, but it's also possible to note that CONFIG_MODULE_SIG_FORCE is
> handled on kernel/module.c and has a kernel cmdline param,
> module.sig_enforce, that is read in case CONFIG_MODULE_SIG_FORCE is not
> set. Wouldn't be better ima_read_file depend on this cmdline param
> instead directly on the CONFIG? That way kernels compiled without
> CONFIG_MODULE_SIG_FORCE set as default would have the option to enable
> the kernel param and use their normal policy (MODULE_CHECK).
> 
> What do you think?

I wasn't aware of the module_param.  Thank you for pointing it out.
 "sig_enforce" is currently defined as static.  Should it be defined
as __initdata?

Mimi