Re: [RFC PATCH 0/2] ima: uncompressed module appraisal support

[Eric Snowberg]

> On Feb 7, 2020, at 7:51 AM, Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote:
> 
> On Thu, 2020-02-06 at 14:40 -0700, Eric Snowberg wrote:
> 
> <snip>
> 
>> Currently the upstream code will fail if the module is uncompressed.
>> If you compress the same module it will load with the current
>> upstream code.
>> 
>>> Lastly, there is nothing in these patches that indicate that the
>> kernel modules being compressed/uncompressed is related to the
>> signature verification.
>>>  
>> 
>> Basically if you have the following setup:
>> 
>> Kernel built with CONFIG_IMA_ARCH_POLICY or kernel booted with
>> module.sig_enforce=1 along with the following ima policy:
>> 
>> appraise func=MODULE_CHECK appraise_type=imasig|modsig
> 
> Enabling CONFIG_IMA_ARCH_POLICY or module.sig_enforce=1 behave totally
> differently.  CONFIG_IMA_ARCH_POLICY coordinates between the IMA
> signature verification and the original module_sig_check()
> verification.  Either one signature verification method is enabled or
> the other, but not both.
> 
> The existing IMA x86 arch policy has not been updated to support
> appended signatures.

That is not what Iâm seeing.  Appended signatures mostly work.  They just
donât work thru the finit_module system call.

> To understand what is happening, we need to analyze each scenario
> separately.
> 
> - If CONFIG_MODULE_SIG is configured or enabled on the boot command
> line ("module.sig_enforce = 1"), then the IMA arch x86 policy WILL NOT
> require an IMA signature.

All tests below are without my change
x86 booted with module.sig_enforce=1

empty ima policy
$ cat /sys/kernel/security/ima/policy
$ insmod ./foo.ko.xz   <â loads ok
$ rmmod foo
$ unxz ./foo.ko.xz
$ insmod ./foo.ko      <â loads ok
$ rmmod foo

add in module appraisal 
$ echo "appraise func=MODULE_CHECK appraise_type=imasig|modsig" > /sys/kernel/security/ima/policy

$ insmod ./foo.ko.xz   <â loads ok
$ rmmod foo

$ insmod ./foo.ko
insmod: ERROR: could not insert module ./foo.ko: Permission denied

last entry from audit log:
type=INTEGRITY_DATA msg=audit(1581089373.076:83): pid=2874 uid=0 auid=0 ses=1 subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 op=appraise_data cause=invalid-signature comm="insmod" name="/root/keys/modules/foo.ko" dev="dm-0" ino=10918365 res=0^]UID="root" AUID=âroot"

This is because modsig_verify() will be called from within ima_appraise_measurement(), 
since try_modsig is true.  Then modsig_verify() will return INTEGRITY_FAIL.

If I build with CONFIG_IMA_ARCH_POLICY & CONFIG_MODULE_SIG all tests work the same above,
I just donât have to add module.sig_enforce=1 when I boot.

Adding my change will allow foo.ko to load above when â|modsigâ is added, since it will now evaluate 
the module. Without my change the âimsig|modsigâ is true for compressed, but the policy is really 
âimasig&modsigâ for uncompressed.


> - If CONFIG_MODULE_SIG is NOT configured or enabled on the boot
> command line, then the IMA arch x86 policy WILL require an IMA
> signature.

Agreed

> - If CONFIG_MODULE_SIG is configured or enabled on the boot command
> line, the IMA arch x86 policy is not configured, and the above policy
> rule is defined, an appended signature will be verified by both IMA
> and module_sig_check().

I think this is the same as what I have done above?


>> If you have a module foo.ko that contains a valid appended signature
>> but is not ima signed, it will fail to load.
> 
> That would only happen in the second scenario or in the last scenario
> if the key is not found.
> 
>> Now if the end user simply compresses the same foo.ko, making it
>> foo.ko.xz.  The module will load.
> 
> This implies that CONFIG_MODULE_SIG is configured or enabled on the
> boot command line, like the first scenario described above, or in the
> last scenario and the key is found.
>> Modules can be loaded thru two different syscalls, finit_module and
>> init_module.  The changes added in [1] work if you use the
>> init_module syscall.  My change adds support when the finit_module
>> syscall gets used instead.
> 
> With the IMA arch x86 policy, without CONFIG_MODULE_SIG configured or
> enabled on the boot command line, IMA will prevent the init_module()
> syscall.  This is intentional.

Agreed

> Your second patch (2/2) changes the arch x86 policy rule to allow
> appended signatures.  The reason for any other changes needs to be
> clearer.  I suggest you look at the audit log and kernel messages, as
> well as the kexec selftests, to better understand what is happening.
> 

I can add more details.  Iâm just trying to make it so the end user has the same 
experience when using the default secure_boot ima policy. I donât see a point in
forcing someone to compress a module to get around security, especially when they
have a policy that contains â|modsigâ.  

Let me know how you would like me to move forward.  Are you ok with the actual code in 
my patches, assuming I add a lot more details? Or do you want more analysis here first?