Messages in this thread | | | Subject | Re: [RFC PATCH 0/2] ima: uncompressed module appraisal support | From | Eric Snowberg <> | Date | Fri, 7 Feb 2020 09:57:18 -0700 |
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> On Feb 7, 2020, at 7:51 AM, Mimi Zohar <zohar@linux.ibm.com> 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 I’m seeing. Appended signatures mostly work. They just don’t 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 <— loads ok $ rmmod foo $ unxz ./foo.ko.xz $ insmod ./foo.ko <— 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 <— 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=“root"
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 don’t have to add module.sig_enforce=1 when I boot.
Adding my change will allow foo.ko to load above when “|modsig” is added, since it will now evaluate the module. Without my change the “imsig|modsig” is true for compressed, but the policy is really “imasig&modsig” 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. I’m just trying to make it so the end user has the same experience when using the default secure_boot ima policy. I don’t see a point in forcing someone to compress a module to get around security, especially when they have a policy that contains “|modsig”.
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?
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