linux-kernel - Re: [PATCH v9 19/25] integrity: Move integrity_kernel_module

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Date: Tue, 13 Feb 2024 11:31:26 -0500
From: Stefan Berger <stefanb@...ux.ibm.com>
To: Roberto Sassu <roberto.sassu@...weicloud.com>,
        Paul Moore <paul@...l-moore.com>
Cc: viro@...iv.linux.org.uk, brauner@...nel.org, chuck.lever@...cle.com,
        jlayton@...nel.org, neilb@...e.de, kolga@...app.com,
        Dai.Ngo@...cle.com, tom@...pey.com, jmorris@...ei.org,
        serge@...lyn.com, zohar@...ux.ibm.com, dmitry.kasatkin@...il.com,
        eric.snowberg@...cle.com, dhowells@...hat.com, jarkko@...nel.org,
        stephen.smalley.work@...il.com, eparis@...isplace.org,
        casey@...aufler-ca.com, shuah@...nel.org, mic@...ikod.net,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-nfs@...r.kernel.org, linux-security-module@...r.kernel.org,
        linux-integrity@...r.kernel.org, keyrings@...r.kernel.org,
        selinux@...r.kernel.org, linux-kselftest@...r.kernel.org,
        Roberto Sassu <roberto.sassu@...wei.com>
Subject: Re: [PATCH v9 19/25] integrity: Move
 integrity_kernel_module_request() to IMA



On 2/13/24 03:57, Roberto Sassu wrote:
> On Mon, 2024-02-12 at 15:28 -0500, Stefan Berger wrote:
>>
>> On 2/12/24 12:56, Paul Moore wrote:
>>> On Mon, Feb 12, 2024 at 12:48 PM Stefan Berger <stefanb@...ux.ibm.com> wrote:
>>>> On 1/15/24 13:18, Roberto Sassu wrote:
>>>
>>> ...
>>>
>>>>> +/**
>>>>> + * ima_kernel_module_request - Prevent crypto-pkcs1pad(rsa,*) requests
>>>>> + * @kmod_name: kernel module name
>>>>> + *
>>>>> + * We have situation, when public_key_verify_signature() in case of RSA > + * algorithm use alg_name to store internal information in order to
>>>>> + * construct an algorithm on the fly, but crypto_larval_lookup() will try
>>>>> + * to use alg_name in order to load kernel module with same name.
>>>>> + * Since we don't have any real "crypto-pkcs1pad(rsa,*)" kernel modules,
>>>>> + * we are safe to fail such module request from crypto_larval_lookup().
>>>>> + *
>>>>> + * In this way we prevent modprobe execution during digsig verification
>>>>> + * and avoid possible deadlock if modprobe and/or it's dependencies
>>>>> + * also signed with digsig.
>>>>
>>>> This text needs to some reformulation at some point..
>>>
>>> There is no time like the present.  If you have a suggestion I would
>>> love to hear it and I'm sure Roberto would too.
>>>
>>
>> My interpretation of the issue after possibly lossy decoding of the
>> above sentences:
>>
>> Avoid a deadlock by rejecting a virtual kernel module with the name
>> "crypto-pkcs1pad(rsa,*)". This module may be requested by
>> crypto_larval_lookup() while trying to verify an RSA signature in
>> public_key_verify_signature(). Since the loading of the RSA module may
>> itself cause the request for an RSA signature verification it will
>> otherwise lead to a deadlock.
> 
> I can be even more precise I guess (I actually reproduced it). >
> Avoid a verification loop where verifying the signature of the modprobe
> binary requires executing modprobe itself. Since the modprobe iint-
>> mutex is already held when the signature verification is performed, a
> deadlock occurs as soon as modprobe is executed within the critical
> region, since the same lock cannot be taken again.

When ecdsa is used for signing files it could get stuck as well and 
would need this patch:

diff --git a/security/integrity/ima/ima_main.c 
b/security/integrity/ima/ima_main.c
index 45f1a102c599..2e71dc977d43 100644
--- a/security/integrity/ima/ima_main.c
+++ b/security/integrity/ima/ima_main.c
@@ -1110,7 +1110,9 @@ EXPORT_SYMBOL_GPL(ima_measure_critical_data);
   */
  static int ima_kernel_module_request(char *kmod_name)
  {
-       if (strncmp(kmod_name, "crypto-pkcs1pad(rsa,", 20) == 0)
+       if (strncmp(kmod_name, "crypto-pkcs1pad(rsa,", 20) == 0 ||
+           strncmp(kmod_name, "crypto-ecdsa-nist-p", 19) == 0 ||
+           strcmp(kmod_name, "cryptomgr") == 0)
                 return -EINVAL;

         return 0;

Rejecting cryptomgr seems necessary in the ecdsa case though I am not 
sure what the side effects of rejecting it all the time could be.

    Stefan

> 
> This happens when public_key_verify_signature(), in case of RSA
> algorithm, use alg_name to store internal information in order to
> construct an algorithm on the fly, but crypto_larval_lookup() will try
> to use alg_name in order to load a kernel module with same name.
> 
> Since we don't have any real "crypto-pkcs1pad(rsa,*)" kernel modules,
> we are safe to fail such module request from crypto_larval_lookup(),
> and avoid the verification loop.
> 
> Roberto
> 
>