linux-kernel - Re: [PATCH v8 21/24] evm: Move to LSM infrastructure

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Date: Tue, 2 Jan 2024 12:56:26 +0100
From: Roberto Sassu <roberto.sassu@...weicloud.com>
To: Mimi Zohar <zohar@...ux.ibm.com>, 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,
 paul@...l-moore.com, jmorris@...ei.org, serge@...lyn.com,
 dmitry.kasatkin@...il.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
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...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 v8 21/24] evm: Move to LSM infrastructure

On 12/26/2023 11:13 PM, Mimi Zohar wrote:
> On Thu, 2023-12-14 at 18:08 +0100, Roberto Sassu wrote:
>> From: Roberto Sassu <roberto.sassu@...wei.com>
>>
>> As for IMA, move hardcoded EVM function calls from various places in the
>> kernel to the LSM infrastructure, by introducing a new LSM named 'evm'
>> (last and always enabled like 'ima'). The order in the Makefile ensures
>> that 'evm' hooks are executed after 'ima' ones.
>>
>> Make EVM functions as static (except for evm_inode_init_security(), which
>> is exported), and register them as hook implementations in init_evm_lsm().
>>
>> Unlike before (see commit to move IMA to the LSM infrastructure),
>> evm_inode_post_setattr(), evm_inode_post_set_acl(),
>> evm_inode_post_remove_acl(), and evm_inode_post_removexattr() are not
>> executed for private inodes.
>>
> 
> Missing is a comment on moving the inline function definitions -
> evm_inode_remove_acl(), evm_inode_post_remove_acl(), and
> evm_inode_post_set_acl() - to evm_main.c.

Ok.

>> Finally, add the LSM_ID_EVM case in lsm_list_modules_test.c
>>
>> Signed-off-by: Roberto Sassu <roberto.sassu@...wei.com>
>> ---
> 
> [...]
>> @@ -2307,9 +2299,7 @@ int security_inode_setxattr(struct mnt_idmap *idmap,
>>   
>>   	if (ret == 1)
>>   		ret = cap_inode_setxattr(dentry, name, value, size, flags);
>> -	if (ret)
>> -		return ret;
>> -	return evm_inode_setxattr(idmap, dentry, name, value, size, flags);
>> +	return ret;
>>   }
> 
> Even though capability will be called after EVM, it doesn't make a
> difference in this instance.
> 
> [...]
> 
>>   /**
>> @@ -2493,9 +2472,7 @@ int security_inode_removexattr(struct mnt_idmap *idmap,
>>   	ret = call_int_hook(inode_removexattr, 1, idmap, dentry, name);
>>   	if (ret == 1)
>>   		ret = cap_inode_removexattr(idmap, dentry, name);
>> -	if (ret)
>> -		return ret;
>> -	return evm_inode_removexattr(idmap, dentry, name);
>> +	return ret;
>>   }
> 
> 'security.capability' is one of the EVM protected xattrs.  As
> capability isn't an LSM, it will now be called after EVM, which is a
> problem.

Uhm, according to this comment in security_inode_removexattr() and 
security_inode_setxattr():

	/*
	 * SELinux and Smack integrate the cap call,
	 * so assume that all LSMs supplying this call do so.
	 */

We can add the call to IMA and EVM as well, to be compliant.

However, I'm missing why the two cases are different. It seems 
cap_inode_set/removexattr() are doing just checks.

Thanks

Roberto