linux-kernel - Re: [PATCH -next] evm: Use __vfs_setxattr() to update security.evm

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Date:   Mon, 30 Jan 2023 09:53:20 +0800
From:   "Guozihua (Scott)" <guozihua@...wei.com>
To:     Mimi Zohar <zohar@...ux.ibm.com>,
        Xiu Jianfeng <xiujianfeng@...wei.com>,
        <dmitry.kasatkin@...il.com>, <paul@...l-moore.com>,
        <jmorris@...ei.org>, <serge@...lyn.com>
CC:     <linux-integrity@...r.kernel.org>,
        <linux-security-module@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next] evm: Use __vfs_setxattr() to update security.evm

On 2023/1/19 5:45, Mimi Zohar wrote:
> On Wed, 2022-12-28 at 11:02 +0800, Xiu Jianfeng wrote:
>> Currently it uses __vfs_setxattr_noperm() to update "security.evm",
>> however there are two lsm hooks(inode_post_setxattr and inode_setsecurity)
>> being called inside this function, which don't make any sense for xattr
>> "security.evm", because the handlers of these two hooks, such as selinux
>> and smack, only care about their own xattr.
> 
> Updating the security.ima hash triggers re-calculating and writing the
> security.evm HMAC.  Refer to evm_inode_post_setxattr().
> 
> Mimi

Hi Mimi,

I believe what Jianfeng is trying to do is to avoid re-triggering
security_inode_post_setxattr if we are updating security.evm. I can't
think of any other xattr that could "absorb" security.evm.
> 
>>
>> On the other hand, there is a literally rather than actually cyclical
>> callchain as follows:
>> security_inode_post_setxattr
>>   ->evm_inode_post_setxattr
>>     ->evm_update_evmxattr
>>       ->__vfs_setxattr_noperm
>>         ->security_inode_post_setxattr
>>
>> So use __vfs_setxattr() to update "security.evm".
>>
>> Signed-off-by: Xiu Jianfeng <xiujianfeng@...wei.com>
>> ---
>>  security/integrity/evm/evm_crypto.c   | 7 +++----
>>  security/integrity/ima/ima_appraise.c | 8 ++++----
>>  2 files changed, 7 insertions(+), 8 deletions(-)
>>
>> diff --git a/security/integrity/evm/evm_crypto.c b/security/integrity/evm/evm_crypto.c
>> index fa5ff13fa8c9..d8275dfa49ef 100644
>> --- a/security/integrity/evm/evm_crypto.c
>> +++ b/security/integrity/evm/evm_crypto.c
>> @@ -376,10 +376,9 @@ int evm_update_evmxattr(struct dentry *dentry, const char *xattr_name,
>>  			   xattr_value_len, &data);
>>  	if (rc == 0) {
>>  		data.hdr.xattr.sha1.type = EVM_XATTR_HMAC;
>> -		rc = __vfs_setxattr_noperm(&init_user_ns, dentry,
>> -					   XATTR_NAME_EVM,
>> -					   &data.hdr.xattr.data[1],
>> -					   SHA1_DIGEST_SIZE + 1, 0);
>> +		rc = __vfs_setxattr(&init_user_ns, dentry, d_inode(dentry),
>> +				    XATTR_NAME_EVM, &data.hdr.xattr.data[1],
>> +				    SHA1_DIGEST_SIZE + 1, 0);
>>  	} else if (rc == -ENODATA && (inode->i_opflags & IOP_XATTR)) {
>>  		rc = __vfs_removexattr(&init_user_ns, dentry, XATTR_NAME_EVM);
>>  	}
>> diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
>> index ee6f7e237f2e..d2de9dc6c345 100644
>> --- a/security/integrity/ima/ima_appraise.c
>> +++ b/security/integrity/ima/ima_appraise.c
>> @@ -98,10 +98,10 @@ static int ima_fix_xattr(struct dentry *dentry,
>>  		iint->ima_hash->xattr.ng.type = IMA_XATTR_DIGEST_NG;
>>  		iint->ima_hash->xattr.ng.algo = algo;
>>  	}
>> -	rc = __vfs_setxattr_noperm(&init_user_ns, dentry, XATTR_NAME_IMA,
>> -				   &iint->ima_hash->xattr.data[offset],
>> -				   (sizeof(iint->ima_hash->xattr) - offset) +
>> -				   iint->ima_hash->length, 0);
>> +	rc = __vfs_setxattr(&init_user_ns, dentry, d_inode(dentry),
>> +			    XATTR_NAME_IMA, &iint->ima_hash->xattr.data[offset],
>> +			    (sizeof(iint->ima_hash->xattr) - offset) +
>> +			    iint->ima_hash->length, 0);
>>  	return rc;
>>  }
>>  
> 
> 

-- 
Best
GUO Zihua