linux-kernel - Re: [PATCH] Smack: check for SMACK xattr max size in smack_inode

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Date:	Wed, 15 Apr 2009 08:26:07 +0200
From:	Etienne Basset <etienne.basset@...ericable.fr>
To:	Casey Schaufler <casey@...aufler-ca.com>
CC:	LSM <linux-security-module@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Smack: check for SMACK xattr max size in  smack_inode_setxattr

Casey Schaufler wrote:
> Etienne Basset wrote:
>> the following patch move the check for SMACK xattr size >= SMK_LABELLEN
>> from  smack_inode_post_setxattr (which cannot return an error to the user)
>> to smack_inode_setxattr (which can return an error).
>>
>> without patch a SMACK setxattr with size >= SMK_LABELLEN returns success although it doesn't 
>>   
> 
> How about an early call to smk_import() to determine if the
> label if legitimate, rather than checking the length? That
> could save grief for other "invalid label" issues.

Hi,

both are needed. And we'll need one call in setxattr to check the validity and one in
postsetxattr/ inode_setsecurity (to set the security blob)
(which make me think we should split smk_import in 2,  smk_check/smk_import_nocheck or whatever)

The harder issue is that smk_import can change the value/size of the xattr
and smk_import can fail for 2 reasons : ENOMEM or EINVAL (not sure this is an issue though)
(we should really split)

root@...enne-desktop:/tmp# attr -S -s SMACK64 -V '123////' toto
Attribute "SMACK64" set to a 7 byte value for toto:
123////
root@...enne-desktop:/tmp# attr -S -g SMACK64 toto
Attribute "SMACK64" had a 4 byte value for toto:
123

maybe it would be nicer to fail than change the label under the foot of the-user-who-hasnt-read-the-doc?

Etienne
> 
>> Signed-off-by: Etienne Basset <etienne.basset@...ericable.fr>
>> ---
>> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
>> index 9215149..da6954d 100644
>> --- a/security/smack/smack_lsm.c
>> +++ b/security/smack/smack_lsm.c
>> @@ -610,7 +610,8 @@ static int smack_inode_setxattr(struct dentry *dentry, const char *name,
>>  		if (!capable(CAP_MAC_ADMIN))
>>  			rc = -EPERM;
>>  		/* a label cannot be void and cannot begin with '-' */
>> -		if (size == 0 || (size > 0 && ((char *)value)[0] == '-'))
>> +		if (size == 0 || size >= SMK_LABELLEN ||
>> +		   (size > 0 && ((char *)value)[0] == '-'))
>>  			rc = -EINVAL;
>>  	} else
>>  		rc = cap_inode_setxattr(dentry, name, value, size, flags);
>> @@ -644,9 +645,6 @@ static void smack_inode_post_setxattr(struct dentry *dentry, const char *name,
>>  	if (strcmp(name, XATTR_NAME_SMACK))
>>  		return;
>>  
>> -	if (size >= SMK_LABELLEN)
>> -		return;
>> -
>>  	isp = dentry->d_inode->i_security;
>>  
>>  	/*
>>
>> --
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>>
>>   
> 
> 

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