Date:   Tue, 18 Apr 2023 09:02:12 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Roberto Sassu <roberto.sassu@...weicloud.com>, zohar@...ux.ibm.com,
        dmitry.kasatkin@...il.com, paul@...l-moore.com, jmorris@...ei.org,
        serge@...lyn.com, stephen.smalley.work@...il.com,
        eparis@...isplace.org
Cc:     reiserfs-devel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org, selinux@...r.kernel.org,
        bpf@...r.kernel.org, kpsingh@...nel.org, keescook@...omium.org,
        nicolas.bouchinet@...p-os.org,
        Roberto Sassu <roberto.sassu@...wei.com>,
        Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [PATCH] Smack modifications for: security: Allow all LSMs to
 provide xattrs for inode_init_security hook

On 4/18/2023 12:05 AM, Roberto Sassu wrote:
> On Mon, 2023-04-17 at 09:41 -0700, Casey Schaufler wrote:
>> On 4/13/2023 12:11 AM, Roberto Sassu wrote:
>>> On Wed, 2023-04-12 at 13:29 -0700, Casey Schaufler wrote:
>>>> On 4/12/2023 12:22 AM, Roberto Sassu wrote:
>>>>> On Tue, 2023-04-11 at 10:54 -0700, Casey Schaufler wrote:
>>>>>> On 4/11/2023 10:23 AM, Roberto Sassu wrote:
>>>>>>> From: Roberto Sassu <roberto.sassu@...wei.com>
>>>>>>>
>>>>>>> Very very quick modification. Not tested.
>>>>>>>
>>>>>>> Signed-off-by: Roberto Sassu <roberto.sassu@...wei.com>
>>>>>>> ---
>>>>>>>  security/smack/smack.h     |  2 +-
>>>>>>>  security/smack/smack_lsm.c | 42 ++++++++++++++++++++------------------
>>>>>>>  2 files changed, 23 insertions(+), 21 deletions(-)
>>>>>>>
>>>>>>> diff --git a/security/smack/smack.h b/security/smack/smack.h
>>>>>>> index e2239be7bd6..f00c8498c60 100644
>>>>>>> --- a/security/smack/smack.h
>>>>>>> +++ b/security/smack/smack.h
>>>>>>> @@ -127,7 +127,7 @@ struct task_smack {
>>>>>>>  
>>>>>>>  #define	SMK_INODE_INSTANT	0x01	/* inode is instantiated */
>>>>>>>  #define	SMK_INODE_TRANSMUTE	0x02	/* directory is transmuting */
>>>>>>> -#define	SMK_INODE_CHANGED	0x04	/* smack was transmuted */
>>>>>>> +#define	SMK_INODE_CHANGED	0x04	/* smack was transmuted (unused) */
>>>>>> See below ...
>>>>>>
>>>>>>>  #define	SMK_INODE_IMPURE	0x08	/* involved in an impure transaction */
>>>>>>>  
>>>>>>>  /*
>>>>>>> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
>>>>>>> index 8392983334b..b43820bdbd0 100644
>>>>>>> --- a/security/smack/smack_lsm.c
>>>>>>> +++ b/security/smack/smack_lsm.c
>>>>>>> @@ -54,12 +54,12 @@
>>>>>>>  
>>>>>>>  /*
>>>>>>>   * Smack uses multiple xattrs.
>>>>>>> - * SMACK64 - for access control, SMACK64EXEC - label for the program,
>>>>>>> - * SMACK64MMAP - controls library loading,
>>>>>>> + * SMACK64 - for access control,
>>>>>>>   * SMACK64TRANSMUTE - label initialization,
>>>>>>> - * Not saved on files - SMACK64IPIN and SMACK64IPOUT
>>>>>>> + * Not saved on files - SMACK64IPIN and SMACK64IPOUT,
>>>>>>> + * Must be set explicitly - SMACK64EXEC and SMACK64MMAP
>>>>>>>   */
>>>>>>> -#define SMACK_INODE_INIT_XATTRS 4
>>>>>>> +#define SMACK_INODE_INIT_XATTRS 2
>>>>>>>  
>>>>>>>  #ifdef SMACK_IPV6_PORT_LABELING
>>>>>>>  static DEFINE_MUTEX(smack_ipv6_lock);
>>>>>>> @@ -957,11 +957,11 @@ static int smack_inode_init_security(struct inode *inode, struct inode *dir,
>>>>>>>  				     const struct qstr *qstr,
>>>>>>>  				     struct xattr *xattrs, int *xattr_count)
>>>>>>>  {
>>>>>>> -	struct inode_smack *issp = smack_inode(inode);
>>>>>>>  	struct smack_known *skp = smk_of_current();
>>>>>>>  	struct smack_known *isp = smk_of_inode(inode);
>>>>>>>  	struct smack_known *dsp = smk_of_inode(dir);
>>>>>>>  	struct xattr *xattr = lsm_get_xattr_slot(xattrs, xattr_count);
>>>>>>> +	struct xattr *xattr2;
>>>>>> I'm going to channel Paul and suggest this be xattr_transmute instead of xattr2.
>>>>>> It also looks like it could move to be declared in the if clause.
>>>>>>
>>>>>>>  	int may;
>>>>>>>  
>>>>>>>  	if (xattr) {
>>>>>>> @@ -979,7 +979,17 @@ static int smack_inode_init_security(struct inode *inode, struct inode *dir,
>>>>>>>  		if (may > 0 && ((may & MAY_TRANSMUTE) != 0) &&
>>>>>>>  		    smk_inode_transmutable(dir)) {
>>>>>>>  			isp = dsp;
>>>>>>> -			issp->smk_flags |= SMK_INODE_CHANGED;
>>>>>> I think you need to keep this. More below.
>>>>>>
>>>>>>> +			xattr2 = lsm_get_xattr_slot(xattrs, xattr_count);
>>>>>>> +			if (xattr2) {
>>>>>>> +				xattr2->value = kmemdup(TRANS_TRUE,
>>>>>>> +							TRANS_TRUE_SIZE,
>>>>>>> +							GFP_NOFS);
>>>>>>> +				if (xattr2->value == NULL)
>>>>>>> +					return -ENOMEM;
>>>>>>> +
>>>>>>> +				xattr2->value_len = TRANS_TRUE_SIZE;
>>>>>>> +				xattr2->name = XATTR_NAME_SMACKTRANSMUTE;
>>>>>>> +			}
>>>>>>>  		}
>>>>>>>  
>>>>>>>  		xattr->value = kstrdup(isp->smk_known, GFP_NOFS);
>>>>>>> @@ -3512,20 +3522,12 @@ static void smack_d_instantiate(struct dentry *opt_dentry, struct inode *inode)
>>>>>>>  			 * If there is a transmute attribute on the
>>>>>>>  			 * directory mark the inode.
>>>>>>>  			 */
>>>>>>> -			if (isp->smk_flags & SMK_INODE_CHANGED) {
>>>>>>> -				isp->smk_flags &= ~SMK_INODE_CHANGED;
>>>>>>> -				rc = __vfs_setxattr(&nop_mnt_idmap, dp, inode,
>>>>>>> -					XATTR_NAME_SMACKTRANSMUTE,
>>>>>>> -					TRANS_TRUE, TRANS_TRUE_SIZE,
>>>>>>> -					0);
>>>>>>> -			} else {
>>>>>>> -				rc = __vfs_getxattr(dp, inode,
>>>>>>> -					XATTR_NAME_SMACKTRANSMUTE, trattr,
>>>>>>> -					TRANS_TRUE_SIZE);
>>>>>>> -				if (rc >= 0 && strncmp(trattr, TRANS_TRUE,
>>>>>>> -						       TRANS_TRUE_SIZE) != 0)
>>>>>>> -					rc = -EINVAL;
>>>>>>> -			}
>>>>>>> +			rc = __vfs_getxattr(dp, inode,
>>>>>>> +					    XATTR_NAME_SMACKTRANSMUTE, trattr,
>>>>>>> +					    TRANS_TRUE_SIZE);
>>>>>>> +			if (rc >= 0 && strncmp(trattr, TRANS_TRUE,
>>>>>>> +					       TRANS_TRUE_SIZE) != 0)
>>>>>>> +				rc = -EINVAL;
>>>>>> Where is the SMACK64_TRANSMUTE attribute going to get set on the file?
>>>>>> It's not going to get set in smack_init_inode_security(). The inode will
>>>>> Isn't that the purpose of the inode_init_security hook?
>>>> No. It initializes the in-memory inode. 
>>> I hope I'm not mistaken here...
>>>
>>> I make a small example. Filesystems call
>>> security_inode_init_security(). Ext4 does:
>>>
>>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/ext4/xattr_security.c?h=v6.3-rc6#n54
>>>
>>> security_inode_init_security() allocates new_xattrs. Each LSM fills
>>> new_xattrs. At the end of the loop, if there is at least one xattr
>>> filled, the initxattrs() callback passed by the caller of
>>> security_inode_init_security() is called.
>>>
>>> The ext4 initxattrs() callback is:
>>>
>>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/ext4/xattr_security.c?h=v6.3-rc6#n35
>>>
>>> which scans the xattr array and, for each xattr,
>>> calls ext4_xattr_set_handle().
>>>
>>> Maybe I'm overlooking it, but ext4_xattr_set_handle() is setting xattrs
>>> on the disk. Am I wrong?
>> Yes, you're wrong. I tried your change, and the SMACK64_TRANSMUTE isn't
>> set on the sub-directory when it's created. The __vfs_setxattr() call really
>> is necessary. 
> Could you please also check if there is any change with this fix:
>
> Replace:
>
> 	xattr2->name = XATTR_NAME_SMACKTRANSMUTE;
>
> with:
>
> 	xattr2->name = XATTR_SMACK_TRANSMUTE;
>
> Thanks

Unless I'm missing something really obvious there's no way that any
of the evm/ima changes would obviate the need for the __vfs_setxattr() call.
It's real easy to verify correct behavior, see the attached script.
(you'll want to change the "notroot" value to a user on your system)

>
> Roberto
>
>>> Thanks
>>>
>>> Roberto
>>>
>>>>> After all LSMs provide one or multiple xattrs, xattrs are going to be
>>>>> written to the disk with the initxattr() callback of filesystems.
>>>>>
>>>>> There is a small mistake above (XATTR_SMACK_TRANSMUTE instead
>>>>> of XATTR_NAME_SMACKTRANSMUTE, as we are providing just the suffix).
>>>> but I'm pretty sure the __vfs_setxattr() call is necessary to get
>>>> the attribute written out. With your change the in-memory inode will
>>>> get the attribute, but if you reboot it won't be on the directory.
>>>>
>>>>> 95 Passed, 0 Failed, 100% Success rate
>>>>>
>>>>> There was a test failing in dir-transmute.sh, before I fixed the xattr
>>>>> name.
>>>>>
>>>>> Thanks
>>>>>
>>>>> Roberto
>>>>>
>>>>>> know it's transmuting, but it won't get to disk without the __vfs_setxattr()
>>>>>> here in smack_d_instantiate(). Now, it's been a long time since that code
>>>>>> was written, so I could be wrong, but I'm pretty sure about that.
>>>>>>
>>>>>> I think that you should be fine with the changes in smack_init_inode_security(),
>>>>>> and leaving smack_d_instantiate() untouched. 
>>>>>>
>>>>>>>  			if (rc >= 0)
>>>>>>>  				transflag = SMK_INODE_TRANSMUTE;
>>>>>>>  		}
View attachment "testtransmute.sh" of type "text/plain" (9694 bytes)

Powered by blists - more mailing lists