Re: [PATCH] Smack modifications for: security: Allow all LSMs to provide xattrs for inode_init_security hook

[Casey Schaufler <casey@schaufler-ca.com>]

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@huawei.com>
>>>>>
>>>>> Very very quick modification. Not tested.
>>>>>
>>>>> Signed-off-by: Roberto Sassu <roberto.sassu@huawei.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. 

>
> 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).
>>>
>>> After fixing that, Smack tests succeed:
>> It's over a decade since I seriously looked at this code path,
>> 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;
>>>>>  		}