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

[Casey Schaufler]

On 4/11/2023 10:23 AM, Roberto Sassu wrote:
> From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
>
> Very very quick modification. Not tested.
>
> Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> ---
>  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
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;
>  		}