Re: [PATCH v11 2/4] smack: Set the SMACK64TRANSMUTE xattr in smack_inode_init_security()

[Mimi Zohar]

Hi Roberto,

On Sat, 2023-06-03 at 21:15 +0200, Roberto Sassu wrote:
> From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> 
> With the newly added ability of LSMs to supply multiple xattrs, set
> SMACK64TRASMUTE in smack_inode_init_security(), instead of d_instantiate().
> Do it by incrementing SMACK_INODE_INIT_XATTRS to 2 and by calling
> lsm_get_xattr_slot() a second time, if the transmuting conditions are met.
> 
> The LSM infrastructure passes all xattrs provided by LSMs to the
> filesystems through the initxattrs() callback, so that filesystems can
> store xattrs in the disk.
> 
> After the change, the SMK_INODE_TRANSMUTE inode flag is always set by
> d_instantiate() after fetching SMACK64TRANSMUTE from the disk. Before it
> was done by smack_inode_post_setxattr() as result of the __vfs_setxattr()
> call.
> 
> Removing __vfs_setxattr() also prevents invalidating the EVM HMAC, by
> adding a new xattr without checking and updating the existing HMAC.
> 
> Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>

Just a few comments/nits inline.

> ---
>  security/smack/smack.h     |  2 +-
>  security/smack/smack_lsm.c | 43 +++++++++++++++++++++++---------------
>  2 files changed, 27 insertions(+), 18 deletions(-)
> 
> diff --git a/security/smack/smack.h b/security/smack/smack.h
> index aa15ff56ed6..041688e5a77 100644
> --- a/security/smack/smack.h
> +++ b/security/smack/smack.h
> @@ -128,7 +128,7 @@ 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) */
>  #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 a1c30275692..b67d901ee74 100644
> --- a/security/smack/smack_lsm.c
> +++ b/security/smack/smack_lsm.c
> @@ -52,7 +52,14 @@
>  #define SMK_RECEIVING	1
>  #define SMK_SENDING	2
>  
> -#define SMACK_INODE_INIT_XATTRS 1
> +/*
> + * Smack uses multiple xattrs.
> + * SMACK64 - for access control,
> + * SMACK64TRANSMUTE - label initialization,
> + * Not saved on files - SMACK64IPIN and SMACK64IPOUT,
> + * Must be set explicitly - SMACK64EXEC and SMACK64MMAP
> + */
> +#define SMACK_INODE_INIT_XATTRS 2
>  
>  #ifdef SMACK_IPV6_PORT_LABELING
>  static DEFINE_MUTEX(smack_ipv6_lock);
> @@ -935,7 +942,6 @@ static int smack_inode_init_security(struct inode *inode, struct inode *dir,
>  				     struct xattr *xattrs, int *xattr_count)
>  {
>  	struct task_smack *tsp = smack_cred(current_cred());
> -	struct inode_smack *issp = smack_inode(inode);
>  	struct smack_known *skp = smk_of_task(tsp);
>  	struct smack_known *isp = smk_of_inode(inode);
>  	struct smack_known *dsp = smk_of_inode(dir);
> @@ -963,6 +969,8 @@ static int smack_inode_init_security(struct inode *inode, struct inode *dir,
>  		if ((tsp->smk_task == tsp->smk_transmuted) ||
>  		    (may > 0 && ((may & MAY_TRANSMUTE) != 0) &&
>  		     smk_inode_transmutable(dir))) {
> +			struct xattr *xattr_transmute;
> +

Variables should be defined at the beginning of the function.

Is there a reason for beginning the function with "if (xattr) {"
instead "if (!xattr) return 0;".  This causes unnecessary indenting.  

>  			/*
>  			 * The caller of smack_dentry_create_files_as()
>  			 * should have overridden the current cred, so the
> @@ -971,7 +979,16 @@ static int smack_inode_init_security(struct inode *inode, struct inode *dir,
>  			 */
>  			if (tsp->smk_task != tsp->smk_transmuted)
>  				isp = dsp;
> -			issp->smk_flags |= SMK_INODE_CHANGED;
> +			xattr_transmute = lsm_get_xattr_slot(xattrs, xattr_count);
> +			if (xattr_transmute) {
> +				xattr_transmute->value = kmemdup(TRANS_TRUE,
> +						TRANS_TRUE_SIZE, GFP_NOFS);

script/checkpatch --strict complains here.

> +				if (xattr_transmute->value == NULL)
> +					return -ENOMEM;
> +
> +				xattr_transmute->value_len = TRANS_TRUE_SIZE;
> +				xattr_transmute->name = XATTR_SMACK_TRANSMUTE;
> +			}
>  		}
>  
>  		xattr->value = kstrdup(isp->smk_known, GFP_NOFS);
> @@ -3518,20 +3535,12 @@ 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;
>  			if (rc >= 0)
>  				transflag = SMK_INODE_TRANSMUTE;
>  		}