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

[Roberto Sassu]

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@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

Isn't that the purpose of the inode_init_security hook?

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:

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;
> >  		}