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

[Roberto Sassu]

On 6/9/2023 9:35 PM, Mimi Zohar wrote:
> 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.

Casey asked to declare the variable in this block.

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

I revisited this part and made few fixes:

https://lore.kernel.org/linux-security-module/20230607123612.2791303-1-roberto.sassu@xxxxxxxxxxxxxxx/

Patch 3 should address your concern.

> >   			/*
> >   			 * 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.

Thanks, I didn't know about it.

It seems that they are more stylistic things. Probably, not worth to 
respin the patch set just for those (unless you prefer I do it).

Thanks

Roberto

> > +				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;
> >   		}