Re: [PATCH v10 3/4] evm: Align evm_inode_init_security() definition with LSM infrastructure

[Roberto Sassu]

On Tue, 2023-04-11 at 03:22 -0400, Mimi Zohar wrote:
> On Fri, 2023-03-31 at 14:32 +0200, Roberto Sassu wrote:
> > From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> > 
> > Change the evm_inode_init_security() definition to align with the LSM
> > infrastructure. Keep the existing behavior of including in the HMAC
> > calculation only the first xattr provided by LSMs.
> > 
> > Changing the evm_inode_init_security() definition requires passing the
> > xattr array allocated by security_inode_init_security(), and the number of
> > xattrs filled by previously invoked LSMs.
> > 
> > Use the newly introduced lsm_get_xattr_slot() to position EVM correctly in
> > the xattrs array, like a regular LSM, and to increment the number of filled
> > slots. For now, the LSM infrastructure allocates enough xattrs slots to
> > store the EVM xattr, without using the reservation mechanism.
> > 
> > Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> > ---
> >  include/linux/evm.h               | 13 +++++++------
> >  security/integrity/evm/evm_main.c | 16 ++++++++++------
> >  security/security.c               |  6 +++---
> >  3 files changed, 20 insertions(+), 15 deletions(-)
> > 
> > diff --git a/include/linux/evm.h b/include/linux/evm.h
> > index 7dc1ee74169..597632c71c7 100644
> > --- a/include/linux/evm.h
> > +++ b/include/linux/evm.h
> > @@ -56,9 +56,9 @@ static inline void evm_inode_post_set_acl(struct dentry *dentry,
> >  {
> >  	return evm_inode_post_setxattr(dentry, acl_name, NULL, 0);
> >  }
> > -extern int evm_inode_init_security(struct inode *inode,
> > -				   const struct xattr *xattr_array,
> > -				   struct xattr *evm);
> > +extern int evm_inode_init_security(struct inode *inode, struct inode *dir,
> > +				   const struct qstr *qstr,
> > +				   struct xattr *xattrs, int *xattr_count);
> >  extern bool evm_revalidate_status(const char *xattr_name);
> >  extern int evm_protected_xattr_if_enabled(const char *req_xattr_name);
> >  extern int evm_read_protected_xattrs(struct dentry *dentry, u8 *buffer,
> > @@ -157,9 +157,10 @@ static inline void evm_inode_post_set_acl(struct dentry *dentry,
> >  	return;
> >  }
> >  
> > -static inline int evm_inode_init_security(struct inode *inode,
> > -					  const struct xattr *xattr_array,
> > -					  struct xattr *evm)
> > +static inline int evm_inode_init_security(struct inode *inode, struct inode *dir,
> > +					  const struct qstr *qstr,
> > +					  struct xattr *xattrs,
> > +					  int *xattr_count)
> >  {
> >  	return 0;
> >  }
> > diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
> > index cf24c525558..475196ce712 100644
> > --- a/security/integrity/evm/evm_main.c
> > +++ b/security/integrity/evm/evm_main.c
> > @@ -21,6 +21,7 @@
> >  #include <linux/evm.h>
> >  #include <linux/magic.h>
> >  #include <linux/posix_acl_xattr.h>
> > +#include <linux/lsm_hooks.h>
> >  
> >  #include <crypto/hash.h>
> >  #include <crypto/hash_info.h>
> > @@ -864,23 +865,26 @@ void evm_inode_post_setattr(struct dentry *dentry, int ia_valid)
> >  /*
> >   * evm_inode_init_security - initializes security.evm HMAC value
> >   */
> > -int evm_inode_init_security(struct inode *inode,
> > -				 const struct xattr *lsm_xattr,
> > -				 struct xattr *evm_xattr)
> > +int evm_inode_init_security(struct inode *inode, struct inode *dir,
> > +			    const struct qstr *qstr, struct xattr *xattrs,
> > +			    int *xattr_count)
> >  {
> >  	struct evm_xattr *xattr_data;
> > +	struct xattr *evm_xattr;
> >  	int rc;
> >  
> > -	if (!(evm_initialized & EVM_INIT_HMAC) ||
> > -	    !evm_protected_xattr(lsm_xattr->name))
> > +	if (!(evm_initialized & EVM_INIT_HMAC) || !xattrs ||
> > +	    !evm_protected_xattr(xattrs->name))
> >  		return 0;
> >  
> > +	evm_xattr = lsm_get_xattr_slot(xattrs, xattr_count);
> > +
> >  	xattr_data = kzalloc(sizeof(*xattr_data), GFP_NOFS);
> >  	if (!xattr_data)
> >  		return -ENOMEM;
> >  
> >  	xattr_data->data.type = EVM_XATTR_HMAC;
> > -	rc = evm_init_hmac(inode, lsm_xattr, xattr_data->digest);
> > +	rc = evm_init_hmac(inode, xattrs, xattr_data->digest);
> >  	if (rc < 0)
> >  		goto out;
> >  
> > diff --git a/security/security.c b/security/security.c
> > index 1aeaa8ce449..ef7779ec8b2 100644
> > --- a/security/security.c
> > +++ b/security/security.c
> > @@ -1645,9 +1645,9 @@ int security_inode_init_security(struct inode *inode, struct inode *dir,
> >  	if (!xattr_count)
> >  		goto out;
> >  
> > -	ret = evm_inode_init_security(inode, new_xattrs,
> > -				      new_xattrs + xattr_count);
> > -	if (ret)
> > +	ret = evm_inode_init_security(inode, dir, qstr, new_xattrs,
> > +				      &xattr_count);
> > +	if (ret && ret != -EOPNOTSUPP)
> 
> As previously discussed, -EOPNOTSUPP originally meant that the
> filesystem itself did not support writing xattrs.  So there was no
> point in trying to write the EVM security xattr.   With the change in
> -EOPNOTSUPP meaning, it will now try to write the EVM security xattr. 
> Instead of glossing over the change in behavior, it needs to at least
> be mentioned in the patch description.

Oh, my mistake. I forgot to update this part (evm_inode_init_security()
now returns zero instead of -EOPNOTSUPP).

Thanks

Roberto

> >  		goto out;
> >  	ret = initxattrs(inode, new_xattrs, fs_data);
> >  out: