Re: [PATCH v9 3/4] evm: Align evm_inode_init_security() definition with LSM infrastructure

[Roberto Sassu <roberto.sassu@huaweicloud.com>]

On Thu, 2023-03-30 at 18:55 -0400, Paul Moore wrote:
> On Wed, Mar 29, 2023 at 9:05 AM Roberto Sassu
> <roberto.sassu@huaweicloud.com> wrote:
> > From: Roberto Sassu <roberto.sassu@huawei.com>
> > 
> > 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_find_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.
> > 
> > Finally, make evm_inode_init_security() return value compatible with the
> > inode_init_security hook conventions, i.e. return -EOPNOTSUPP if it is not
> > setting an xattr.
> > 
> > EVM is a bit tricky, because xattrs is both an input and an output. If it
> > was just output, EVM should have returned zero if xattrs is NULL. But,
> > since xattrs is also input, EVM is unable to do its calculations, so return
> > -EOPNOTSUPP and handle this error in security_inode_init_security().
> 
> I don't quite understand why EVM would return EOPNOTSUPP if it is
> enabled but there are not xattrs to measure.  It seems like EVM should
> return success/0 in the no-xattr case; there were no xattrs to
> measure, so it succeeded in measuring nothing.  Am I missing
> something?

From a very quick look at what other LSMs do, it seems that they return
zero even if they are not initialized.

So, it makes sense to return zero also here.

Thanks

Roberto

> > Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
> > ---
> >  include/linux/evm.h               | 14 ++++++++------
> >  security/integrity/evm/evm_main.c | 18 +++++++++++-------
> >  security/security.c               |  6 +++---
> >  3 files changed, 22 insertions(+), 16 deletions(-)
> > 
> > diff --git a/include/linux/evm.h b/include/linux/evm.h
> > index 7dc1ee74169..3c0e8591b69 100644
> > --- a/include/linux/evm.h
> > +++ b/include/linux/evm.h
> > @@ -56,9 +56,10 @@ 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 *num_filled_xattrs);
> >  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 +158,10 @@ 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 *num_filled_xattrs)
> >  {
> >         return 0;
> >  }
> > diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
> > index cf24c525558..9e75759150c 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 *num_filled_xattrs)
> >  {
> >         struct evm_xattr *xattr_data;
> > +       struct xattr *evm_xattr;
> >         int rc;
> > 
> > -       if (!(evm_initialized & EVM_INIT_HMAC) ||
> > -           !evm_protected_xattr(lsm_xattr->name))
> > -               return 0;
> > +       if (!(evm_initialized & EVM_INIT_HMAC) || !xattrs ||
> > +           !evm_protected_xattr(xattrs->name))
> > +               return -EOPNOTSUPP;
> > +
> > +       evm_xattr = lsm_find_xattr_slot(xattrs, num_filled_xattrs);
> > 
> >         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 be33d643a81..22ab4fb7ebf 100644
> > --- a/security/security.c
> > +++ b/security/security.c
> > @@ -1674,9 +1674,9 @@ int security_inode_init_security(struct inode *inode, struct inode *dir,
> >         if (!num_filled_xattrs)
> >                 goto out;
> > 
> > -       ret = evm_inode_init_security(inode, new_xattrs,
> > -                                     new_xattrs + num_filled_xattrs);
> > -       if (ret)
> > +       ret = evm_inode_init_security(inode, dir, qstr, new_xattrs,
> > +                                     &num_filled_xattrs);
> > +       if (ret && ret != -EOPNOTSUPP)
> >                 goto out;
> >         ret = initxattrs(inode, new_xattrs, fs_data);
> >  out:
> > --
> > 2.25.1
> > 
> 
>