Re: [PATCH v6 4/6] security: Allow all LSMs to provide xattrs for inode_init_security hook

[Mimi Zohar]

On Thu, 2022-11-24 at 09:17 +0100, Roberto Sassu wrote:
> On Wed, 2022-11-23 at 20:14 -0500, Mimi Zohar wrote:
> > Hi Roberto,
> > 
> > On Wed, 2022-11-23 at 16:47 +0100, Roberto Sassu wrote:
> > >  int security_inode_init_security(struct inode *inode, struct inode *dir,
> > >                                  const struct qstr *qstr,
> > >                                  const initxattrs initxattrs, void *fs_data)
> > >  {
> > > -       struct xattr new_xattrs[MAX_LSM_EVM_XATTR + 1];
> > > -       struct xattr *lsm_xattr, *evm_xattr, *xattr;
> > > -       int ret;
> > > +       struct security_hook_list *P;
> > > +       struct xattr *new_xattrs;
> > > +       struct xattr *xattr;
> > > +       int ret = -EOPNOTSUPP, num_filled_xattrs = 0;
> > >  
> > >         if (unlikely(IS_PRIVATE(inode)))
> > >                 return 0;
> > >  
> > > +       if (!blob_sizes.lbs_xattr)
> > > +               return 0;
> > > +
> > >         if (!initxattrs)
> > >                 return call_int_hook(inode_init_security, -EOPNOTSUPP, inode,
> > > -                                    dir, qstr, NULL, NULL, NULL);
> > > -       memset(new_xattrs, 0, sizeof(new_xattrs));
> > > -       lsm_xattr = new_xattrs;
> > > -       ret = call_int_hook(inode_init_security, -EOPNOTSUPP, inode, dir, qstr,
> > > -                                               &lsm_xattr->name,
> > > -                                               &lsm_xattr->value,
> > > -                                               &lsm_xattr->value_len);
> > > -       if (ret)
> > > +                                   dir, qstr, NULL);
> > > +       /* Allocate +1 for EVM and +1 as terminator. */
> > > +       new_xattrs = kcalloc(blob_sizes.lbs_xattr + 2, sizeof(*new_xattrs),
> > > +                            GFP_NOFS);
> > > +       if (!new_xattrs)
> > > +               return -ENOMEM;
> > > +
> > > +       hlist_for_each_entry(P, &security_hook_heads.inode_init_security,
> > > +                            list) {
> > > +               ret = P->hook.inode_init_security(inode, dir, qstr, new_xattrs);
> > > +               if (ret && ret != -EOPNOTSUPP)
> > > +                       goto out;
> > > +               if (ret == -EOPNOTSUPP)
> > > +                       continue;
> > 
> > In this context, -EOPNOTSUPP originally signified that the filesystem
> > does not support writing xattrs.  Writing any xattr would fail. 
> > Returning -ENODATA for no LSM xattr(s) data would seem to be more
> > appropriate than -EOPNOTSUPP.
> 
> Hi Mimi
> 
> I thought about adding new return values. Currently only -EOPNOTSUPP
> and -ENOMEM are expected as errors.
> 
> However, changing the conventions would mean revisiting the LSMs code
> and ensuring that they follow the new conventions.
> 
> I would be more in favor of not touching it.

Casey, Paul, any comment?

> > 
> > > +               /*
> > > +                * As the number of xattrs reserved by LSMs is not directly
> > > +                * available, directly use the total number blob_sizes.lbs_xattr
> > > +                * to keep the code simple, while being not the most efficient
> > > +                * way.
> > > +                */
> > > +               ret = security_check_compact_filled_xattrs(new_xattrs,
> > > +                                                          blob_sizes.lbs_xattr,
> > > +                                                          &num_filled_xattrs);
> > > +               if (ret < 0) {
> > > +                       ret = -ENOMEM;
> > > +                       goto out;
> > > +               }
> > > +       }
> > > +
> > > +       if (!num_filled_xattrs)
> > >                 goto out;
> > >  
> > > -       evm_xattr = lsm_xattr + 1;
> > > -       ret = evm_inode_init_security(inode, lsm_xattr, evm_xattr);
> > > +       ret = evm_inode_init_security(inode, new_xattrs,
> > > +                                     new_xattrs + num_filled_xattrs);
> > >         if (ret)
> > >                 goto out;
> > >         ret = initxattrs(inode, new_xattrs, fs_data);
> > >  out:
> > >         for (xattr = new_xattrs; xattr->value != NULL; xattr++)
> > >                 kfree(xattr->value);
> > > +       kfree(new_xattrs);
> > >         return (ret == -EOPNOTSUPP) ? 0 : ret;
> > >  }
> > b
>