linux-kernel - Re: [PATCH v6 4/6] security: Allow all LSMs to provide xattrs for inode_init

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Date:   Thu, 24 Nov 2022 09:17:51 +0100
From:   Roberto Sassu <roberto.sassu@...weicloud.com>
To:     Mimi Zohar <zohar@...ux.ibm.com>, mark@...heh.com,
        jlbec@...lplan.org, joseph.qi@...ux.alibaba.com,
        dmitry.kasatkin@...il.com, paul@...l-moore.com, jmorris@...ei.org,
        serge@...lyn.com, stephen.smalley.work@...il.com,
        eparis@...isplace.org, casey@...aufler-ca.com
Cc:     ocfs2-devel@....oracle.com, reiserfs-devel@...r.kernel.org,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org, selinux@...r.kernel.org,
        linux-kernel@...r.kernel.org, keescook@...omium.org,
        nicolas.bouchinet@...p-os.org,
        Roberto Sassu <roberto.sassu@...wei.com>
Subject: Re: [PATCH v6 4/6] security: Allow all LSMs to provide xattrs for
 inode_init_security hook

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.

Thanks

Roberto

> thanks,
> 
> Mimi
> 
> > +               /*
> > +                * 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