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

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Date:   Wed, 23 Nov 2022 20:14:01 -0500
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Roberto Sassu <roberto.sassu@...weicloud.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

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.

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