Re: [PATCH v6 2/2] overlayfs: override_creds=off option bypass creator_cred

[Amir Goldstein]

On Mon, Nov 5, 2018 at 8:22 PM Mark Salyzyn <salyzyn@xxxxxxxxxxx> wrote:
>
> By default, all access to the upper, lower and work directories is the
> recorded mounter's MAC and DAC credentials.  The incoming accesses are
> checked against the caller's credentials.
>
> If the principles of least privilege are applied, the mounter's
> credentials might not overlap the credentials of the caller's when
> accessing the overlayfs filesystem.  For example, a file that a lower
> DAC privileged caller can execute, is MAC denied to the generally
> higher DAC privileged mounter, to prevent an attack vector.
>
> We add the option to turn off override_creds in the mount options; all
> subsequent operations after mount on the filesystem will be only the
> caller's credentials.  The module boolean parameter and mount option
> override_creds is also added as a presence check for this "feature",
> existence of /sys/module/overlay/parameters/overlay_creds.
>
> Signed-off-by: Mark Salyzyn <salyzyn@xxxxxxxxxxx>
> Cc: Miklos Szeredi <miklos@xxxxxxxxxx>
> Cc: Jonathan Corbet <corbet@xxxxxxx>
> Cc: Vivek Goyal <vgoyal@xxxxxxxxxx>
> Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> Cc: Amir Goldstein <amir73il@xxxxxxxxx>
> Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
> Cc: Stephen Smalley <sds@xxxxxxxxxxxxx>
> Cc: linux-unionfs@xxxxxxxxxxxxxxx
> Cc: linux-doc@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: kernel-team@xxxxxxxxxxx
>
> v2:
> - Forward port changed attr to stat, resulting in a build error.
> - altered commit message.
>
> v3:
> - Change name from caller_credentials / creator_credentials to the
>   boolean override_creds.
> - Changed from creator to mounter credentials.
> - Updated and fortified the documentation.
> - Added CONFIG_OVERLAY_FS_OVERRIDE_CREDS
>
> v4:
> - spelling and grammar errors in text
>
> v5:
> - beefed up the caveats in the Documentation
> - Is dependent on
>   "overlayfs: check CAP_DAC_READ_SEARCH before issuing exportfs_decode_fh"
>   "overlayfs: check CAP_MKNOD before issuing vfs_whiteout"
> - Added prwarn when override_creds=off
>
> v6:
> - Drop CONFIG_OVERLAY_FS_OVERRIDE_CREDS.
> - Do better with the documentation.
> - pr_warn message adjusted to report consequences.

same comment about patch revision - not in commit message.

> ---
>  Documentation/filesystems/overlayfs.txt | 17 +++++++++++++++++
>  fs/overlayfs/copy_up.c                  |  2 +-
>  fs/overlayfs/dir.c                      |  9 +++++----
>  fs/overlayfs/inode.c                    | 16 ++++++++--------
>  fs/overlayfs/namei.c                    |  6 +++---
>  fs/overlayfs/overlayfs.h                |  1 +
>  fs/overlayfs/ovl_entry.h                |  1 +
>  fs/overlayfs/readdir.c                  |  4 ++--
>  fs/overlayfs/super.c                    | 23 ++++++++++++++++++++++-
>  fs/overlayfs/util.c                     | 12 ++++++++++--
>  10 files changed, 70 insertions(+), 21 deletions(-)
>
> diff --git a/Documentation/filesystems/overlayfs.txt b/Documentation/filesystems/overlayfs.txt
> index eef7d9d259e8..5cc299df4436 100644
> --- a/Documentation/filesystems/overlayfs.txt
> +++ b/Documentation/filesystems/overlayfs.txt
> @@ -102,6 +102,23 @@ Only the lists of names from directories are merged.  Other content
>  such as metadata and extended attributes are reported for the upper
>  directory only.  These attributes of the lower directory are hidden.
>
> +credentials
> +-----------
> +
> +By default, all access to the upper, lower and work directories is the
> +recorded mounter's MAC and DAC credentials.  The incoming accesses are
> +checked against the caller's credentials.
> +
> +override_creds mount flag turned off is reserved for when mounter and
> +caller MAC or DAC credentials do not overlap.  Several unintended side
> +effects will occur.  The caller with a lower privilege will not be
> +able to delete files or directories, create nodes, or search some
> +directories.  The caller with higher privilege can perform unexpected
> +or unsecured operations.  The uneven security model where upperdir
> +and workdir are opened at privilege, but accessed without, should only
> +be used with strict understanding of the side effects and of the
> +security policies.
> +
>  whiteouts and opaque directories
>  --------------------------------
>
> diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
> index 9e62dcf06fc4..dfab62ce7504 100644
> --- a/fs/overlayfs/copy_up.c
> +++ b/fs/overlayfs/copy_up.c
> @@ -860,7 +860,7 @@ int ovl_copy_up_flags(struct dentry *dentry, int flags)
>                 dput(parent);
>                 dput(next);
>         }
> -       revert_creds(old_cred);
> +       ovl_revert_creds(old_cred);
>
>         return err;
>  }
> diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> index c6289147c787..b7052e23c467 100644
> --- a/fs/overlayfs/dir.c
> +++ b/fs/overlayfs/dir.c
> @@ -566,7 +566,8 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
>                 override_cred->fsgid = inode->i_gid;
>                 if (!attr->hardlink) {
>                         err = security_dentry_create_files_as(dentry,
> -                                       attr->mode, &dentry->d_name, old_cred,
> +                                       attr->mode, &dentry->d_name,
> +                                       old_cred ? old_cred : current_cred(),
>                                         override_cred);
>                         if (err) {
>                                 put_cred(override_cred);
> @@ -582,7 +583,7 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
>                         err = ovl_create_over_whiteout(dentry, inode, attr);
>         }
>  out_revert_creds:
> -       revert_creds(old_cred);
> +       ovl_revert_creds(old_cred);
>         return err;
>  }
>
> @@ -842,7 +843,7 @@ static int ovl_do_remove(struct dentry *dentry, bool is_dir)
>                 err = ovl_remove_upper(dentry, is_dir, &list);
>         else
>                 err = ovl_remove_and_whiteout(dentry, &list);
> -       revert_creds(old_cred);
> +       ovl_revert_creds(old_cred);
>         if (!err) {
>                 if (is_dir)
>                         clear_nlink(dentry->d_inode);
> @@ -1212,7 +1213,7 @@ static int ovl_rename(struct inode *olddir, struct dentry *old,
>  out_unlock:
>         unlock_rename(new_upperdir, old_upperdir);
>  out_revert_creds:
> -       revert_creds(old_cred);
> +       ovl_revert_creds(old_cred);
>         if (update_nlink)
>                 ovl_nlink_end(new);
>  out_drop_write:
> diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
> index 6bcc9dedc342..192f5508ed45 100644
> --- a/fs/overlayfs/inode.c
> +++ b/fs/overlayfs/inode.c
> @@ -64,7 +64,7 @@ int ovl_setattr(struct dentry *dentry, struct iattr *attr)
>                 inode_lock(upperdentry->d_inode);
>                 old_cred = ovl_override_creds(dentry->d_sb);
>                 err = notify_change(upperdentry, attr, NULL);
> -               revert_creds(old_cred);
> +               ovl_revert_creds(old_cred);
>                 if (!err)
>                         ovl_copyattr(upperdentry->d_inode, dentry->d_inode);
>                 inode_unlock(upperdentry->d_inode);
> @@ -260,7 +260,7 @@ int ovl_getattr(const struct path *path, struct kstat *stat,
>                 stat->nlink = dentry->d_inode->i_nlink;
>
>  out:
> -       revert_creds(old_cred);
> +       ovl_revert_creds(old_cred);
>
>         return err;
>  }
> @@ -303,7 +303,7 @@ int ovl_permission(struct inode *inode, int mask)
>
>         old_cred = ovl_override_creds(inode->i_sb);
>         err = inode_permission(realinode, mask);
> -       revert_creds(old_cred);
> +       ovl_revert_creds(old_cred);
>
>         return err;
>  }
> @@ -320,7 +320,7 @@ static const char *ovl_get_link(struct dentry *dentry,
>
>         old_cred = ovl_override_creds(dentry->d_sb);
>         p = vfs_get_link(ovl_dentry_real(dentry), done);
> -       revert_creds(old_cred);
> +       ovl_revert_creds(old_cred);
>         return p;
>  }
>
> @@ -363,7 +363,7 @@ int ovl_xattr_set(struct dentry *dentry, struct inode *inode, const char *name,
>                 WARN_ON(flags != XATTR_REPLACE);
>                 err = vfs_removexattr(realdentry, name);
>         }
> -       revert_creds(old_cred);
> +       ovl_revert_creds(old_cred);
>
>         /* copy c/mtime */
>         ovl_copyattr(d_inode(realdentry), inode);
> @@ -384,7 +384,7 @@ int ovl_xattr_get(struct dentry *dentry, struct inode *inode, const char *name,
>
>         old_cred = ovl_override_creds(dentry->d_sb);
>         res = vfs_getxattr(realdentry, name, value, size);
> -       revert_creds(old_cred);
> +       ovl_revert_creds(old_cred);
>         return res;
>  }
>
> @@ -408,7 +408,7 @@ ssize_t ovl_listxattr(struct dentry *dentry, char *list, size_t size)
>
>         old_cred = ovl_override_creds(dentry->d_sb);
>         res = vfs_listxattr(realdentry, list, size);
> -       revert_creds(old_cred);
> +       ovl_revert_creds(old_cred);
>         if (res <= 0 || size == 0)
>                 return res;
>
> @@ -443,7 +443,7 @@ struct posix_acl *ovl_get_acl(struct inode *inode, int type)
>
>         old_cred = ovl_override_creds(inode->i_sb);
>         acl = get_acl(realinode, type);
> -       revert_creds(old_cred);
> +       ovl_revert_creds(old_cred);
>
>         return acl;
>  }
> diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
> index aa012b6bd46e..b73e5f7aea2e 100644
> --- a/fs/overlayfs/namei.c
> +++ b/fs/overlayfs/namei.c
> @@ -1074,7 +1074,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
>                         goto out_free_oe;
>         }
>
> -       revert_creds(old_cred);
> +       ovl_revert_creds(old_cred);
>         if (origin_path) {
>                 dput(origin_path->dentry);
>                 kfree(origin_path);
> @@ -1101,7 +1101,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
>         kfree(upperredirect);
>  out:
>         kfree(d.redirect);
> -       revert_creds(old_cred);
> +       ovl_revert_creds(old_cred);
>         return ERR_PTR(err);
>  }
>
> @@ -1155,7 +1155,7 @@ bool ovl_lower_positive(struct dentry *dentry)
>                         dput(this);
>                 }
>         }
> -       revert_creds(old_cred);
> +       ovl_revert_creds(old_cred);
>
>         return positive;
>  }
> diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
> index 5e45cb3630a0..6f8b6f9ff357 100644
> --- a/fs/overlayfs/overlayfs.h
> +++ b/fs/overlayfs/overlayfs.h
> @@ -208,6 +208,7 @@ int ovl_want_write(struct dentry *dentry);
>  void ovl_drop_write(struct dentry *dentry);
>  struct dentry *ovl_workdir(struct dentry *dentry);
>  const struct cred *ovl_override_creds(struct super_block *sb);
> +void ovl_revert_creds(const struct cred *oldcred);
>  struct super_block *ovl_same_sb(struct super_block *sb);
>  int ovl_can_decode_fh(struct super_block *sb);
>  struct dentry *ovl_indexdir(struct super_block *sb);
> diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
> index ec237035333a..e38eea8104be 100644
> --- a/fs/overlayfs/ovl_entry.h
> +++ b/fs/overlayfs/ovl_entry.h
> @@ -20,6 +20,7 @@ struct ovl_config {
>         bool nfs_export;
>         int xino;
>         bool metacopy;
> +       bool override_creds;
>  };
>
>  struct ovl_sb {
> diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
> index cc8303a806b4..ec591b49e902 100644
> --- a/fs/overlayfs/readdir.c
> +++ b/fs/overlayfs/readdir.c
> @@ -289,7 +289,7 @@ static int ovl_check_whiteouts(struct dentry *dir, struct ovl_readdir_data *rdd)
>                 }
>                 inode_unlock(dir->d_inode);
>         }
> -       revert_creds(old_cred);
> +       ovl_revert_creds(old_cred);
>
>         return err;
>  }
> @@ -921,7 +921,7 @@ int ovl_check_empty_dir(struct dentry *dentry, struct list_head *list)
>
>         old_cred = ovl_override_creds(dentry->d_sb);
>         err = ovl_dir_read_merged(dentry, list, &root);
> -       revert_creds(old_cred);
> +       ovl_revert_creds(old_cred);
>         if (err)
>                 return err;
>
> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> index 0116735cc321..1669d4fa7ad8 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -56,6 +56,11 @@ module_param_named(xino_auto, ovl_xino_auto_def, bool, 0644);
>  MODULE_PARM_DESC(ovl_xino_auto_def,
>                  "Auto enable xino feature");
>
> +static bool __read_mostly ovl_default_override_creds = true;

Please stick to conventions - ovl_override_creds_def.

> +module_param_named(override_creds, ovl_default_override_creds, bool, 0644);
> +MODULE_PARM_DESC(ovl_default_override_creds,
> +                "Use mounter's credentials for accesses");
> +
>  static void ovl_entry_stack_free(struct ovl_entry *oe)
>  {
>         unsigned int i;
> @@ -362,6 +367,8 @@ static int ovl_show_options(struct seq_file *m, struct dentry *dentry)
>         if (ofs->config.metacopy != ovl_metacopy_def)
>                 seq_printf(m, ",metacopy=%s",
>                            ofs->config.metacopy ? "on" : "off");
> +       seq_show_option(m, "override_creds",

show only if != ovl_override_creds_def

> +                       ofs->config.override_creds ? "on" : "off");
>         return 0;
>  }
>
> @@ -401,6 +408,8 @@ enum {
>         OPT_XINO_AUTO,
>         OPT_METACOPY_ON,
>         OPT_METACOPY_OFF,
> +       OPT_OVERRIDE_CREDS_ON,
> +       OPT_OVERRIDE_CREDS_OFF,
>         OPT_ERR,
>  };
>
> @@ -419,6 +428,8 @@ static const match_table_t ovl_tokens = {
>         {OPT_XINO_AUTO,                 "xino=auto"},
>         {OPT_METACOPY_ON,               "metacopy=on"},
>         {OPT_METACOPY_OFF,              "metacopy=off"},
> +       {OPT_OVERRIDE_CREDS_ON,         "override_creds=on"},
> +       {OPT_OVERRIDE_CREDS_OFF,        "override_creds=off"},
>         {OPT_ERR,                       NULL}
>  };
>
> @@ -477,6 +488,7 @@ static int ovl_parse_opt(char *opt, struct ovl_config *config)
>         config->redirect_mode = kstrdup(ovl_redirect_mode_def(), GFP_KERNEL);
>         if (!config->redirect_mode)
>                 return -ENOMEM;
> +       config->override_creds = ovl_default_override_creds;
>
>         while ((p = ovl_next_opt(&opt)) != NULL) {
>                 int token;
> @@ -557,6 +569,14 @@ static int ovl_parse_opt(char *opt, struct ovl_config *config)
>                         config->metacopy = false;
>                         break;
>
> +               case OPT_OVERRIDE_CREDS_ON:
> +                       config->override_creds = true;
> +                       break;
> +
> +               case OPT_OVERRIDE_CREDS_OFF:
> +                       config->override_creds = false;
> +                       break;
> +
>                 default:
>                         pr_err("overlayfs: unrecognized mount option \"%s\" or missing value\n", p);
>                         return -EINVAL;
> @@ -1549,7 +1569,8 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
>                        ovl_dentry_lower(root_dentry), NULL);
>
>         sb->s_root = root_dentry;
> -
> +       if (!ofs->config.override_creds)
> +               pr_warn("overlayfs: override_creds=off, caller credentials may not be enough to delete file or directories, create nodes, or search directories.\n");

The audience is someone that has this feature on by mistake or someone
that turn it
on without understanding what it does. I am not sure that this is
scary enough, but
I don't have a better suggestion.
Will let others state their opinion.

Thanks,
Amir.