Date:   Tue, 6 Nov 2018 09:39:35 +0100
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     Mark Salyzyn <salyzyn@...roid.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        Vivek Goyal <vgoyal@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        overlayfs <linux-unionfs@...r.kernel.org>,
        linux-doc@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v6 2/2] overlayfs: override_creds=off option bypass creator_cred

On Mon, Nov 5, 2018 at 7:47 PM, Amir Goldstein <amir73il@...il.com> wrote:
> On Mon, Nov 5, 2018 at 8:22 PM Mark Salyzyn <salyzyn@...roid.com> 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@...roid.com>
>> Cc: Miklos Szeredi <miklos@...redi.hu>
>> Cc: Jonathan Corbet <corbet@....net>
>> Cc: Vivek Goyal <vgoyal@...hat.com>
>> Cc: Eric W. Biederman <ebiederm@...ssion.com>
>> Cc: Amir Goldstein <amir73il@...il.com>
>> Cc: Randy Dunlap <rdunlap@...radead.org>
>> Cc: Stephen Smalley <sds@...ho.nsa.gov>
>> Cc: linux-unionfs@...r.kernel.org
>> Cc: linux-doc@...r.kernel.org
>> Cc: linux-kernel@...r.kernel.org
>> Cc: kernel-team@...roid.com
>>
>> 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.

I don't think we need any warning message, writing down the rules in
the documentation should be enough.

Thanks,
Miklos

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