Re: [PATCH v8 2/2] overlayfs: override_creds=off option bypass creator_cred

[Vivek Goyal]

On Tue, Nov 06, 2018 at 03:01:15PM -0800, Mark Salyzyn 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.

Some random things. Not sure what's the correct answer. It might not
even be a issue, just trying to think loud.

- ovl_permission() does not do the check for permission on underlying
  inode if only MAY_EXEC is being asked for. This kind of sounds like
  a problem. That means one can create an overlay mount with context=<foo>
  and allow a process to execute a file which it could not execute
  outside overlay mount. If this is an issue, it probably is an issue
  both with override_creds=on/off.

- ovl_permission() does not check for permission on underlying inode
  for special file. Is it a problem where one can not do an operation
  on special device on host but can do it through overlay context
  mount.

- What about creds for copy up. ovl_prep_cu_creds(). Looks like even
  with override_creds=off, we will be switching to the creds as returned
  by security_inode_copy_up(). This basically sets ->create_sid if
  it is a context mount so that new inode gets created with same 
  label as context=<label>. I was worried about being able to create
  files as context=label (while mounter itself probaly could not do
  that). Dan Walsh mentioned that it might not be an issue because
  even if ->create_sid has been set, selinux will still check whether
  caller is allowed to create a file with that label in target dir
  or not.

Thanks
Vivek
> 
> 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/override_creds.
> 
> It was not always this way.  Circa 4.4 there was no recorded mounter's
> credentials, instead privileged access to upper or work directories
> were temporarily increased to perform the operations.  The MAC
> (selinux) policies were caller's in all cases.  override_creds=off
> partially returns us to this older access model minus the insecure
> temporary credential increases.  This is to permit use in a system
> with non-overlapping security models for each executable including
> the agent that mounts the overlayfs filesystem.  In Android
> this is the case since init, which performs the mount operations,
> has a minimal MAC set of privileges to reduce any attack surface,
> and services that use the content have a different set of MAC
> privileges (eg: read, for vendor labelled configuration, execute for
> vendor libraries and modules).
> 
> 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
> ---
> v8:
> - drop pr_warn message after straw poll to remove it.
> - added a use case in the commit message
> 
> v7:
> - change name of internal parameter to ovl_override_creds_def
> - report override_creds only if different than default
> 
> v6:
> - Drop CONFIG_OVERLAY_FS_OVERRIDE_CREDS.
> - Do better with the documentation.
> - pr_warn message adjusted to report consequences.
> 
> 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
> 
> v4:
> - spelling and grammar errors in text
> 
> 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
> 
> v2:
> - Forward port changed attr to stat, resulting in a build error.
> - altered 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                    | 22 +++++++++++++++++++++-
>  fs/overlayfs/util.c                     | 12 ++++++++++--
>  10 files changed, 69 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 3ac9dc8f6cc0..c32fa8ed72e6 100644
> --- a/fs/overlayfs/namei.c
> +++ b/fs/overlayfs/namei.c
> @@ -1072,7 +1072,7 @@ 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);
> @@ -1099,7 +1099,7 @@ 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);
>  }
>  
> @@ -1153,7 +1153,7 @@ 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..933829ca7d7d 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_override_creds_def = true;
> +module_param_named(override_creds, ovl_override_creds_def, bool, 0644);
> +MODULE_PARM_DESC(ovl_override_creds_def,
> +		 "Use mounter's credentials for accesses");
> +
>  static void ovl_entry_stack_free(struct ovl_entry *oe)
>  {
>  	unsigned int i;
> @@ -362,6 +367,9 @@ 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");
> +	if (ofs->config.override_creds != ovl_override_creds_def)
> +		seq_show_option(m, "override_creds",
> +				ofs->config.override_creds ? "on" : "off");
>  	return 0;
>  }
>  
> @@ -401,6 +409,8 @@ enum {
>  	OPT_XINO_AUTO,
>  	OPT_METACOPY_ON,
>  	OPT_METACOPY_OFF,
> +	OPT_OVERRIDE_CREDS_ON,
> +	OPT_OVERRIDE_CREDS_OFF,
>  	OPT_ERR,
>  };
>  
> @@ -419,6 +429,8 @@ 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 +489,7 @@ 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_override_creds_def;
>  
>  	while ((p = ovl_next_opt(&opt)) != NULL) {
>  		int token;
> @@ -557,6 +570,14 @@ 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 +1570,6 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
>  		       ovl_dentry_lower(root_dentry), NULL);
>  
>  	sb->s_root = root_dentry;
> -
>  	return 0;
>  
>  out_free_oe:
> diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
> index 7c01327b1852..484d7f76ac9c 100644
> --- a/fs/overlayfs/util.c
> +++ b/fs/overlayfs/util.c
> @@ -40,9 +40,17 @@ const struct cred *ovl_override_creds(struct super_block *sb)
>  {
>  	struct ovl_fs *ofs = sb->s_fs_info;
>  
> +	if (!ofs->config.override_creds)
> +		return NULL;
>  	return override_creds(ofs->creator_cred);
>  }
>  
> +void ovl_revert_creds(const struct cred *old_cred)
> +{
> +	if (old_cred)
> +		revert_creds(old_cred);
> +}
> +
>  struct super_block *ovl_same_sb(struct super_block *sb)
>  {
>  	struct ovl_fs *ofs = sb->s_fs_info;
> @@ -782,7 +790,7 @@ int ovl_nlink_start(struct dentry *dentry)
>  	 * value relative to the upper inode nlink in an upper inode xattr.
>  	 */
>  	err = ovl_set_nlink_upper(dentry);
> -	revert_creds(old_cred);
> +	ovl_revert_creds(old_cred);
>  
>  out:
>  	if (err)
> @@ -800,7 +808,7 @@ void ovl_nlink_end(struct dentry *dentry)
>  
>  		old_cred = ovl_override_creds(dentry->d_sb);
>  		ovl_cleanup_index(dentry);
> -		revert_creds(old_cred);
> +		ovl_revert_creds(old_cred);
>  	}
>  
>  	ovl_inode_unlock(inode);
> -- 
> 2.19.1.930.g4563a0d9d0-goog
>