Re: [PATCH v4] overlayfs: override_creds=off option bypass creator_cred

[Vivek Goyal]

On Fri, Jun 22, 2018 at 10:16:02AM -0700, 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.
> 
> 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.

Hi Mark,

I am wondering, what does it mean that caller is privileged enough to do
mknod and set trusted xattrs but it does not have privileges to do mount.
If caller is privileged, then it can do mount as well?

Or, what does it mean that a mounter can mount (hence providing access
to certain resources on the system) but then mounter itself does not
have access to those resources. If mounter does not have access to
those resources, then mounter should not be allowed to do the mount
and provide access to those resources to a third person?

For example, SELinux context= mount option. So here mounter can create
a mount point with label context=foo, and provide access to underlying
files/dirs to the caller. Now if mounter itself does not have access
to resources on which mount is being created, then how it is supposed
to provide that access to unprivileged caller?

Going by your analogy of init being attacked, then one simply have to
attack init and trick it to mount something with context=foo and gain
access to resources mounter itself could not access.

While my example is fully valid for disks, it is not fully valid for
overlay as we do two level of checks for many operations. So while overlay
inode level check will pass due to context=, underlying file system check
will fail. But this two level of checks does not happen outside overlay.
SELinux is not aware of stacking of filesystems so it could just do check
on overlay inode. So if a caller opens a file and passes file descriptor
to another process who is not supposed to access file, with context= mounts,
I think SELinux will allow access as second process is allowed to access
overlay inode.

IOW, if mounter is a separate process and if mounter itself can not
access a certain resource, then it should not allow other lower privileged
processes access to that resource. (Linux SELinux context= mounts). And
I am concerned that by taking away checks for mounter's creds later, how
do we ensure that privlege escalation did not happen by tricking mounter.

Thanks
Vivek

> 
> 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.  This option default is set in the CONFIG
> OVERLAY_FS_OVERRIDE_CREDS or in the module option override_creds.
> 
> The module boolean parameter and mount option override_creds is also
> added as a presence check for this "feature" by checking existence of
> /sys/module/overlay/parameters/overlay_creds.  This will allow user
> space to determine if the option can be supplied successfully to the
> mount(2) operation.
> 
> 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: linux-unionfs@xxxxxxxxxxxxxxx
> Cc: linux-doc@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> 
> ---
> 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
> 
>  Documentation/filesystems/overlayfs.txt | 17 +++++++++++++++++
>  fs/overlayfs/Kconfig                    | 20 ++++++++++++++++++++
>  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                    | 21 +++++++++++++++++++++
>  fs/overlayfs/util.c                     | 12 ++++++++++--
>  11 files changed, 89 insertions(+), 20 deletions(-)
> 
> diff --git a/Documentation/filesystems/overlayfs.txt b/Documentation/filesystems/overlayfs.txt
> index 72615a2c0752..18e6d70ea4c9 100644
> --- a/Documentation/filesystems/overlayfs.txt
> +++ b/Documentation/filesystems/overlayfs.txt
> @@ -106,6 +106,23 @@ 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.
> +
> +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.  One
> +option is to turn off override_creds in the mount options; all
> +subsequent operations after mount on the filesystem will be only the
> +caller's credentials.  This option default is set in the CONFIG
> +OVERLAY_FS_OVERRIDE_CREDS or in the module option override_creds.
> +
>  whiteouts and opaque directories
>  --------------------------------
>  
> diff --git a/fs/overlayfs/Kconfig b/fs/overlayfs/Kconfig
> index 9384164253ac..d21dde046b8d 100644
> --- a/fs/overlayfs/Kconfig
> +++ b/fs/overlayfs/Kconfig
> @@ -103,3 +103,23 @@ config OVERLAY_FS_XINO_AUTO
>  	  For more information, see Documentation/filesystems/overlayfs.txt
>  
>  	  If unsure, say N.
> +
> +config OVERLAY_FS_OVERRIDE_CREDS
> +	bool "Overlay filesystem override credentials"
> +	depends on OVERLAY_FS
> +	default y
> +	help
> +	  If set, 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.  The mount option
> +	  "override_creds=off" drops the mounter's credential check, so that
> +	  all subsequent operations, after mount, on the filesystem will only
> +	  be the caller's credentials.  This option sets the default for the
> +	  module option override_creds, and thus the default for all mounts
> +	  that do not specify this option.
> +
> +	  For more information see Documentation/filesystems/overlayfs.txt
> diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
> index ddaddb4ce4c3..7a841718ff2e 100644
> --- a/fs/overlayfs/copy_up.c
> +++ b/fs/overlayfs/copy_up.c
> @@ -790,7 +790,7 @@ 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 f480b1a2cd2e..a9f10cd38e32 100644
> --- a/fs/overlayfs/dir.c
> +++ b/fs/overlayfs/dir.c
> @@ -561,7 +561,8 @@ 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);
> @@ -577,7 +578,7 @@ 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;
>  }
>  
> @@ -824,7 +825,7 @@ 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);
> @@ -1150,7 +1151,7 @@ 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);
>  	ovl_nlink_end(new, locked);
>  out_drop_write:
>  	ovl_drop_write(old);
> diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
> index ed16a898caeb..afb0af1a24e9 100644
> --- a/fs/overlayfs/inode.c
> +++ b/fs/overlayfs/inode.c
> @@ -49,7 +49,7 @@ 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);
> @@ -208,7 +208,7 @@ 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;
>  }
> @@ -242,7 +242,7 @@ int ovl_permission(struct inode *inode, int mask)
>  		mask |= MAY_READ;
>  	}
>  	err = inode_permission(realinode, mask);
> -	revert_creds(old_cred);
> +	ovl_revert_creds(old_cred);
>  
>  	return err;
>  }
> @@ -259,7 +259,7 @@ 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;
>  }
>  
> @@ -302,7 +302,7 @@ 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);
>  
>  out_drop_write:
>  	ovl_drop_write(dentry);
> @@ -320,7 +320,7 @@ 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;
>  }
>  
> @@ -344,7 +344,7 @@ 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;
>  
> @@ -379,7 +379,7 @@ 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 c993dd8db739..c53e0b127332 100644
> --- a/fs/overlayfs/namei.c
> +++ b/fs/overlayfs/namei.c
> @@ -1024,7 +1024,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
>  		OVL_I(inode)->redirect = upperredirect;
>  	}
>  
> -	revert_creds(old_cred);
> +	ovl_revert_creds(old_cred);
>  	dput(index);
>  	kfree(stack);
>  	kfree(d.redirect);
> @@ -1043,7 +1043,7 @@ 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);
>  }
>  
> @@ -1097,7 +1097,7 @@ 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 7538b9b56237..81968e574264 100644
> --- a/fs/overlayfs/overlayfs.h
> +++ b/fs/overlayfs/overlayfs.h
> @@ -195,6 +195,7 @@ 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 41655a7d6894..ee4cc3802147 100644
> --- a/fs/overlayfs/ovl_entry.h
> +++ b/fs/overlayfs/ovl_entry.h
> @@ -19,6 +19,7 @@ struct ovl_config {
>  	bool index;
>  	bool nfs_export;
>  	int xino;
> +	bool override_creds;
>  };
>  
>  struct ovl_sb {
> diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
> index ef1fe42ff7bb..150c7ee2f7f7 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;
>  }
> @@ -906,7 +906,7 @@ 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 704b37311467..9f1e0cc85d27 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -56,6 +56,12 @@ 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 =
> +	IS_ENABLED(CONFIG_OVERLAY_FS_OVERRIDE_CREDS);
> +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;
> @@ -376,6 +382,8 @@ static int ovl_show_options(struct seq_file *m, struct dentry *dentry)
>  						"on" : "off");
>  	if (ofs->config.xino != ovl_xino_def())
>  		seq_printf(m, ",xino=%s", ovl_xino_str[ofs->config.xino]);
> +	seq_show_option(m, "override_creds",
> +			ofs->config.override_creds ? "on" : "off");
>  	return 0;
>  }
>  
> @@ -413,6 +421,8 @@ enum {
>  	OPT_XINO_ON,
>  	OPT_XINO_OFF,
>  	OPT_XINO_AUTO,
> +	OPT_OVERRIDE_CREDS_ON,
> +	OPT_OVERRIDE_CREDS_OFF,
>  	OPT_ERR,
>  };
>  
> @@ -429,6 +439,8 @@ static const match_table_t ovl_tokens = {
>  	{OPT_XINO_ON,			"xino=on"},
>  	{OPT_XINO_OFF,			"xino=off"},
>  	{OPT_XINO_AUTO,			"xino=auto"},
> +	{OPT_OVERRIDE_CREDS_ON,		"override_creds=on"},
> +	{OPT_OVERRIDE_CREDS_OFF,	"override_creds=off"},
>  	{OPT_ERR,			NULL}
>  };
>  
> @@ -485,6 +497,7 @@ 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;
> @@ -555,6 +568,14 @@ static int ovl_parse_opt(char *opt, struct ovl_config *config)
>  			config->xino = OVL_XINO_AUTO;
>  			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;
> diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
> index 6f1078028c66..0a59de9b4088 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;
> @@ -630,7 +638,7 @@ int ovl_nlink_start(struct dentry *dentry, bool *locked)
>  	 * 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)
> @@ -650,7 +658,7 @@ void ovl_nlink_end(struct dentry *dentry, bool locked)
>  
>  			old_cred = ovl_override_creds(dentry->d_sb);
>  			ovl_cleanup_index(dentry);
> -			revert_creds(old_cred);
> +			ovl_revert_creds(old_cred);
>  		}
>  
>  		mutex_unlock(&OVL_I(d_inode(dentry))->lock);
> -- 
> 2.18.0.rc2.346.g013aa6912e-goog
>