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Date: Fri, 22 Jun 2018 08:56:29 -0700
From: Randy Dunlap <rdunlap@...radead.org>
To: Mark Salyzyn <salyzyn@...roid.com>, linux-kernel@...r.kernel.org
Cc: Miklos Szeredi <miklos@...redi.hu>,
Jonathan Corbet <corbet@....net>,
Vivek Goyal <vgoyal@...hat.com>,
"Eric W . Biederman" <ebiederm@...ssion.com>,
Amir Goldstein <amir73il@...il.com>,
linux-unionfs@...r.kernel.org, linux-doc@...r.kernel.org,
kernel-team@...roid.com
Subject: Re: [PATCH v3] overlayfs: override_creds=off option bypass
creator_cred
Hi Mark,
On 06/22/2018 08:20 AM, 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 principals of least privilege are applied, the mounter's
principles
> credentials might not overlap the credential 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. This option default is set in the CONFIG
> OVERLAY_FS_OVERRIDE_CREDS or in the module option override_creds.
>
> The module bool parameter and mount option override_creds is also
boolean
> 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@...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: 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
>
> Documentation/filesystems/overlayfs.txt | 17 +++++++++++++++++
> fs/overlayfs/Kconfig | 21 +++++++++++++++++++++
> 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, 90 insertions(+), 20 deletions(-)
>
> diff --git a/Documentation/filesystems/overlayfs.txt b/Documentation/filesystems/overlayfs.txt
> index 72615a2c0752..5c646f993a4b 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 principals of least privilege are applied, the mounter's
principles
> +credentials might not overlap the credential of the caller's when
credentials (?)
> +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
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..1ecb910f0300 100644
> --- a/fs/overlayfs/Kconfig
> +++ b/fs/overlayfs/Kconfig
> @@ -103,3 +103,24 @@ 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. The check of both access
> + credentials.
last "sentence" is incomplete.
> +
> + If the principals of least privilege are applied, the mounter's
principles
> + credentials might not overlap the credential of the caller's when
> + accessing the overlayfs filesystem. The mount option override_creds=n
"override_creds=n"
> + 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
--
~Randy
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