From: Miklos Szeredi <mszeredi@redhat.com>
To: "Eric W . Biederman" <ebiederm@xmission.com>
Cc: linux-unionfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: [RFC PATCH 1/5] ovl: document permission model
Date: Fri, 25 Oct 2019 13:29:13 +0200 [thread overview]
Message-ID: <20191025112917.22518-2-mszeredi@redhat.com> (raw)
In-Reply-To: <20191025112917.22518-1-mszeredi@redhat.com>
Add missing piece of documentation regarding how permissions are checked in
overlayfs.
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
---
Documentation/filesystems/overlayfs.txt | 44 +++++++++++++++++++++++++
1 file changed, 44 insertions(+)
diff --git a/Documentation/filesystems/overlayfs.txt b/Documentation/filesystems/overlayfs.txt
index 845d689e0fd7..674fc8b1e420 100644
--- a/Documentation/filesystems/overlayfs.txt
+++ b/Documentation/filesystems/overlayfs.txt
@@ -246,6 +246,50 @@ overlay filesystem (though an operation on the name of the file such as
rename or unlink will of course be noticed and handled).
+Permission model
+----------------
+
+Permission checking in the overlay filesystem follows these principles:
+
+ 1) permission check SHOULD return the same result before and after copy up
+
+ 2) task creating the overlay mount MUST NOT gain additional privileges
+
+ 3) non-mounting task MAY gain additional privileges through the overlay,
+ compared to direct access on underlying lower or upper filesystems
+
+This is achieved by performing two permission checks on each access
+
+ a) check if current task is allowed access based on local DAC (owner,
+ group, mode and posix acl), as well as MAC checks
+
+ b) check if mounting task would be allowed real operation on lower or
+ upper layer based on underlying filesystem permissions, again including
+ MAC checks
+
+Check (a) ensures consistency (1) since owner, group, mode and posix acls
+are copied up. On the other hand it can result in server enforced
+permissions (used by NFS, for example) being ignored (3).
+
+Check (b) ensures that no task gains permissions to underlying layers that
+the mounting task does not have (2). This also means that it is possible
+to create setups where the consistency rule (1) does not hold; normally,
+however, the mounting task will have sufficient privileges to perform all
+operations.
+
+Another way to demonstrate this model is drawing parallels between
+
+ mount -t overlay overlay -olowerdir=/lower,upperdir=/upper,... /merged
+
+and
+
+ cp -a /lower /upper
+ mount --bind /upper /merged
+
+The resulting access permissions should be the same. The difference is in
+the time of copy (on-demand vs. up-front).
+
+
Multiple lower layers
---------------------
--
2.21.0
next prev parent reply other threads:[~2019-10-25 11:29 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-25 11:29 [RFC PATCH 0/5] allow unprivileged overlay mounts Miklos Szeredi
2019-10-25 11:29 ` Miklos Szeredi [this message]
2019-10-25 11:29 ` [RFC PATCH 2/5] ovl: ignore failure to copy up unknown xattrs Miklos Szeredi
2019-10-25 11:29 ` [RFC PATCH 3/5] vfs: allow unprivileged whiteout creation Miklos Szeredi
2019-10-25 11:29 ` [RFC PATCH 4/5] ovl: user xattr Miklos Szeredi
2019-10-26 13:50 ` Amir Goldstein
2019-10-25 11:29 ` [RFC PATCH 5/5] ovl: unprivieged mounts Miklos Szeredi
2019-10-25 11:35 ` [RFC PATCH 0/5] allow unprivileged overlay mounts Miklos Szeredi
2019-10-29 17:01 ` Serge E. Hallyn
2020-02-24 2:45 ` Ian Kent
2019-10-25 13:42 ` Eric W. Biederman
2019-11-25 15:14 ` Miklos Szeredi
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