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| -rw-r--r-- | Documentation/filesystems/overlayfs.rst | 44 |
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diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst index e443be7928db..e398fdf7353e 100644 --- a/Documentation/filesystems/overlayfs.rst +++ b/Documentation/filesystems/overlayfs.rst @@ -248,6 +248,50 @@ 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 --------------------- |