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Is it possible to mount an Overlayfs filesystem as an unprivileged user in a user namespace in Linux kernels >4.3.3; it seems that the fix to this vulnerability has blocked this functionality entirely.

When I create a new user namespace with clone(), passing the CLONE_NEWNS flag and attempt to invoke mount with an overlayfs filesystem, I'm given permission denied. I can mount any other filesystem though.

Is there a way to work around this/am I missing something?

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It was not allowed to mount overlayfs within unprivileged user namespace up until and including kernel version v5.10. Not sure but if you can try a bind mount of the overlayfs into the user namespace.

Starting with kernel 5.11 however, the appropriate support has been added and you can now finally use them.

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