By Thomas Leonard - 2019-04-26
For general information regarding MirageOS Security Advisories, please visit https://mirage.io/security.
MirageOS is a library operating system using cooperative multitasking, which can be executed as a guest of the Xen hypervisor. Virtual machines running on a Xen host can communicate by sharing pages of memory. For example, when a Mirage VM wants to use a virtual network device provided by a Linux dom0:
The Mirage VM and dom0 can now communicate using this shared memory. When dom0 has finished with the memory:
Mirage removes the entry by calling the gnttab_end_access function in Mini-OS. This function checks whether the remote domain still has the memory mapped. If so, it returns 0 to indicate that the entry cannot be removed yet. To make this function available to OCaml code, the stub_gntshr_end_access C stub in mirage-xen wrapped this with the OCaml calling conventions. Unfortunately, it ignored the return code and reported success in all cases.
A malicious VM can tell a MirageOS unikernel that it has finished using some shared memory while it is still mapped. The Mirage unikernel will think that the unshare operation has succeeded and may reuse the memory, or allow it to be garbage collected. The malicious VM will still have access to the memory.
In many cases (such as in the example above) the remote domain will be dom0, which is already fully trusted. However, if a unikernel shares memory with an untrusted domain then there is a problem.
No workaround is available.
Returning the result from the C stub required changes to the OCaml grant API to deal with the result. This turned out to be difficult because, for historical reasons, the OCaml part of the API was in the ocaml-gnt package while the C stubs were in mirage-xen, and because the C stubs are also shared with the Unix backend.
We instead created a new grant API in mirage-xen, migrated all existing Mirage drivers to use it, and then dropped support for the old API. mirage-xen 3.3.0 added support for the new API and 3.4.0 removed support for the old one.
The recommended way to upgrade is:
opam update opam upgrade mirage-xen
The following PRs were part of the fix:
You can find the latest version of this advisory online at https://mirage.io/blog/MSA02.
This advisory is signed using OpenPGP, you can verify the signature
by downloading our public key from a keyserver (
gpg --recv-key 4A732D757C0EDA74),
downloading the raw markdown source of this advisory from
gpg --verify 02.txt.asc.