The grant table code in Xen has a bespoke semi-lockfree allocator for
recording grant mappings ("maptrack" entries). This allocator has a
race which allows the free list to be corrupted.
Specifically: the code for removing an entry from the free list, prior
to use, assumes (without locking) that if inspecting head item shows
that it is not the tail, it will continue to not be the tail of the
list if it is later found to be still the head and removed with
cmpxchg. But the entry might have been removed and replaced, with the
result that it might be the tail by then. (The invariants for the
semi-lockfree data structure were never formally documented.)
Additionally, a stolen entry is put on the free list with an incorrect
link field, which will very likely corrupt the list.
A malicious guest administrator can crash the host, and can probably
escalate their privilege to that of the host.
Xen 4.6 and later are vulnerable.
Xen 4.5 and earlier are not vulnerable.
There is no mitigation for this vulnerability.
Name: the Xen project
Upstream: Ian Jackson (Citrix)
Created xen tracking bugs for this issue:
Affects: fedora-all [bug 1481765]