CVE-2021-28687

HVM soft-reset crashes toolstack libxl requires all data structures passed
across its public interface to be initialized before use and disposed of
afterwards by calling a specific set of functions. Many internal data
structures also require this initialize / dispose discipline, but not all
of them. When the “soft reset” feature was implemented, the
libxl__domain_suspend_state structure didn’t require any initialization or
disposal. At some point later, an initialization function was introduced
for the structure; but the “soft reset” path wasn’t refactored to call the
initialization function. When a guest nwo initiates a “soft reboot”,
uninitialized data structure leads to an assert() when later code finds the
structure in an unexpected state. The effect of this is to crash the
process monitoring the guest. How this affects the system depends on the
structure of the toolstack. For xl, this will have no security-relevant
effect: every VM has its own independent monitoring process, which contains
no state. The domain in question will hang in a crashed state, but can be
destroyed by xl destroy just like any other non-cooperating domain. For
daemon-based toolstacks linked against libxl, such as libvirt, this will
crash the toolstack, losing the state of any in-progress operations
(localized DoS), and preventing further administrator operations unless the
daemon is configured to restart automatically (system-wide DoS). If crashes
“leak” resources, then repeated crashes could use up resources, also
causing a system-wide DoS.

Notes