CVE intelligence and bounded remediation

CVE-2021-28701 — Another race in XENMAPSPACE_grant_table handling Guests are permitted access to certain Xen-owned pages of memory

High CVSS 7.8

Another race in XENMAPSPACE_grant_table handling Guests are permitted access to certain Xen-owned pages of memory. The majority of such pages remain allocated / associated with a guest for its entire lifetime. Grant table v2 status pages, however, are de-allocated when a guest switches (back) from v2 to v1. Freeing such pages requires that the hypervisor enforce that no parallel request can result in the addition of a mapping of such a page to a guest. That enforcement was missing, allowing guests to retain access to pages that were freed and perhaps re-used for other purposes. Unfortunately, when XSA-379 was being prepared, this similar issue was not noticed.

Severity
High
CVSS
7.8 (3.1)
Published
2021-09-08
CISA KEV
Not currently listed
Ecosystem
operating-system
Weaknesses
CWE-362

Affected products

  • xen / xen
  • debian / debian_linux / 11.0
  • fedoraproject / fedora / 33
  • fedoraproject / fedora / 34
  • fedoraproject / fedora / 35

Matched remediation archetype

Race condition, TOCTOU, and lifecycle synchronization

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Check exposure

  • Map concurrent actors, shared state, lock boundaries, signals, callbacks, retries, and check-then-use sequences in the affected path.
  • Determine whether untrusted users can influence timing, object names, filesystem state, or repeated state transitions.
  • Identify clustered and multi-process behavior that repository-local tests may not represent.

Remediate safely

  • Make the sensitive state transition atomic or protect it with a consistently ordered synchronization primitive.
  • Perform authorization and invariant checks on the same authoritative object and transaction used for the operation.
  • Use unique private resources, safe ownership transfer, and idempotent operations; add deterministic concurrency regression tests.

Authoritative sources

Complete CVE record and remediation plan

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