CVE-2026-72380: xen/pvcalls: bound backend response req_id before indexing rsp[]

In the Linux kernel, the following vulnerability has been resolved:

xen/pvcalls: bound backend response reqid before indexing rsp[]

pvcallsfronteventhandler() takes reqid directly from the backend-supplied ring response and uses it to index the fixed-size bedata->rsp[] array for a memcpy() and a store, with no range check. A malicious or buggy backend can set reqid past PVCALLSNRRSPPERRING and drive an out-of-bounds write past the bedata allocation.

reqid was also declared int while the wire field rsp->reqid is u32, so a range check on the signed value alone is insufficient: a backend reqid of 0xffffffff becomes -1, passes a >= PVCALLSNRRSPPERRING test and indexes bedata->rsp[-1]. Declare reqid as u32 so a single bound covers both ends.

A backend that sends an out-of-range reqid has violated the wire protocol, so rather than silently dropping the response, log once and stop trusting the backend: set bedata->disabled. The event handler then ignores further responses, and the request paths that wait for a response return -EIO instead of blocking forever. This mirrors the fatal-error handling xen-netback uses (xenviffataltxerr()).

The pvcalls frontend currently trusts its backend, so this is not a classic-Xen security issue, but it matters for hardening PV frontends against malicious backends (confidential and disaggregated deployments).