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

Official description

In the Linux kernel, the following vulnerability has been resolved: xen/pvcalls: bound backend response req_id before indexing rsp[] pvcalls_front_event_handler() takes req_id 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 req_id past PVCALLS_NR_RSP_PER_RING and drive an out-of-bounds write past the bedata allocation. req_id was also declared int while the wire field rsp->req_id is u32, so a range check on the signed value alone is insufficient: a backend req_id of 0xffffffff becomes -1, passes a >= PVCALLS_NR_RSP_PER_RING test and indexes bedata->rsp[-1]. Declare req_id as u32 so a single bound covers both ends. A backend that sends an out-of-range req_id 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 (xenvif_fatal_tx_err()). 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).

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/4aaf7f32f1569e2c0475e073b8d7ec63b231d4c6
• https://git.kernel.org/stable/c/72e27ef70a20298e60a1de438f3628c515dea895
• https://git.kernel.org/stable/c/ede7a60b046d8b7288651c55198574ed7c259da7
• https://git.kernel.org/stable/c/bc8f2d1c6f7cf6e6551c00eb582baf4b52a04794
• https://git.kernel.org/stable/c/d1297a9e2fd6ce08678b370d41bc980ca798f809
• https://git.kernel.org/stable/c/d33846c8dcc06b83b7acdeac1e8bfbb5c0c26cb2