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

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

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

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).

The Linux kernel CVE team has assigned CVE-2026-72380 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 4.15 with commit 2195046bfd69e487d9a76dc47840f15c8412840c and fixed in 7.1.5 with commit d1297a9e2fd6ce08678b370d41bc980ca798f809
	Issue introduced in 4.15 with commit 2195046bfd69e487d9a76dc47840f15c8412840c and fixed in 7.2-rc2 with commit d33846c8dcc06b83b7acdeac1e8bfbb5c0c26cb2

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-72380
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	drivers/xen/pvcalls-front.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/d1297a9e2fd6ce08678b370d41bc980ca798f809
	https://git.kernel.org/stable/c/d33846c8dcc06b83b7acdeac1e8bfbb5c0c26cb2