CVE-2026-64036: cgroup/rstat: validate cpu before css_rstat_cpu() access

From: Greg Kroah-Hartman <[email protected]>

Description
===========

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

cgroup/rstat: validate cpu before css_rstat_cpu() access

css_rstat_updated() is exposed as a BPF kfunc and accepts a
caller-provided cpu argument. The function uses cpu for per-cpu rstat
lookups without checking whether it refers to a valid possible CPU.

A BPF iter/cgroup program with CAP_BPF and CAP_PERFMON can pass an
invalid cpu value. On an unfixed UBSCAN_BOUNDS test kernel, cpu ==
0x7fffffff triggers:

  UBSAN: array-index-out-of-bounds in kernel/cgroup/rstat.c:31:9
  index 2147483647 is out of range for type 'long unsigned int [64]'
  Call Trace:
    css_rstat_updated
    bpf_iter_run_prog
    cgroup_iter_seq_show
    bpf_seq_read

Add cpu validation to the BPF-facing css_rstat_updated() kfunc and
move the common implementation to __css_rstat_updated() for in-kernel
callers.

The Linux kernel CVE team has assigned CVE-2026-64036 to this issue.


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

	Issue introduced in 6.1 with commit a319185be9f5ad13c2a296d448ac52ffe45d194c and fixed in 6.18.34 with commit 6a01413a4e8fcb0263d7bef5075c5f8f4eb3a8b6
	Issue introduced in 6.1 with commit a319185be9f5ad13c2a296d448ac52ffe45d194c and fixed in 7.0.11 with commit fd2bd9fa7700ddf28296486b2598cff2f80cc819
	Issue introduced in 6.1 with commit a319185be9f5ad13c2a296d448ac52ffe45d194c and fixed in 7.1 with commit 8817005efbdfdf5d4e4814cb5dc52b53d12917d7

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-2026-64036
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:
	block/blk-cgroup.c
	include/linux/cgroup.h
	kernel/cgroup/rstat.c
	mm/memcontrol.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/6a01413a4e8fcb0263d7bef5075c5f8f4eb3a8b6
	https://git.kernel.org/stable/c/fd2bd9fa7700ddf28296486b2598cff2f80cc819
	https://git.kernel.org/stable/c/8817005efbdfdf5d4e4814cb5dc52b53d12917d7