CVE-2026-98613: bpf: Bound ownership depth through local kptrs and graph roots

CVE-2026-98613: bpf: Bound ownership depth through local kptrs and graph roots

[Greg Kroah-Hartman]

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

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

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

bpf: Bound ownership depth through local kptrs and graph roots

Program-allocated objects can own other local objects through referenced
kptrs. bpf_obj_free_fields() follows those pointers through
__bpf_obj_drop_impl() synchronously, before the object storage is freed
through RCU. A self-referential local kptr type therefore permits arbitrarily
deep object chains, and dropping the head can exhaust the kernel stack.
Long acyclic type chains have the same problem.

btf_check_and_fixup_fields() still assumes referenced kptrs only point to
kernel types and checks ownership through list and rbtree roots only. Its
existing rule is sufficient for graph-only cycles: the target of each graph
edge must contain a node, so every type in a cycle has both a root and a
node. The rule rejects such a type owning another root, breaking every
cycle. It also limits graph-only chains to three types, or two if the first
type contains a node, and conservatively rejects longer acyclic chains.
The missing local-kptr edges, rather than a missed graph-only cycle, are the
bug introduced by support for bpf_kptr_xchg() into local kptrs.

Replace that restriction with one bounded ownership walk covering graph
roots and local referenced kptrs. Run it after all BTF records have been
fixed up, reject cycles and paths deeper than eight record-bearing types,
and cache each type's suffix depth while checking it against the remaining
budget. This also permits the longer acyclic graph-only layouts rejected
by the old rule; update their existing BTF tests accordingly.

Keep the bound independent of MAX_CALL_FRAMES because recursive destruction
can run below a BPF call chain. A plain local pointee without special-field
metadata adds only a final non-recursing drop. Non-owning kptrs and
kernel-BTF kptrs do not recurse through local records and remain outside the
walk. Include local percpu-kptr edges too, although allocation of percpu
objects with special fields is currently forbidden, so that relaxing that
restriction cannot bypass the ownership bound.

btf_check_and_fixup_fields() continues to initialize graph_root.value_rec,
including for separately allocated map records. The ownership relationships
belong to immutable program BTF and only need validation at BTF load time.

The Linux kernel CVE team has assigned CVE-2026-98613 to this issue.


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

	Issue introduced in 6.12 with commit b0966c724584a5a9fd7fb529de19807c31f27a45 and fixed in 6.12.112 with commit 92aa0502c781d72bb9685245c3280efb3766a110
	Issue introduced in 6.12 with commit b0966c724584a5a9fd7fb529de19807c31f27a45 and fixed in 6.18.55 with commit 9c11e543fec32e92b3aa3f693339ca3ee79ea9dd
	Issue introduced in 6.12 with commit b0966c724584a5a9fd7fb529de19807c31f27a45 and fixed in 7.2.9 with commit 4105870a4b8afe852aeb24a6dd58e1836cc4ce58
	Issue introduced in 6.12 with commit b0966c724584a5a9fd7fb529de19807c31f27a45 and fixed in 7.3-rc5 with commit bfc888f04588f591851e95c974954cfca58e6c19

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-2026-98613
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:
	kernel/bpf/btf.c
	tools/testing/selftests/bpf/prog_tests/linked_list.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/92aa0502c781d72bb9685245c3280efb3766a110
	https://git.kernel.org/stable/c/9c11e543fec32e92b3aa3f693339ca3ee79ea9dd
	https://git.kernel.org/stable/c/4105870a4b8afe852aeb24a6dd58e1836cc4ce58
	https://git.kernel.org/stable/c/bfc888f04588f591851e95c974954cfca58e6c19