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

bpf: Infer zext_dst based on static register liveness analysis

As reported in the thread [1], the verifier's 32-bit operations zero
extension logic is broken. This logic is responsible for correct
semantics of 32-bit operations on s390 architecture.

According to BPF semantics, operation `w1 += 1` is supposed to zero
extend the upper half of the register `r1`. On s390 the JIT relies on
the verifier emitting explicit zero extension before such operations.

The verifier attempts to minimize the amount of zero extensions
inserted by tracking whether upper halves of the 64-bit registers are
ever used. Previously such tracking worked as follows:

- bpf_reg_state->subreg_def field was set by do_check_insn()
for each operation defining lower but not the upper halves
of the register.
- Whenever an operation reading the whole register was verified,
the verifier checked register's subreg_def and set
bpf_insn_aux_data->zext_dst flag as true via a call to
mark_insn_zext() function.
- After the verification was complete, a special pass
bpf_opt_subreg_zext_lo32_rnd_hi32() extended 32-bit operations
with bpf_insn_aux_data->zext_dst set as true by adding
explicit zero extension.

Note that the logic above relies on bpf_reg_state->subreg_def,
which is a property of a current verifier state.
Before the commit [2] two additional steps happened:

- The verifier tracked upper and lower register halves' liveness as
flags REG_LIVE_READ{32,64} in bpf_reg_state->live.
- The function propagate_liveness() called mark_insn_zext()
in order to transfer the knowledge about which registers have
their upper halves alive (and thus might require zero extension).

The commit [2] removed the two steps described above,
hence making possible a situation like below:

- The register's upper half is set and is used on some verification
path P1 and the register happens not to be marked as precise.
- The checkpoint C is created while processing some instruction
between register initialization and usage.
- On some other verification path P2 the register's upper half is not
initialized and that path ends hitting the checkpoint C.
- In such a case the register's initialization on path P2 would lack
zext_dst mark, making it possible for the program to inject
an arbitrary value in the register's upper half.

This commit replaces subreg_def based logic with computing zext_dst
statically, as a part of the bpf_compute_live_registers() analysis:

- The analysis now tracks usage of upper and lower halves of the
registers separately.
- If some instruction defines a 32-bit subregister, but not the whole
register, *and* the upper half of the register is alive after that
instruction, the instruction is marked as zext_dst.

There is one notable drop in precision: whenever a BPF subprogram is
called, all 64 bits of parameter registers are presumed to be used.
The assumption is that such a drop in precision would not inflict
a noticeable performance penalty.

[1] https://lore.kernel.org/bpf/CAGKGUv=sOuqQtA1Ub-5JXfA4FPosJFYKAQE4B79cK+P1erxqtg@mail.gmail.com/
[2] commit 107e16979905 ("bpf: disable and remove registers chain based liveness")
Published: 2026-10-21
Score: 7.8 High
EPSS: < 1% Very Low
KEV: No
Impact: Kernel memory corruption
Action: Patch immediately
AI Analysis

Impact

The Linux kernel’s BPF verifier has a flaw in the zero‑extension logic on s390. The verifier incorrectly determines when to apply a zero‑extension to the upper half of a 64‑bit register during 32‑bit operations. As a result, a maliciously crafted eBPF program can cause the kernel to treat an arbitrary value as the upper half of a register, potentially corrupting kernel memory or enabling code execution.

Affected Systems

This issue exists in all Linux kernel releases for the s390 architecture that lack the commit 107e16979905, which corrected the liveness tracking used by the verifier. Systems running such kernels and allowing user‑supplied eBPF code are susceptible. It does not affect other architectures.

Risk and Exploitability

The CVSS score is 7.8, and the EPSS indicates a very low but still present likelihood of exploitation. Because the vulnerability requires the attacker to load a malicious eBPF program, it is limited to environments where such loading is permitted. The lack of a public exploit (not listed in KEV) suggests the threat is currently low, but the potential for kernel memory corruption or privilege escalation remains high if exploited.

Generated by OpenCVE AI on October 24, 2026 at 15:39 UTC.

Remediation

No solution or workaround provided in the CVE record.

OpenCVE Recommended Actions

  • Upgrade the Linux kernel to a version that contains commit 107e16979905, which fixes the zero‑extension logic.
  • Restrict eBPF program loading to privileged or trusted users only.
  • Disable eBPF feature through sysctl settings or by removing the eBPF subsystem when untrusted code execution must be prevented.

Generated by OpenCVE AI on October 24, 2026 at 15:39 UTC.

Tracking

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Advisories

No advisories yet.

History

Sat, 24 Oct 2026 14:45:00 +0000

Type Values Removed Values Added
Weaknesses CWE-20
CWE-665

Sat, 24 Oct 2026 12:30:00 +0000

Type Values Removed Values Added
Weaknesses CWE-681
References
Metrics threat_severity

None

 cvssV3_1

{'score': 7.8, 'vector': 'CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H'}

threat_severity

Important

Thu, 22 Oct 2026 08:00:00 +0000

Type Values Removed Values Added
Weaknesses CWE-20
CWE-665

Thu, 22 Oct 2026 04:45:00 +0000

Type Values Removed Values Added
First Time appeared Linux linux
Vendors & Products Linux linux

Thu, 22 Oct 2026 01:45:00 +0000

Type Values Removed Values Added
Weaknesses CWE-682

Wed, 21 Oct 2026 23:00:00 +0000

Type Values Removed Values Added
Weaknesses CWE-682

Wed, 21 Oct 2026 17:15:00 +0000

Type Values Removed Values Added
Description In the Linux kernel, the following vulnerability has been resolved: bpf: Infer zext_dst based on static register liveness analysis As reported in the thread [1], the verifier's 32-bit operations zero extension logic is broken. This logic is responsible for correct semantics of 32-bit operations on s390 architecture. According to BPF semantics, operation `w1 += 1` is supposed to zero extend the upper half of the register `r1`. On s390 the JIT relies on the verifier emitting explicit zero extension before such operations. The verifier attempts to minimize the amount of zero extensions inserted by tracking whether upper halves of the 64-bit registers are ever used. Previously such tracking worked as follows: - bpf_reg_state->subreg_def field was set by do_check_insn() for each operation defining lower but not the upper halves of the register. - Whenever an operation reading the whole register was verified, the verifier checked register's subreg_def and set bpf_insn_aux_data->zext_dst flag as true via a call to mark_insn_zext() function. - After the verification was complete, a special pass bpf_opt_subreg_zext_lo32_rnd_hi32() extended 32-bit operations with bpf_insn_aux_data->zext_dst set as true by adding explicit zero extension. Note that the logic above relies on bpf_reg_state->subreg_def, which is a property of a current verifier state. Before the commit [2] two additional steps happened: - The verifier tracked upper and lower register halves' liveness as flags REG_LIVE_READ{32,64} in bpf_reg_state->live. - The function propagate_liveness() called mark_insn_zext() in order to transfer the knowledge about which registers have their upper halves alive (and thus might require zero extension). The commit [2] removed the two steps described above, hence making possible a situation like below: - The register's upper half is set and is used on some verification path P1 and the register happens not to be marked as precise. - The checkpoint C is created while processing some instruction between register initialization and usage. - On some other verification path P2 the register's upper half is not initialized and that path ends hitting the checkpoint C. - In such a case the register's initialization on path P2 would lack zext_dst mark, making it possible for the program to inject an arbitrary value in the register's upper half. This commit replaces subreg_def based logic with computing zext_dst statically, as a part of the bpf_compute_live_registers() analysis: - The analysis now tracks usage of upper and lower halves of the registers separately. - If some instruction defines a 32-bit subregister, but not the whole register, *and* the upper half of the register is alive after that instruction, the instruction is marked as zext_dst. There is one notable drop in precision: whenever a BPF subprogram is called, all 64 bits of parameter registers are presumed to be used. The assumption is that such a drop in precision would not inflict a noticeable performance penalty. [1] https://lore.kernel.org/bpf/CAGKGUv=sOuqQtA1Ub-5JXfA4FPosJFYKAQE4B79cK+P1erxqtg@mail.gmail.com/ [2] commit 107e16979905 ("bpf: disable and remove registers chain based liveness")
Title bpf: Infer zext_dst based on static register liveness analysis
First Time appeared Linux
Linux linux Kernel
CPEs cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*
Vendors & Products Linux
Linux linux Kernel
References

Subscriptions

Linux Linux Linux Kernel
cve-icon MITRE

Status: PUBLISHED

Assigner: Linux

Published:

Updated: 2026-10-24T14:09:25.680Z

Reserved: 2026-09-25T10:25:14.377Z

Link: CVE-2026-98869

cve-icon Vulnrichment

No data.

cve-icon NVD

Status : Received

Published: 2026-10-21T15:17:19.827

Modified: 2026-10-24T14:16:47.630

Link: CVE-2026-98869

cve-icon Redhat

Severity : Important

Publid Date: 2026-10-21T00:00:00Z

Links: CVE-2026-98869 - Bugzilla

cve-icon OpenCVE Enrichment

Updated: 2026-10-24T15:45:07Z

Weaknesses
  • CWE-681

    Incorrect Conversion between Numeric Types