Linux Kernel BPF Verifier bpf_compute_live_registers zext_dst input validation

Summaryinfo

A vulnerability, which was classified as very critical, has been found in Linux Kernel. This vulnerability affects the function bpf_compute_live_registers of the component BPF Verifier. Performing a manipulation of the argument zext_dst results in input validation. This vulnerability is known as CVE-2026-98869. Remote exploitation of the attack is possible. No exploit is available. Applying a patch is the recommended action to fix this issue.

Detailsinfo

A vulnerability classified as very critical has been found in Linux Kernel (affected version unknown). This affects the function bpf_compute_live_registers of the component BPF Verifier. The manipulation of the argument zext_dst with an unknown input leads to a input validation vulnerability. CWE is classifying the issue as CWE-20. The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly. This is going to have an impact on confidentiality, integrity, and availability. The summary by CVE is:

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")

The advisory is shared at git.kernel.org. This vulnerability is uniquely identified as CVE-2026-98869 since 09/25/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. The exploitation requires an enhanced level of successful authentication. It demands that the victim is doing some kind of user interaction. Technical details are known, but no exploit is available. The price for an exploit might be around USD $0-$5k at the moment (estimation calculated on 10/21/2026).

Applying the patch 7ce090afbf725e25fff29caacce1eaf8459b1117 is able to eliminate this problem.

The vulnerability is also documented in the vulnerability database at EUVD (EUVD-2026-107288). If you want to get the best quality for vulnerability data then you always have to consider VulDB.

Productinfo

Type

• Operating System

Vendor

• Linux

Name

• Kernel

License

• open-source

Website

• Vendor: https://www.kernel.org/

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Entryinfo

If you want to get the best quality for vulnerability data then you always have to consider VulDB.

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