# CVE-2026-45886: bpf: Fix bpf_xdp_store_bytes proto for read-only arg

Status: published. Published: 2026-05-27. Updated: 2026-11-06.

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

bpf: Fix bpf_xdp_store_bytes proto for read-only arg

While making some maps in Cilium read-only from the BPF side, we noticed
that the bpf_xdp_store_bytes proto is incorrect. In particular, the
verifier was throwing the following error:

  ; ret = ctx_store_bytes(ctx, l3_off + offsetof(struct iphdr, saddr),
                          &nat->address, 4, 0);
  635: (79) r1 = *(u64 *)(r10 -144)     ; R1=ctx() R10=fp0 fp-144=ctx()
  636: (b4) w2 = 26                     ; R2=26
  637: (b4) w4 = 4                      ; R4=4
  638: (b4) w5 = 0                      ; R5=0
  639: (85) call bpf_xdp_store_bytes#190
  write into map forbidden, value_size=6 off=0 size=4

nat comes from a BPF_F_RDONLY_PROG map, so R3 is a PTR_TO_MAP_VALUE.
The verifier checks the helper's memory access to R3 in
check_mem_size_reg, as it reaches ARG_CONST_SIZE argument. The third
argument has expected type ARG_PTR_TO_UNINIT_MEM, which includes the
MEM_WRITE flag. The verifier thus checks for a BPF_WRITE access on R3.
Given R3 points to a read-only map, the check fails.

Conversely, ARG_PTR_TO_UNINIT_MEM can also lead to the helper reading
from uninitialized memory.

This patch simply fixes the expected argument type to match that of
bpf_skb_store_bytes.

## Remediation

No remediation reported in collected sources

## Affected products and versions

- Linux / Linux: ≥ 3f364222d032eea6b245780e845ad213dab28cdd; < ffb5d1c5e3933b947fc7303ad68bf0c536d0c85e (affected, git); ≥ 3f364222d032eea6b245780e845ad213dab28cdd; < ddc34a1b85505c919026ddc82fafdada9a160b15 (affected, git); ≥ 3f364222d032eea6b245780e845ad213dab28cdd; < 0db169a91381a473b7974021d1c02f8da72c5775 (affected, git); ≥ 3f364222d032eea6b245780e845ad213dab28cdd; < d7b87adeb0eb539b9b824b101bb14fb01e41240b (affected, git); ≥ 3f364222d032eea6b245780e845ad213dab28cdd; < 57f7f6a0ad04a65c8a7a067b2f56cbbf2aec9e52 (affected, git); ≥ 3f364222d032eea6b245780e845ad213dab28cdd; < 6557f1565d779851c4db9c488c49c05a47a6e72f (affected, git). Source: CNA: Linux
- Linux / Linux: 5.18 (affected); ≥ 0; < 5.18 (unaffected, semver); ≥ 6.1.165; ≤ 6.1.* (unaffected, semver); ≥ 6.6.128; ≤ 6.6.* (unaffected, semver); ≥ 6.12.75; ≤ 6.12.* (unaffected, semver); ≥ 6.18.14; ≤ 6.18.* (unaffected, semver); ≥ 6.19.4; ≤ 6.19.* (unaffected, semver); ≥ 7.0; ≤ * (unaffected, original_commit_for_fix). Source: CNA: Linux

## Severity and prioritization

- CISA KEV: No listing in collected sources

- EPSS: 0.16% as of 2026-11-06; percentile 5.27%

- SSVC: exploitation none; automatable no; technical impact partial; source ADP: CISA-ADP

- Weaknesses: Not reported

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://git.kernel.org/stable/c/ffb5d1c5e3933b947fc7303ad68bf0c536d0c85e (CNA: Linux)

- https://git.kernel.org/stable/c/ddc34a1b85505c919026ddc82fafdada9a160b15 (CNA: Linux)

- https://git.kernel.org/stable/c/0db169a91381a473b7974021d1c02f8da72c5775 (CNA: Linux)

- https://git.kernel.org/stable/c/d7b87adeb0eb539b9b824b101bb14fb01e41240b (CNA: Linux)

- https://git.kernel.org/stable/c/57f7f6a0ad04a65c8a7a067b2f56cbbf2aec9e52 (CNA: Linux)

- https://git.kernel.org/stable/c/6557f1565d779851c4db9c488c49c05a47a6e72f (CNA: Linux)

## Provenance

- CNA: Linux: https://www.cve.org/CVERecord?id=CVE-2026-45886; retrieved 2026-11-06; source updated 2026-11-06

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-45886; retrieved 2026-11-06; source updated Not reported

Revision: 3fa4c133f64409a91016b274d6990b6dca00c33d7afd05929874fd9c22ed721d

Canonical: https://y2.dev/cve/CVE-2026-45886

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