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Medium severity5.5NVD Advisory· Published Nov 5, 2024· Updated May 17, 2026

CVE-2023-52920

CVE-2023-52920

Description

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

bpf: support non-r10 register spill/fill to/from stack in precision tracking

Use instruction (jump) history to record instructions that performed register spill/fill to/from stack, regardless if this was done through read-only r10 register, or any other register after copying r10 into it *and* potentially adjusting offset.

To make this work reliably, we push extra per-instruction flags into instruction history, encoding stack slot index (spi) and stack frame number in extra 10 bit flags we take away from prev_idx in instruction history. We don't touch idx field for maximum performance, as it's checked most frequently during backtracking.

This change removes basically the last remaining practical limitation of precision backtracking logic in BPF verifier. It fixes known deficiencies, but also opens up new opportunities to reduce number of verified states, explored in the subsequent patches.

There are only three differences in selftests' BPF object files according to veristat, all in the positive direction (less states).

File Program Insns (A) Insns (B) Insns (DIFF) States (A) States (B) States (DIFF) -------------------------------------- ------------- --------- --------- ------------- ---------- ---------- ------------- test_cls_redirect_dynptr.bpf.linked3.o cls_redirect 2987 2864 -123 (-4.12%) 240 231 -9 (-3.75%) xdp_synproxy_kern.bpf.linked3.o syncookie_tc 82848 82661 -187 (-0.23%) 5107 5073 -34 (-0.67%) xdp_synproxy_kern.bpf.linked3.o syncookie_xdp 85116 84964 -152 (-0.18%) 5162 5130 -32 (-0.62%)

Note, I avoided renaming jmp_history to more generic insn_hist to minimize number of lines changed and potential merge conflicts between bpf and bpf-next trees.

Notice also cur_hist_entry pointer reset to NULL at the beginning of instruction verification loop. This pointer avoids the problem of relying on last jump history entry's insn_idx to determine whether we already have entry for current instruction or not. It can happen that we added jump history entry because current instruction is_jmp_point(), but also we need to add instruction flags for stack access. In this case, we don't want to entries, so we need to reuse last added entry, if it is present.

Relying on insn_idx comparison has the same ambiguity problem as the one that was fixed recently in [0], so we avoid that.

[0] https://patchwork.kernel.org/project/netdevbpf/patch/20231110002638.4168352-3-andrii@kernel.org/

AI Insight

LLM-synthesized narrative grounded in this CVE's description and references.

BPF verifier now supports non-r10 register spill/fill to/from stack in precision tracking, fixing a limitation that could cause false positives.

The Linux kernel's BPF verifier precision tracking previously had a limitation where it only tracked register spill/fill operations through register r10. This meant that when a BPF program copied r10 into another register and used that register to spill/fill stack slots, the precision backtracking could not correctly track these operations, potentially leading to verification errors [1].

This vulnerability is not exploitable in the traditional sense but could cause the verifier to incorrectly reject valid BPF programs or allow unsafe programs. The fix extends precision tracking to record spill/fill operations regardless of the register used, by storing stack slot index and frame number in instruction history flags [1].

The improvement reduces the number of verified states in BPF programs, as shown by selftests where instruction counts and state counts decreased [1]. This enhances the efficiency and correctness of BPF verification.

The fix has been backported to stable kernel versions via commits [2][3][4]. Users should update their kernels to the latest stable releases to benefit from this improvement.

References
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AI Insight generated on May 20, 2026. Synthesized from this CVE's description and the cited reference URLs; citations are validated against the source bundle.

Affected products

97

Patches

0

No patches discovered yet.

Vulnerability mechanics

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References

4

News mentions

0

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