CVE-2026-31525

Summary

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

bpf: Fix undefined behavior in interpreter sdiv/smod for INT_MIN

The BPF interpreter's signed 32-bit division and modulo handlers use the kernel abs() macro on s32 operands. The abs() macro documentation (include/linux/math.h) explicitly states the result is undefined when the input is the type minimum. When DST contains S32_MIN (0x80000000), abs((s32)DST) triggers undefined behavior and returns S32_MIN unchanged on arm64/x86. This value is then sign-extended to u64 as 0xFFFFFFFF80000000, causing do_div() to compute the wrong result.

The verifier's abstract interpretation (scalar32_min_max_sdiv) computes the mathematically correct result for range tracking, creating a verifier/interpreter mismatch that can be exploited for out-of-bounds map value access.

Introduce abs_s32() which handles S32_MIN correctly by casting to u32 before negating, avoiding signed overflow entirely. Replace all 8 abs((s32)…) call sites in the interpreter's sdiv32/smod32 handlers.

s32 is the only affected case – the s64 division/modulo handlers do not use abs().

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/694ea55f1b1c74f9942d91ec366ae9e822422e42
• https://git.kernel.org/stable/c/9ab1227765c446942f290c83382f0b19887c55cf
• https://git.kernel.org/stable/c/f14ca604c0ff274fba19f73f1f0485c0047c1396
• https://git.kernel.org/stable/c/0d5d8c3ce45c734aaf3c51cbef59155a6746157d
• https://git.kernel.org/stable/c/c77b30bd1dcb61f66c640ff7d2757816210c7cb0

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