bpf: Fix abs(INT_MIN) undefined behavior in interpreter sdiv/smod#11321
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bpf: Fix abs(INT_MIN) undefined behavior in interpreter sdiv/smod#11321kernel-patches-daemon-bpf[bot] wants to merge 2 commits intobpf-next_basefrom
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March 9, 2026 09:45
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 __safe_abs32() which handles S32_MIN correctly and replace all 8 abs((s32)...) call sites in the interpreter's sdiv32/smod32 handlers. Fixes: ec0e2da ("bpf: Support new signed div/mod instructions.") Signed-off-by: Jenny Guanni Qu <qguanni@gmail.com>
Add tests to verify that signed 32-bit division and modulo operations produce correct results when the dividend is INT_MIN (0x80000000). These test the fix in the previous commit which replaced abs() with a safe helper to avoid undefined behavior for S32_MIN. Test cases: - SDIV32 INT_MIN / 2 = -1073741824 (imm and reg divisor) - SMOD32 INT_MIN % 2 = 0 (positive and negative divisor) Signed-off-by: Jenny Guanni Qu <qguanni@gmail.com>
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Pull request for series with
subject: bpf: Fix abs(INT_MIN) undefined behavior in interpreter sdiv/smod
version: 2
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=1062802