bpf: Fix abs(INT_MIN) undefined behavior in interpreter sdiv/smod#11343
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bpf: Fix abs(INT_MIN) undefined behavior in interpreter sdiv/smod#11343kernel-patches-daemon-bpf[bot] wants to merge 2 commits intobpf-next_basefrom
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March 10, 2026 11:58
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 by casting to u32 before negating, avoiding signed overflow entirely. Replace all 8 abs((s32)...) call sites in the interpreter's sdiv32/smod32 handlers. Fixes: ec0e2da ("bpf: Support new signed div/mod instructions.") Acked-by: Yonghong Song <yonghong.song@linux.dev> Signed-off-by: Jenny Guanni Qu <qguanni@gmail.com> Acked-by: Mykyta Yatsenko <yatsenko@meta.com>
Add tests to verify that signed 32-bit division and modulo operations produce correct results when the dividend is INT_MIN (0x80000000). The bug fixed in the previous commit only affects the BPF interpreter path. When JIT is enabled (the default on most architectures), the native CPU division instruction produces the correct result and these tests pass regardless. With bpf_jit_enable=0, the interpreter is used and without the previous fix, INT_MIN / 2 incorrectly returns 0x40000000 instead of 0xC0000000 due to abs(S32_MIN) undefined behavior, causing these tests to fail. 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> Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev> Acked-by: Yonghong Song <yonghong.song@linux.dev>
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At least one diff in series https://patchwork.kernel.org/project/netdevbpf/list/?series=1063949 expired. Closing PR. |
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Pull request for series with
subject: bpf: Fix abs(INT_MIN) undefined behavior in interpreter sdiv/smod
version: 4
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=1063949