bpf: Reject negative offsets for ALU ops

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: bpf: Reject negative offsets for ALU ops When verifying BPF programs, the check_alu_op() function validates instructions with ALU operations. The 'offset' field in these instructions is a signed 16-bit integer. The existing check 'insn->off > 1' was intended to ensure the offset is either 0, or 1 for BPF_MOD/BPF_DIV. However, because 'insn->off' is signed, this check incorrectly accepts all negative values (e.g., -1). This commit tightens the validation by changing the condition to '(insn->off != 0 && insn->off != 1)'. This ensures that any value other than the explicitly permitted 0 and 1 is rejected, hardening the verifier against malformed BPF programs.

References

• https://git.kernel.org/stable/c/3bce44b344040e5eef3d64d38b157c15304c0aab
• https://git.kernel.org/stable/c/5017c302ca4b2a45149ad64e058fa2d5623c068f
• https://git.kernel.org/stable/c/21167bf70dbe400563e189ac632258d35eda38b5
• https://git.kernel.org/stable/c/55c0ced59fe17dee34e9dfd5f7be63cbab207758