From 55c0ced59fe17dee34e9dfd5f7be63cbab207758 Mon Sep 17 00:00:00 2001 From: Yazhou Tang Date: Tue, 30 Sep 2025 23:04:33 +0800 Subject: [PATCH] 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. Co-developed-by: Shenghao Yuan Signed-off-by: Shenghao Yuan Co-developed-by: Tianci Cao Signed-off-by: Tianci Cao Signed-off-by: Yazhou Tang Acked-by: Yonghong Song Fixes: ec0e2da95f72 ("bpf: Support new signed div/mod instructions.") Link: https://lore.kernel.org/r/tencent_70D024BAE70A0A309A4781694C7B764B0608@qq.com Signed-off-by: Alexei Starovoitov --- kernel/bpf/verifier.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index a5c52d53502a2..ff40e5e65c435 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -15804,7 +15804,7 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn) } else { /* all other ALU ops: and, sub, xor, add, ... */ if (BPF_SRC(insn->code) == BPF_X) { - if (insn->imm != 0 || insn->off > 1 || + if (insn->imm != 0 || (insn->off != 0 && insn->off != 1) || (insn->off == 1 && opcode != BPF_MOD && opcode != BPF_DIV)) { verbose(env, "BPF_ALU uses reserved fields\n"); return -EINVAL; @@ -15814,7 +15814,7 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn) if (err) return err; } else { - if (insn->src_reg != BPF_REG_0 || insn->off > 1 || + if (insn->src_reg != BPF_REG_0 || (insn->off != 0 && insn->off != 1) || (insn->off == 1 && opcode != BPF_MOD && opcode != BPF_DIV)) { verbose(env, "BPF_ALU uses reserved fields\n"); return -EINVAL; -- 2.47.3