arm64: bpf: fix mod-by-zero case

Turns out in the case of modulo by zero in a BPF program:
	A = A % X;  (X == 0)
the expected behavior is to terminate with return value 0.

The bug in JIT is exposed by a new test case [1].

[1] https://lkml.org/lkml/2015/11/4/499

Signed-off-by: Zi Shen Lim <zlim.lnx@gmail.com>
Reported-by: Yang Shi <yang.shi@linaro.org>
Reported-by: Xi Wang <xi.wang@gmail.com>
CC: Alexei Starovoitov <ast@plumgrid.com>
Fixes: e54bcde3d69d ("arm64: eBPF JIT compiler")
Cc: <stable@vger.kernel.org> # 3.18+
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
index 9ae6f23..6217f80 100644 (file)
--- a/arch/arm64/net/bpf_jit_comp.c
+++ b/arch/arm64/net/bpf_jit_comp.c
@@ -269,6 +269,8 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx)
                break;
        case BPF_ALU | BPF_DIV | BPF_X:
        case BPF_ALU64 | BPF_DIV | BPF_X:
+       case BPF_ALU | BPF_MOD | BPF_X:
+       case BPF_ALU64 | BPF_MOD | BPF_X:
        {
                const u8 r0 = bpf2a64[BPF_REG_0];
 
@@ -281,16 +283,19 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx)
                check_imm26(jmp_offset);
                emit(A64_B(jmp_offset), ctx);
                /* else */
-               emit(A64_UDIV(is64, dst, dst, src), ctx);
+               switch (BPF_OP(code)) {
+               case BPF_DIV:
+                       emit(A64_UDIV(is64, dst, dst, src), ctx);
+                       break;
+               case BPF_MOD:
+                       ctx->tmp_used = 1;
+                       emit(A64_UDIV(is64, tmp, dst, src), ctx);
+                       emit(A64_MUL(is64, tmp, tmp, src), ctx);
+                       emit(A64_SUB(is64, dst, dst, tmp), ctx);
+                       break;
+               }
                break;
        }
-       case BPF_ALU | BPF_MOD | BPF_X:
-       case BPF_ALU64 | BPF_MOD | BPF_X:
-               ctx->tmp_used = 1;
-               emit(A64_UDIV(is64, tmp, dst, src), ctx);
-               emit(A64_MUL(is64, tmp, tmp, src), ctx);
-               emit(A64_SUB(is64, dst, dst, tmp), ctx);
-               break;
        case BPF_ALU | BPF_LSH | BPF_X:
        case BPF_ALU64 | BPF_LSH | BPF_X:
                emit(A64_LSLV(is64, dst, dst, src), ctx);