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>

1 files changed, 13 insertions, 8 deletions

diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
index 9ae6f2373a6d..6217f80702d2 100644
--- 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);