ARM: net: bpf: fix zero right shift

The LSR instruction cannot be used to perform a zero right shift since a
0 as the immediate value (imm5) in the LSR instruction encoding means
that a shift of 32 is perfomed.  See DecodeIMMShift() in the ARM ARM.

Make the JIT skip generation of the LSR if a zero-shift is requested.

This was found using american fuzzy lop.

Signed-off-by: Rabin Vincent <rabin@rab.in>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

arch/arm/net/bpf_jit_32.c

@@ -756,7 +756,8 @@ static int build_body(struct jit_ctx *ctx)
 		case BPF_ALU | BPF_RSH | BPF_K:
 			if (unlikely(k > 31))
 				return -1;
-			emit(ARM_LSR_I(r_A, r_A, k), ctx);
+			if (k)
+				emit(ARM_LSR_I(r_A, r_A, k), ctx);
 			break;
 		case BPF_ALU | BPF_RSH | BPF_X:
 			update_on_xread(ctx);