bpf: enable direct packet data write for xdp progs

For forwarding to be effective, XDP programs should be allowed to
rewrite packet data.

This requires that the drivers supporting XDP must all map the packet
memory as TODEVICE or BIDIRECTIONAL before invoking the program.
Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

kernel/bpf/verifier.c

@@ -653,6 +653,16 @@ static int check_map_access(struct verifier_env *env, u32 regno, int off,
 
 #define MAX_PACKET_OFF 0xffff
 
+static bool may_write_pkt_data(enum bpf_prog_type type)
+{
+	switch (type) {
+	case BPF_PROG_TYPE_XDP:
+		return true;
+	default:
+		return false;
+	}
+}
+
 static int check_packet_access(struct verifier_env *env, u32 regno, int off,
 			       int size)
 {
@@ -806,10 +816,15 @@ static int check_mem_access(struct verifier_env *env, u32 regno, int off,
 			err = check_stack_read(state, off, size, value_regno);
 		}
 	} else if (state->regs[regno].type == PTR_TO_PACKET) {
-		if (t == BPF_WRITE) {
+		if (t == BPF_WRITE && !may_write_pkt_data(env->prog->type)) {
 			verbose("cannot write into packet\n");
 			return -EACCES;
 		}
+		if (t == BPF_WRITE && value_regno >= 0 &&
+		    is_pointer_value(env, value_regno)) {
+			verbose("R%d leaks addr into packet\n", value_regno);
+			return -EACCES;
+		}
 		err = check_packet_access(env, regno, off, size);
 		if (!err && t == BPF_READ && value_regno >= 0)
 			mark_reg_unknown_value(state->regs, value_regno);