net/9p: Check errno validity

While working on a modified server I had the Linux clients crash
a few times. This lead me to find this:

Some error codes are directly extracted from the server replies.
A malformed server reply could contain an invalid error code, with a
very large value. If this value is then passed to ERR_PTR() it will
not be properly detected as an error code by IS_ERR() and as a result
the kernel will dereference an invalid pointer.

This patch tries to avoid this.
Signed-off-by: Simon Derr <simon.derr@bull.net>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

net/9p/client.c

@@ -76,6 +76,20 @@ inline int p9_is_proto_dotu(struct p9_client *clnt)
 }
 EXPORT_SYMBOL(p9_is_proto_dotu);
 
+/*
+ * Some error codes are taken directly from the server replies,
+ * make sure they are valid.
+ */
+static int safe_errno(int err)
+{
+	if ((err > 0) || (err < -MAX_ERRNO)) {
+		p9_debug(P9_DEBUG_ERROR, "Invalid error code %d\n", err);
+		return -EPROTO;
+	}
+	return err;
+}
+
+
 /* Interpret mount option for protocol version */
 static int get_protocol_version(char *s)
 {
@@ -782,7 +796,7 @@ p9_client_rpc(struct p9_client *c, int8_t type, const char *fmt, ...)
 		return req;
 reterr:
 	p9_free_req(c, req);
-	return ERR_PTR(err);
+	return ERR_PTR(safe_errno(err));
 }
 
 /**
@@ -865,7 +879,7 @@ static struct p9_req_t *p9_client_zc_rpc(struct p9_client *c, int8_t type,
 		return req;
 reterr:
 	p9_free_req(c, req);
-	return ERR_PTR(err);
+	return ERR_PTR(safe_errno(err));
 }
 
 static struct p9_fid *p9_fid_create(struct p9_client *clnt)