[PATCH] nbd: Don't let client send oversize strings

[Eric Blake <eblake@redhat.com>]

Qemu as server currently won't accept export names larger than 256
bytes, so most uses of qemu as client have no reason to get anywhere
near the NBD spec maximum of a 4k limit per string.  However, we
didn't actually have any code that prevented the client from violating
the protocol, which, while useful for testing corner-case server
reactions, is probably not ideal.

Signed-off-by: Eric Blake <eblake@redhat.com>
---
 include/block/nbd.h | 1 +
 nbd/client.c        | 8 ++++++++
 2 files changed, 9 insertions(+)

diff --git a/include/block/nbd.h b/include/block/nbd.h
index 316fd705a9e4..fcabdf0f37c3 100644
--- a/include/block/nbd.h
+++ b/include/block/nbd.h
@@ -232,6 +232,7 @@ enum {
  * going larger would require an audit of more code to make sure we
  * aren't overflowing some other buffer. */
 #define NBD_MAX_NAME_SIZE 256
+#define NBD_MAX_STRING_SIZE 4096

 /* Two types of reply structures */
 #define NBD_SIMPLE_REPLY_MAGIC      0x67446698
diff --git a/nbd/client.c b/nbd/client.c
index f6733962b49b..3f21722dd914 100644
--- a/nbd/client.c
+++ b/nbd/client.c
@@ -648,6 +648,10 @@ static int nbd_send_meta_query(QIOChannel *ioc, uint32_t opt,
     if (query) {
         query_len = strlen(query);
         data_len += sizeof(query_len) + query_len;
+        if (query_len > NBD_MAX_STRING_SIZE) {
+            error_setg(errp, "x_dirty_bitmap query too long to send to server");
+            return -1;
+        }
     } else {
         assert(opt == NBD_OPT_LIST_META_CONTEXT);
     }
@@ -1010,6 +1014,10 @@ int nbd_receive_negotiate(AioContext *aio_context, QIOChannel *ioc,
     bool base_allocation = info->base_allocation;

     assert(info->name);
+    if (strlen(info->name) > NBD_MAX_STRING_SIZE) {
+        error_setg(errp, "name too long to send to server");
+        return -EINVAL;
+    }
     trace_nbd_receive_negotiate_name(info->name);

     result = nbd_start_negotiate(aio_context, ioc, tlscreds, hostname, outioc,
-- 
2.21.0