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Re: [Qemu-block] [PATCH for-2.12] nbd/client: Correctly handle bad serve

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-block] [PATCH for-2.12] nbd/client: Correctly handle bad server REP_META_CONTEXT
Date: Fri, 30 Mar 2018 19:59:49 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0

30.03.2018 02:18, Eric Blake wrote:
It's never a good idea to blindly read for size bytes as
returned by the server without first validating that the size
is within bounds; a malicious or buggy server could cause us
to hang or get out of sync from reading further messages.

It may be smarter to try and teach the client to cope with
unexpected context ids by silently ignoring them instead of
hanging up on the server, but for now, if the server doesn't
reply with exactly the one context we expect, it's easier to
just give up - however, if we give up for any reason other
than an I/O failure, we might as well try to politely tell
the server we are quitting rather than continuing.

Fix some typos in the process.

Signed-off-by: Eric Blake <address@hidden>
  nbd/client.c | 19 +++++++++++++++++--
  1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/nbd/client.c b/nbd/client.c
index 9b9b7f0ea29..4ee1d9a4a2c 100644
--- a/nbd/client.c
+++ b/nbd/client.c
@@ -599,8 +599,8 @@ static QIOChannel *nbd_receive_starttls(QIOChannel *ioc,
   * Set one meta context. Simple means that reply must contain zero (not
   * negotiated) or one (negotiated) contexts. More contexts would be considered
   * as a protocol error. It's also implied that meta-data query equals queried
- * context name, so, if server replies with something different then @context,
- * it considered as error too.
+ * context name, so, if server replies with something different than @context,
+ * it is considered an error too.
   * return 1 for successful negotiation, context_id is set
   *        0 if operation is unsupported,
   *        -1 with errp set for any other error
@@ -651,6 +651,14 @@ static int nbd_negotiate_simple_meta_context(QIOChannel 
          char *name;
          size_t len;

+        if (reply.length != sizeof(received_id) + context_len) {
+            error_setg(errp, "Failed to negotiate meta context '%s', server "
+                       "answered with unexpected length %u", context,

uint32_t, is it worth PRIu32 ? Or %u is absolutely portable in this case?

+                       reply.length);
+            nbd_send_opt_abort(ioc);
+            return -1;
+        }

hmm, after this check, len variable is not actually needed, we can use context_len

          if (nbd_read(ioc, &received_id, sizeof(received_id), errp) < 0) {
              return -1;
@@ -668,6 +676,7 @@ static int nbd_negotiate_simple_meta_context(QIOChannel 
                         "answered with different context '%s'", context,
+            nbd_send_opt_abort(ioc);
              return -1;
@@ -690,6 +699,12 @@ static int nbd_negotiate_simple_meta_context(QIOChannel 
      if (reply.type != NBD_REP_ACK) {
          error_setg(errp, "Unexpected reply type %" PRIx32 " expected %x",
                     reply.type, NBD_REP_ACK);
+        nbd_send_opt_abort(ioc);
+        return -1;
+    }
+    if (reply.length) {

this check is very common for REP_ACK, it may be better to move it to nbd_handle_reply_err... (and rename this function? and  combine it somehow with _option_request() and _option_reply()?)

+        error_setg(errp, "Unexpected length to ACK response");
+        nbd_send_opt_abort(ioc);

hmm, looks like we want nbd_send_opt_abort() before most of return -1. Looks like it lacks some generalization, may be want to send it at some common point..

          return -1;

mostly, just ideas for future refactoring, so:
Reviewed-by: Vladimir Sementsov-Ogievskiy <address@hidden>

Best regards,

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