From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:33362) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gYunF-0007LO-Kv for qemu-devel@nongnu.org; Mon, 17 Dec 2018 10:26:54 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gYunE-0006XG-Lk for qemu-devel@nongnu.org; Mon, 17 Dec 2018 10:26:53 -0500 References: <20181215135324.152629-1-eblake@redhat.com> <20181215135324.152629-13-eblake@redhat.com> <20181215151929.GY27120@redhat.com> From: Eric Blake Message-ID: <9b3c7c13-93cf-55e1-d512-7ed73b008720@redhat.com> Date: Mon, 17 Dec 2018 09:26:41 -0600 MIME-Version: 1.0 In-Reply-To: <20181215151929.GY27120@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v2 12/22] nbd/client: Improve error handling in nbd_negotiate_simple_meta_context() List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Richard W.M. Jones" Cc: qemu-devel@nongnu.org, nsoffer@redhat.com, jsnow@redhat.com, vsementsov@virtuozzo.com, qemu-block@nongnu.org On 12/15/18 9:19 AM, Richard W.M. Jones wrote: > On Sat, Dec 15, 2018 at 07:53:14AM -0600, Eric Blake wrote: >> Always allocate space for the reply returned by the server and >> hoist the trace earlier, as it is more interesting to trace the >> server's reply (even if it is unexpected) than parroting our >> request only on success. After all, skipping the allocation >> for a wrong size was merely a micro-optimization that only >> benefitted a broken server, rather than the common case of a >> compliant server that meets our expectations. >> >> Then turn the reply handling into a loop (even though we still >> never iterate more than once), to make this code easier to use >> when later patches do support multiple server replies. This >> changes the error message for a server with two replies (a >> corner case we are unlikely to hit in practice) from: >> >> Unexpected reply type 4 (meta context), expected 0 (ack) >> >> to: >> >> Server replied with more than one context >> >> Signed-off-by: Eric Blake >> >> --- >> v2: split patch into easier-to-review pieces [Rich, Vladimir] >> --- >> nbd/client.c | 16 ++++++++++++---- >> 1 file changed, 12 insertions(+), 4 deletions(-) >> >> diff --git a/nbd/client.c b/nbd/client.c >> index bcccd5f555e..b6a85fc3ef8 100644 >> --- a/nbd/client.c >> +++ b/nbd/client.c >> @@ -684,10 +684,11 @@ static int nbd_negotiate_simple_meta_context(QIOChannel *ioc, >> return ret; >> } >> >> - if (reply.type == NBD_REP_META_CONTEXT) { >> + while (reply.type == NBD_REP_META_CONTEXT) { > > I'm not sure I understand why this change is safe. > > As far as I can see reply.type is only updated in the loop by > nbd_receive_option_reply, and that reads from the server, and so the > server might keep sending NBD_REP_META_CONTEXT packets (instead of the > expected NBD_REP_ACK), so it could now loop forever against a > malicious server? (This is not taking into account any later patches) The loop can execute at most twice: > > Rich. > >> char *name; >> >> - if (reply.length != sizeof(info->context_id) + context_len) { >> + if (reply.length <= sizeof(info->context_id) || >> + reply.length > NBD_MAX_BUFFER_SIZE) { >> error_setg(errp, "Failed to negotiate meta context '%s', server " >> "answered with unexpected length %" PRIu32, context, >> reply.length); >> @@ -708,6 +709,15 @@ static int nbd_negotiate_simple_meta_context(QIOChannel *ioc, >> return -1; >> } >> name[reply.length] = '\0'; >> + trace_nbd_opt_meta_reply(name, info->context_id); >> + >> + if (received) { >> + error_setg(errp, "Server replied with more than one context"); >> + g_free(name); >> + nbd_send_opt_abort(ioc); >> + return -1; >> + } If the server replies with a second context, we break the loop by complaining. The old code accepted at most one context, by complaining if the server's second reply was not ACK; the new code accepts at most one context, by complaining if the server sent more than one context, so the net effect of killing the connection for a misbehaving server response to SET is unchanged. However, your point about a misbehaving server providing an infinite stream of responses to NBD_OPT_LIST or NBD_OPT_LIST_META_CONTEXT is an interesting question, and may be worth asking upstream to see if the NBD protocol should be tweaked to document any boundaries at how many listings a server might send before a client should worry about the server being malicious. (Does not affect this patch, but pre-existing when we call nbd_receive_list() for servers that lack NBD_OPT_GO, and does impact the later patches in this series that call NBD_OPT_LIST_META_CONTEXT for 'qemu-nbd --list'). -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org