From dd7511a6effe839c73ed8f71ceaa3c53f16ebdbd Mon Sep 17 00:00:00 2001
From: Eric Blake <eblake@redhat.com>
Date: Wed, 13 Dec 2017 18:17:30 +0100
Subject: [PATCH 5/6] nbd/server: CVE-2017-15118 Stack smash on large export
 name

RH-Author: Eric Blake <eblake@redhat.com>
Message-id: <20171213181730.1278-3-eblake@redhat.com>
Patchwork-id: 78393
O-Subject: [RHEV-7.5 qemu-kvm-rhev PATCH 2/2] nbd/server: CVE-2017-15118 Stack smash on large export name
Bugzilla: 1516545 1518548
CVE: CVE-2017-15118/20171128
RH-Acked-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
RH-Acked-by: John Snow <jsnow@redhat.com>
RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>

Introduced in commit f37708f6b8 (2.10).  The NBD spec says a client
can request export names up to 4096 bytes in length, even though
they should not expect success on names longer than 256.  However,
qemu hard-codes the limit of 256, and fails to filter out a client
that probes for a longer name; the result is a stack smash that can
potentially give an attacker arbitrary control over the qemu
process.

The smash can be easily demonstrated with this client:
$ qemu-io f raw nbd://localhost:10809/$(printf %3000d 1 | tr ' ' a)

If the qemu NBD server binary (whether the standalone qemu-nbd, or
the builtin server of QMP nbd-server-start) was compiled with
-fstack-protector-strong, the ability to exploit the stack smash
into arbitrary execution is a lot more difficult (but still
theoretically possible to a determined attacker, perhaps in
combination with other CVEs).  Still, crashing a running qemu (and
losing the VM) is bad enough, even if the attacker did not obtain
full execution control.

CC: qemu-stable@nongnu.org
Signed-off-by: Eric Blake <eblake@redhat.com>
(cherry picked from commit 51ae4f8455c9e32c54770c4ebc25bf86a8128183)
Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
---
 nbd/server.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/nbd/server.c b/nbd/server.c
index b93cb88..56aed3a 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -393,6 +393,10 @@ static int nbd_negotiate_handle_info(NBDClient *client, uint32_t length,
         msg = "name length is incorrect";
         goto invalid;
     }
+    if (namelen >= sizeof(name)) {
+        msg = "name too long for qemu";
+        goto invalid;
+    }
     if (nbd_read(client->ioc, name, namelen, errp) < 0) {
         return -EIO;
     }
-- 
1.8.3.1