From 0ce164b7410c1c4a75eefaef48896ee245eeba5c Mon Sep 17 00:00:00 2001
From: Eric Blake <eblake@redhat.com>
Date: Wed, 13 Dec 2017 18:17:29 +0100
Subject: [PATCH 4/6] nbd/server: CVE-2017-15119 Reject options larger than 32M
RH-Author: Eric Blake <eblake@redhat.com>
Message-id: <20171213181730.1278-2-eblake@redhat.com>
Patchwork-id: 78394
O-Subject: [RHEV-7.5 qemu-kvm-rhev PATCH 1/2] nbd/server: CVE-2017-15119 Reject options larger than 32M
Bugzilla: 1518529 1518551
CVE: CVE-2017-15119/20171128
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>
The NBD spec gives us permission to abruptly disconnect on clients
that send outrageously large option requests, rather than having
to spend the time reading to the end of the option. No real
option request requires that much data anyways; and meanwhile, we
already have the practice of abruptly dropping the connection on
any client that sends NBD_CMD_WRITE with a payload larger than 32M.
For comparison, nbdkit drops the connection on any request with
more than 4096 bytes; however, that limit is probably too low
(as the NBD spec states an export name can theoretically be up
to 4096 bytes, which means a valid NBD_OPT_INFO could be even
longer) - even if qemu doesn't permit exports longer than 256
bytes.
It could be argued that a malicious client trying to get us to
read nearly 4G of data on a bad request is a form of denial of
service. In particular, if the server requires TLS, but a client
that does not know the TLS credentials sends any option (other
than NBD_OPT_STARTTLS or NBD_OPT_EXPORT_NAME) with a stated
payload of nearly 4G, then the server was keeping the connection
alive trying to read all the payload, tying up resources that it
would rather be spending on a client that can get past the TLS
handshake. Hence, this warranted a CVE.
Present since at least 2.5 when handling known options, and made
worse in 2.6 when fixing support for NBD_FLAG_C_FIXED_NEWSTYLE
to handle unknown options.
CC: qemu-stable@nongnu.org
Signed-off-by: Eric Blake <eblake@redhat.com>
(cherry picked from commit fdad35ef6c5839d50dfc14073364ac893afebc30)
Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
---
nbd/server.c | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/nbd/server.c b/nbd/server.c
index 993ade3..b93cb88 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -661,6 +661,12 @@ static int nbd_negotiate_options(NBDClient *client, uint16_t myflags,
}
length = be32_to_cpu(length);
+ if (length > NBD_MAX_BUFFER_SIZE) {
+ error_setg(errp, "len (%" PRIu32" ) is larger than max len (%u)",
+ length, NBD_MAX_BUFFER_SIZE);
+ return -EINVAL;
+ }
+
trace_nbd_negotiate_options_check_option(option,
nbd_opt_lookup(option));
if (client->tlscreds &&
--
1.8.3.1