[Qemu-devel] [PULL 0/2] nbd patches for -rc3

[Qemu-devel] [PULL 0/2] nbd patches for -rc3

[Eric Blake]

The following changes since commit c7e1f823aed63f49e559e7463da76d5b320be35b:

  Merge remote-tracking branch 'remotes/jasowang/tags/net-pull-request' into staging (2017-11-28 10:03:26 +0000)

are available in the Git repository at:

  git://repo.or.cz/qemu/ericb.git tags/pull-nbd-2017-11-28

for you to fetch changes up to 51ae4f8455c9e32c54770c4ebc25bf86a8128183:

  nbd/server: CVE-2017-15118 Stack smash on large export name (2017-11-28 06:58:01 -0600)

Patches were reviewed off-list during the CVE embargo.

----------------------------------------------------------------
nbd patches for 2017-11-28

Eric Blake - 0/2 fix two NBD server CVEs

----------------------------------------------------------------
Eric Blake (2):
      nbd/server: CVE-2017-15119 Reject options larger than 32M
      nbd/server: CVE-2017-15118 Stack smash on large export name

 nbd/server.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

-- 
2.14.3

[Qemu-devel] [PULL 1/2] nbd/server: CVE-2017-15119 Reject options larger than 32M

[Eric Blake]

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>
---
 nbd/server.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/nbd/server.c b/nbd/server.c
index 7d6801b427..a81801e3bc 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -673,6 +673,12 @@ 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 &&
-- 
2.14.3

[Qemu-devel] [PULL 2/2] nbd/server: CVE-2017-15118 Stack smash on large export name

[Eric Blake]

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>
---
 nbd/server.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/nbd/server.c b/nbd/server.c
index a81801e3bc..92c0fdd03b 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -386,6 +386,10 @@ 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;
     }
-- 
2.14.3

Re: [Qemu-devel] [PULL 0/2] nbd patches for -rc3

[Peter Maydell]

On 28 November 2017 at 13:02, Eric Blake <eblake@redhat.com> wrote:
> The following changes since commit c7e1f823aed63f49e559e7463da76d5b320be35b:
>
>   Merge remote-tracking branch 'remotes/jasowang/tags/net-pull-request' into staging (2017-11-28 10:03:26 +0000)
>
> are available in the Git repository at:
>
>   git://repo.or.cz/qemu/ericb.git tags/pull-nbd-2017-11-28
>
> for you to fetch changes up to 51ae4f8455c9e32c54770c4ebc25bf86a8128183:
>
>   nbd/server: CVE-2017-15118 Stack smash on large export name (2017-11-28 06:58:01 -0600)
>
> Patches were reviewed off-list during the CVE embargo.
>
> ----------------------------------------------------------------
> nbd patches for 2017-11-28
>
> Eric Blake - 0/2 fix two NBD server CVEs
>
> ----------------------------------------------------------------
> Eric Blake (2):
>       nbd/server: CVE-2017-15119 Reject options larger than 32M
>       nbd/server: CVE-2017-15118 Stack smash on large export name
>
>  nbd/server.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)

Applied, thanks.

-- PMM