nbd/server: Avoid long error message assertions CVE-2020-10761

Ever since commit 36683283 (v2.8), the server code asserts that error
strings sent to the client are well-formed per the protocol by not
exceeding the maximum string length of 4096.  At the time the server
first started sending error messages, the assertion could not be
triggered, because messages were completely under our control.
However, over the years, we have added latent scenarios where a client
could trigger the server to attempt an error message that would
include the client's information if it passed other checks first:

- requesting NBD_OPT_INFO/GO on an export name that is not present
  (commit 0cfae925 in v2.12 echoes the name)

- requesting NBD_OPT_LIST/SET_META_CONTEXT on an export name that is
  not present (commit e7b1948d in v2.12 echoes the name)

At the time, those were still safe because we flagged names larger
than 256 bytes with a different message; but that changed in commit
93676c88 (v4.2) when we raised the name limit to 4096 to match the NBD
string limit.  (That commit also failed to change the magic number
4096 in nbd_negotiate_send_rep_err to the just-introduced named
constant.)  So with that commit, long client names appended to server
text can now trigger the assertion, and thus be used as a denial of
service attack against a server.  As a mitigating factor, if the
server requires TLS, the client cannot trigger the problematic paths
unless it first supplies TLS credentials, and such trusted clients are
less likely to try to intentionally crash the server.

We may later want to further sanitize the user-supplied strings we
place into our error messages, such as scrubbing out control
characters, but that is less important to the CVE fix, so it can be a
later patch to the new nbd_sanitize_name.

Consideration was given to changing the assertion in
nbd_negotiate_send_rep_verr to instead merely log a server error and
truncate the message, to avoid leaving a latent path that could
trigger a future CVE DoS on any new error message.  However, this
merely complicates the code for something that is already (correctly)
flagging coding errors, and now that we are aware of the long message
pitfall, we are less likely to introduce such errors in the future,
which would make such error handling dead code.

Reported-by: Xueqiang Wei <xuwei@redhat.com>
CC: qemu-stable@nongnu.org
Fixes: https://bugzilla.redhat.com/1843684 CVE-2020-10761
Fixes: 93676c88d7
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200610163741.3745251-2-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

2 files changed, 6 insertions, 0 deletions

diff --git a/tests/qemu-iotests/143 b/tests/qemu-iotests/143
index f649b36195..d2349903b1 100755
--- a/tests/qemu-iotests/143
+++ b/tests/qemu-iotests/143
@@ -58,6 +58,10 @@ _send_qemu_cmd $QEMU_HANDLE \
 $QEMU_IO_PROG -f raw -c quit \
     "nbd+unix:///no_such_export?socket=$SOCK_DIR/nbd" 2>&1 \
     | _filter_qemu_io | _filter_nbd
+# Likewise, with longest possible name permitted in NBD protocol
+$QEMU_IO_PROG -f raw -c quit \
+    "nbd+unix:///$(printf %4096d 1 | tr ' ' a)?socket=$SOCK_DIR/nbd" 2>&1 \
+    | _filter_qemu_io | _filter_nbd | sed 's/aaaa*aa/aa--aa/'
 
 _send_qemu_cmd $QEMU_HANDLE \
     "{ 'execute': 'quit' }" \
diff --git a/tests/qemu-iotests/143.out b/tests/qemu-iotests/143.out
index 1f4001c601..fc9c0a761f 100644
--- a/tests/qemu-iotests/143.out
+++ b/tests/qemu-iotests/143.out
@@ -5,6 +5,8 @@ QA output created by 143
 {"return": {}}
 qemu-io: can't open device nbd+unix:///no_such_export?socket=SOCK_DIR/nbd: Requested export not available
 server reported: export 'no_such_export' not present
+qemu-io: can't open device nbd+unix:///aa--aa1?socket=SOCK_DIR/nbd: Requested export not available
+server reported: export 'aa--aa...' not present
 { 'execute': 'quit' }
 {"return": {}}
 {"timestamp": {"seconds":  TIMESTAMP, "microseconds":  TIMESTAMP}, "event": "SHUTDOWN", "data": {"guest": false, "reason": "host-qmp-quit"}}