On Thu, Dec 20, 2018 at 08:57:02PM -0600, Eric Blake wrote:

Fuzzing with afl found a bug where a 27 byte client sequence can cause nbdkit to report a strange error message:

$ printf %s $'000\1IHAVEOPT000\6'$'000\7'$'000\1x00' | tr 0 '\0' | ./nbdkit -s memory size=1m >/dev/null nbdkit: memory: error: client exceeded maximum number of options (32)

The culprit? The client is hanging up on a message boundary, so conn->recv() returns 0 for EOF instead of -1 for read error, and our code blindly continues on in a for loop (re-)parsing the leftover data from the previous option.

Let's fix things to uniformly quit trying to negotiate with a client that has early EOF at a message boundary, just as we do for one that quits mid-field, with the one difference that we treat a message boundary as a warning instead of an error because a client hanging up after an option response that it didn't like (rather than sending NBD_OPT_ABORT to inform the server that it won't be negotiating further) is a surprisingly common behavior among some existing clients.

Signed-off-by: Eric Blake <eblake@redhat.com> --- src/connections.c | 60 ++++++++++++++++++++++++++++++----------------- 1 file changed, 39 insertions(+), 21 deletions(-)

diff --git a/src/connections.c b/src/connections.c index 58ed6b0..577f466 100644 --- a/src/connections.c +++ b/src/connections.c @@ -600,6 +600,31 @@ send_newstyle_option_reply_info_export (struct connection *conn, return 0; }

+/* Sub-function during _negotiate_handshake_newstyle, to uniformly handle + * a client hanging up on a message boundary. + */ +static int __attribute__ ((format (printf, 4, 5))) +conn_recv_full (struct connection *conn, void *buf, size_t len, + const char *fmt, ...) +{ + int r = conn->recv (conn, buf, len); + va_list args; + + if (r == -1) { + va_start (args, fmt); + nbdkit_verror (fmt, args); + va_end (args); + return -1; + } + if (r == 0) { + /* During negotiation, client EOF on message boundary is less + * severe than failure in the middle of the buffer. */ + debug ("client closed input socket, closing connection"); + return -1; + } + return r; +} + /* Sub-function of _negotiate_handshake_newstyle_options below. It * must be called on all non-error paths out of the options for-loop * in that function. @@ -639,10 +664,9 @@ _negotiate_handshake_newstyle_options (struct connection *conn) const char *optname;

for (nr_options = 0; nr_options < MAX_NR_OPTIONS; ++nr_options) { - if (conn->recv (conn, &new_option, sizeof new_option) == -1) { - nbdkit_error ("read: %m"); + if (conn_recv_full (conn, &new_option, sizeof new_option, + "read: %m") == -1) return -1; - }

version = be64toh (new_option.version); if (version != NEW_VERSION) { @@ -675,10 +699,9 @@ _negotiate_handshake_newstyle_options (struct connection *conn)

switch (option) { case NBD_OPT_EXPORT_NAME: - if (conn->recv (conn, data, optlen) == -1) { - nbdkit_error ("read: %s: %m", name_of_nbd_opt (option)); + if (conn_recv_full (conn, data, optlen, + "read: %s: %m", name_of_nbd_opt (option)) == -1) return -1; - } /* Apart from printing it, ignore the export name. */ data[optlen] = '\0'; debug ("newstyle negotiation: %s: " @@ -715,10 +738,9 @@ _negotiate_handshake_newstyle_options (struct connection *conn) if (send_newstyle_option_reply (conn, option, NBD_REP_ERR_INVALID) == -1) return -1; - if (conn->recv (conn, data, optlen) == -1) { - nbdkit_error ("read: %s: %m", name_of_nbd_opt (option)); + if (conn_recv_full (conn, data, optlen, + "read: %s: %m", name_of_nbd_opt (option)) == -1) return -1; - } continue; }

@@ -738,10 +760,9 @@ _negotiate_handshake_newstyle_options (struct connection *conn) if (send_newstyle_option_reply (conn, option, NBD_REP_ERR_INVALID) == -1) return -1; - if (conn->recv (conn, data, optlen) == -1) { - nbdkit_error ("read: %s: %m", name_of_nbd_opt (option)); + if (conn_recv_full (conn, data, optlen, + "read: %s: %m", name_of_nbd_opt (option)) == -1) return -1; - } continue; }

@@ -780,10 +801,9 @@ _negotiate_handshake_newstyle_options (struct connection *conn) case NBD_OPT_INFO: case NBD_OPT_GO: optname = name_of_nbd_opt (option); - if (conn->recv (conn, data, optlen) == -1) { - nbdkit_error ("read: %s: %m", optname); + if (conn_recv_full (conn, data, optlen, + "read: %s: %m", optname) == -1) return -1; - }

if (optlen < 6) { /* 32 bit export length + 16 bit nr info */ debug ("newstyle negotiation: %s option length < 6", optname); @@ -880,10 +900,9 @@ _negotiate_handshake_newstyle_options (struct connection *conn) /* Unknown option. */ if (send_newstyle_option_reply (conn, option, NBD_REP_ERR_UNSUP) == -1) return -1; - if (conn->recv (conn, data, optlen) == -1) { - nbdkit_error ("read: %m"); + if (conn_recv_full (conn, data, optlen, + "read: %m") == -1) return -1; - } }

/* Note, since it's not very clear from the protocol doc, that the @@ -931,10 +950,9 @@ _negotiate_handshake_newstyle (struct connection *conn) }

/* Client now sends us its 32 bit flags word ... */ - if (conn->recv (conn, &conn->cflags, sizeof conn->cflags) == -1) { - nbdkit_error ("read: %m"); + if (conn_recv_full (conn, &conn->cflags, sizeof conn->cflags, + "read: %m") == -1) return -1; - } conn->cflags = be32toh (conn->cflags); /* ... which we check for accuracy. */ debug ("newstyle negotiation: client flags: 0x%x", conn->cflags); -- 2.17.2

Thanks for diagnosing this. I have pushed the patch because I wanted to continue with fuzzing. There are some small changes (only in the error message text) because I rebased it on top of a patch I had queued up to improve error reporting. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-top is 'top' for virtual machines. Tiny program with many powerful monitoring features, net stats, disk stats, logging, etc. http://people.redhat.com/~rjones/virt-top