TCP multipoint servers with Linux-DCO can crash under yet-unknown
circumstances where a TCP socket gets handed to the kernel (= userland
shall not acceess it again) but the socket still lands in the event
polling mechanism, and is passed to link_socket_read() with
sock->fd being "-1" (SOCKET_UNDEFINED).
This is a bug, but it happens very unfrequently so not fixed yet.
When this happens, the server gets stuck in an endless loop of
"trying recvfrom(-1, ..), getting an error, looging that error,
continue" until the server's disk is full.
The situation is being made a bit more complex by the dco-win
approach of treating "all kernel sockets as UDP", so the Linux
implementation tries to access the -1 socket as UDP, confusing
the picture more.
As a bandaid to avoid the crash, this patch changes
- socket.h: only do the "if dco_installed, treat as UDP" for WIN32
(link_socket_read())
- socket.c: add ASSERT(sock->fd >= 0); checks to all UDP socket paths
(we should never even hit those as this is a TCP specific problem,
but in the "sock->fd = -1" case, doing a clean server abort is
preferred to "the disk is full with non-helpful logfiles, and then
the server crashes anyway")
- socket.c: in the TCP read function, link_socket_read_tcp(),
check for sock->fd < 0 and trigger "sock->stream_reset = true"
(+ write to the log what happened).
This change will kill this particular TCP client instance (SIGTERM),
but leave the rest of the server running fine - and given that
in our tests this issue seems to be triggered by inbound TCP RST
in just the wrong moment, it seems to be "a properly-sized bandaid".
v2: rebase on top of "move dco_installed back to link_socket"
v3: move sock->fd check inside !residual_fully_formed clause (so
we can still handle already-read packets)
Github: OpenVPN/openvpn#190
Signed-off-by: Gert Doering <g...@greenie.muc.de>
---
src/openvpn/socket.c | 17 +++++++++++++++++
src/openvpn/socket.h | 4 ++++
2 files changed, 21 insertions(+)
diff --git a/src/openvpn/socket.c b/src/openvpn/socket.c
index c7ec0e06..2230052f 100644
--- a/src/openvpn/socket.c
+++ b/src/openvpn/socket.c
@@ -3228,6 +3228,18 @@ link_socket_read_tcp(struct link_socket *sock,
if (!sock->stream_buf.residual_fully_formed)
{
+ /* with Linux-DCO, we sometimes try to access a socket that is
+ * already installed in the kernel and has no valid file descriptor
+ * anymore. This is a bug.
+ * Handle by resetting client instance instead of crashing.
+ */
+ if (sock->sd == SOCKET_UNDEFINED)
+ {
+ msg(M_INFO, "BUG: link_socket_read_tcp(): sock->sd==-1, reset
client instance" );
+ sock->stream_reset = true; /* reset client instance */
+ return buf->len = 0; /* nothing to read */
+ }
+
#ifdef _WIN32
sockethandle_t sh = { .s = sock->sd };
len = sockethandle_finalize(sh, &sock->reads, buf, NULL);
@@ -3285,6 +3297,8 @@ link_socket_read_udp_posix_recvmsg(struct link_socket
*sock,
struct msghdr mesg;
socklen_t fromlen = sizeof(from->dest.addr);
+ ASSERT(sock->sd >= 0); /* can't happen */
+
iov.iov_base = BPTR(buf);
iov.iov_len = buf_forward_capacity_total(buf);
mesg.msg_iov = &iov;
@@ -3351,6 +3365,9 @@ link_socket_read_udp_posix(struct link_socket *sock,
socklen_t fromlen = sizeof(from->dest.addr);
socklen_t expectedlen = af_addr_size(sock->info.af);
addr_zero_host(&from->dest);
+
+ ASSERT(sock->sd >= 0); /* can't happen */
+
#if ENABLE_IP_PKTINFO
/* Both PROTO_UDPv4 and PROTO_UDPv6 */
if (sock->info.proto == PROTO_UDP && sock->sockflags & SF_USE_IP_PKTINFO)
diff --git a/src/openvpn/socket.h b/src/openvpn/socket.h
index 05c31b10..3b9c1ba3 100644
--- a/src/openvpn/socket.h
+++ b/src/openvpn/socket.h
@@ -1058,7 +1058,11 @@ link_socket_read(struct link_socket *sock,
struct buffer *buf,
struct link_socket_actual *from)
{
+#ifdef _WIN32
if (proto_is_udp(sock->info.proto) || sock->dco_installed)
+#else
+ if (proto_is_udp(sock->info.proto))
+#endif
/* unified UDPv4 and UDPv6, for DCO the kernel
* will strip the length header */
{
--
2.25.1
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