[PULL 2/4] io: prevent unbounded read of websocket data from client

["Daniel P. Berrangé" <berrange@redhat.com>]

The QIOChannelWebsock class wraps an underlying QIOChannel instance,
providing a websocket server handshake and packet encoding/decoding.

When a watch is added to QIOChannelWebsock, this in turn creates a watch
on the underlying QIOChannel that is always monitoring G_IO_IN, as long
as the 'encinput' buffer has space.

When this watch fires, it reads data off the wire into 'encinput' and
then decodes as much of 'encinput' as possible into the 'rawinput'
buffer.

The qio_channel_websock_set_watch method checks to see that 'encinput'
does not exceed a max limit, but fails to check that 'rawinput' size.
Since any data read into 'encinput', can usually be immediately decoded
into 'rawinput', the 'encinput' size check is largely a no-op, and so
'rawinput' will grow without bound.

This is readily seen with the chardev socket backend if enabling
websockets, connecting a client that sends data, while not running any
guest OS to consume data from the chardev via a virtual serial port.

The qio_channel_websock_read_wire() method will also unconditionally
read more data into 'encinput' if it has less than 4k present, which
is then decoded into 'rawinput'.

IOW, if qio_channel_websock_read_wire() is triggered for lots of small
reads, it will be reading and decoding more data to 'rawinput' than is
consumed. Skip reading more data if 'rawinput' is already at its max
size.

Reported-by: Aisle Research
Fixes: CVE-2026-101291
Fixes: 2d1d0e70cf3e ("io: add QIOChannelWebsock class")
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
---
 io/channel-websock.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/io/channel-websock.c b/io/channel-websock.c
index 461abcae487..646cd45be2a 100644
--- a/io/channel-websock.c
+++ b/io/channel-websock.c
@@ -970,7 +970,8 @@ static ssize_t qio_channel_websock_read_wire(QIOChannelWebsock *ioc,
 {
     ssize_t ret;
 
-    if (ioc->encinput.offset < 4096) {
+    if (ioc->encinput.offset < 4096 &&
+        ioc->rawinput.offset < QIO_CHANNEL_WEBSOCK_MAX_BUFFER) {
         size_t want = 4096 - ioc->encinput.offset;
 
         buffer_reserve(&ioc->encinput, want);
@@ -1091,6 +1092,7 @@ static void qio_channel_websock_set_watch(QIOChannelWebsock *ioc)
         cond |= G_IO_OUT;
     }
     if (ioc->encinput.offset < QIO_CHANNEL_WEBSOCK_MAX_BUFFER &&
+        ioc->rawinput.offset < QIO_CHANNEL_WEBSOCK_MAX_BUFFER &&
         !ioc->io_eof) {
         cond |= G_IO_IN;
     }
-- 
2.55.0