On Tue, Jun 23, 2026 at 08:36:30AM -0400, Michael S. Tsirkin wrote: > On Mon, Jun 22, 2026 at 05:30:33PM +0100, Daniel P. Berrangé wrote: > > On Mon, Jun 22, 2026 at 06:11:44PM +0200, Laurent Vivier wrote: > > > A malicious guest can craft virtqueue descriptors with arbitrary lengths. > > > control_out() calls iov_size() on the guest-supplied scatter-gather list > > > and passes the result directly to g_malloc(), allowing a guest to force > > > QEMU to attempt multi-gigabyte allocations and crash the host process. > > > > > > Fix this by copying at most sizeof(struct virtio_console_control) into a > > > stack-local variable instead of allocating a buffer sized by the guest. > > > handle_control_message() only accesses the fixed-size id, event, and > > > value fields, so no data beyond the struct was ever needed. > > > > Does anyone have thoughts on whether we should treat guest initiated > > unbounded allocs as a security issue ? > > > > IIUC, this flaw would require root in the guest OS in order to craft > > the malicious virtqueue descriptors. > > > > A self-initiated crash triggered by root would not historically > > be enough justification for CVE. We would require it to be triggered > > by unprivileged user. > > > > Nested virt with device assignment could change that equation though > > as the L2 guest could be considered an unpriv user from the L1 POV. > > > > Also in theory the large alloc might be large enough to consume all > > host RAM but not large enough to trigger OOM kill of QEMU. This might > > impact operation of other co-located VMs on the same host. > > Don't see why not. We can treat it as low priority and no embargo.
Is there any point in assigning "low" severity CVEs ? I'm not convinced downstream vendors have capacity to deal with "low" CVEs anymore even if we published them. > > > Anyone think this is bad enough to justify a CVE ? Or should we treat > > these OOM scenarios maerely as "hardening" bugs, where they require > > 'root' in the L1 guest ? > > > > > Cc: [email protected] > > > Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3585 > > > Signed-off-by: Laurent Vivier <[email protected]> > > > --- > > > hw/char/virtio-serial-bus.c | 34 +++++++--------------------------- > > > 1 file changed, 7 insertions(+), 27 deletions(-) > > > > > > diff --git a/hw/char/virtio-serial-bus.c b/hw/char/virtio-serial-bus.c > > > index cd234dc6db1d..c1973f0248fc 100644 > > > --- a/hw/char/virtio-serial-bus.c > > > +++ b/hw/char/virtio-serial-bus.c > > > @@ -344,22 +344,16 @@ void virtio_serial_throttle_port(VirtIOSerialPort > > > *port, bool throttle) > > > } > > > > > > /* Guest wants to notify us of some event */ > > > -static void handle_control_message(VirtIOSerial *vser, void *buf, size_t > > > len) > > > +static void handle_control_message(VirtIOSerial *vser, > > > + struct virtio_console_control *gcpkt) > > > { > > > VirtIODevice *vdev = VIRTIO_DEVICE(vser); > > > struct VirtIOSerialPort *port; > > > VirtIOSerialPortClass *vsc; > > > - struct virtio_console_control cpkt, *gcpkt; > > > + struct virtio_console_control cpkt; > > > uint8_t *buffer; > > > size_t buffer_len; > > > > > > - gcpkt = buf; > > > - > > > - if (len < sizeof(cpkt)) { > > > - /* The guest sent an invalid control packet */ > > > - return; > > > - } > > > - > > > cpkt.event = virtio_lduw_p(vdev, &gcpkt->event); > > > cpkt.value = virtio_lduw_p(vdev, &gcpkt->value); > > > > > > @@ -457,41 +451,27 @@ static void control_in(VirtIODevice *vdev, > > > VirtQueue *vq) > > > > > > static void control_out(VirtIODevice *vdev, VirtQueue *vq) > > > { > > > + struct virtio_console_control cpkt; > > > VirtQueueElement *elem; > > > VirtIOSerial *vser; > > > - uint8_t *buf; > > > size_t len; > > > > > > vser = VIRTIO_SERIAL(vdev); > > > > > > - len = 0; > > > - buf = NULL; > > > for (;;) { > > > - size_t cur_len; > > > - > > > elem = virtqueue_pop(vq, sizeof(VirtQueueElement)); > > > if (!elem) { > > > break; > > > } > > > > > > - cur_len = iov_size(elem->out_sg, elem->out_num); > > > - /* > > > - * Allocate a new buf only if we didn't have one previously or > > > - * if the size of the buf differs > > > - */ > > > - if (cur_len > len) { > > > - g_free(buf); > > > - > > > - buf = g_malloc(cur_len); > > > - len = cur_len; > > > + len = iov_to_buf(elem->out_sg, elem->out_num, 0, &cpkt, > > > sizeof(cpkt)); > > > + if (len == sizeof(cpkt)) { > > > + handle_control_message(vser, &cpkt); > > > } > > > - iov_to_buf(elem->out_sg, elem->out_num, 0, buf, cur_len); > > > > > > - handle_control_message(vser, buf, cur_len); > > > virtqueue_push(vq, elem, 0); > > > g_free(elem); > > > } > > > - g_free(buf); > > > virtio_notify(vdev, vq); > > > } > > > > > > -- > > > 2.54.0 > > > > > > > > > > With regards, > > Daniel > > -- > > |: https://berrange.com ~~ https://hachyderm.io/@berrange :| > > |: https://libvirt.org ~~ https://entangle-photo.org :| > > |: https://pixelfed.art/berrange ~~ https://fstop138.berrange.com :| > With regards, Daniel -- |: https://berrange.com ~~ https://hachyderm.io/@berrange :| |: https://libvirt.org ~~ https://entangle-photo.org :| |: https://pixelfed.art/berrange ~~ https://fstop138.berrange.com :|
