[RFC 4/5] libvhost-user: prevent over-running max RAM slots

[Raphael Norwitz]

When VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS support was added to
libvhost-user, no guardrails were added to protect against QEMU
attempting to hot-add too many RAM slots to a VM with a libvhost-user
based backed attached.

This change adds the missing error handling by introducing a check on
the number of RAM slots the device has available before proceeding to
process the VHOST_USER_ADD_MEM_REG message.

Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
---
 subprojects/libvhost-user/libvhost-user.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/subprojects/libvhost-user/libvhost-user.c 
b/subprojects/libvhost-user/libvhost-user.c
index 714cc7e08b..74a9980194 100644
--- a/subprojects/libvhost-user/libvhost-user.c
+++ b/subprojects/libvhost-user/libvhost-user.c
@@ -690,6 +690,11 @@ vu_add_mem_reg(VuDev *dev, VhostUserMsg *vmsg) {
     VuDevRegion *dev_region = &dev->regions[dev->nregions];
     void *mmap_addr;
 
+    if (dev->nregions == VHOST_USER_MAX_RAM_SLOTS) {
+        vu_panic(dev, "No free ram slots available");
+        return true;
+    }
+
     if (vmsg->fd_num != 1 ||
         vmsg->size != sizeof(vmsg->payload.memreg)) {
         vu_panic(dev, "VHOST_USER_REM_MEM_REG received multiple regions");
-- 
2.20.1