Xen: integer overflow in xen-netback xenvif_set_hash_mapping The xen-netback linux kernel module is the default backend for Xen's virtual network devices. Since commit <a href="https://crrev.com/40d8abdee806d496a60ee607a6d01b1cd7fabaf0" title="" class="" rel="nofollow">40d8abdee806d496a60ee607a6d01b1cd7fabaf0</a> the backend supports an additional control protocol described in (include/xen/interface/io/netif.h) to enable receive side scaling (RSS) for Windows guests. When a guest sends a control request with the type XEN_NETIF_CTRL_TYPE_SET_HASH_MAPPING to the backend, the function xenvif_set_hash_mapping is called with guest controlled values in gref, off and len: u32 xenvif_set_hash_mapping(struct xenvif *vif, u32 gref, u32 len, u32 off) { u32 *mapping = &vif->hash.mapping[off]; struct gnttab_copy copy_op = { .source.u.ref = gref, .source.domid = vif->domid, .dest.u.gmfn = virt_to_gfn(mapping), .dest.domid = DOMID_SELF, .dest.offset = xen_offset_in_page(mapping), .len = len * sizeof(u32), .flags = GNTCOPY_source_gref }; (A) if ((off + len > vif->hash.size) || copy_op.len > XEN_PAGE_SIZE) return XEN_NETIF_CTRL_STATUS_INVALID_PARAMETER; (B) while (len-- != 0) if (mapping[off++] >= vif->num_queues) return XEN_NETIF_CTRL_STATUS_INVALID_PARAMETER; if (copy_op.len != 0) { (C) gnttab_batch_copy(&copy_op, 1); if (copy_op.status != GNTST_okay) return XEN_NETIF_CTRL_STATUS_INVALID_PARAMETER; } return XEN_NETIF_CTRL_STATUS_SUCCESS; } A malicious guest can trigger an integer overflow for off + len and bypass the check at (A). This leads to OOB read access in (B) and might be turned into a controlled OOB write in dom0 when reaching (C) Found by: fwilhelm