CVE-2021-3748 (published 2022-03-23T20:15Z)

A use-after-free vulnerability was found in the virtio-net device of QEMU. It could occur when the descriptor's address belongs to the non direct access region, due to num_buffers being set after the virtqueue elem has been unmapped. A malicious guest could use this flaw to crash QEMU, resulting in a denial of service condition, or potentially execute code on the host with the privileges of the QEMU process.

Severity: MEDIUM
CVSS v2 base score: 6.9 (AV:L/AC:M/Au:N/C:C/I:C/A:C)
CVSS v3 base score: 7.5, HIGH (CVSS:3.1/AV:L/AC:H/PR:H/UI:N/S:C/C:H/I:H/A:H)

Weakness information:
cwe_id: CWE-416
cwe_name: Use After Free
description: The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.
extended_description: 
             
             
             
			 
             
             
         
         

Fix information:
repo_url: https://github.com/qemu/qemu
commit_details: repo_url: https://github.com/qemu/qemu
author: Jason Wang
author_date: 2021-09-02 13:44:12+08:00
msg: virtio-net: fix use after unmap/free for sg

When mergeable buffer is enabled, we try to set the num_buffers after
the virtqueue elem has been unmapped. This will lead several issues,
E.g a use after free when the descriptor has an address which belongs
to the non direct access region. In this case we use bounce buffer
that is allocated during address_space_map() and freed during
address_space_unmap().

Fixing this by storing the elems temporarily in an array and delay the
unmap after we set the the num_buffers.

This addresses CVE-2021-3748.

Reported-by: Alexander Bulekov <alxndr@bu.edu>
Fixes: fbe78f4f55c6 ("virtio-net support")
Cc: qemu-stable@nongnu.org
Signed-off-by: Jason Wang <jasowang@redhat.com>
repository: repo_url: https://github.com/qemu/qemu
repo_name: qemu/qemu
description: Official QEMU mirror. Please see https://www.qemu.org/contribute/ for how to submit changes to QEMU. Pull Requests are ignored. Please only use release tarballs from the QEMU website.
repo_language: C
owner: qemu
file_changes: filename: virtio-net.c
change_type: ModificationType.MODIFY
diff: @@ -1746,10 +1746,13 @@ static ssize_t virtio_net_receive_rcu(NetClientState *nc, const uint8_t *buf,
     VirtIONet *n = qemu_get_nic_opaque(nc);
     VirtIONetQueue *q = virtio_net_get_subqueue(nc);
     VirtIODevice *vdev = VIRTIO_DEVICE(n);
+    VirtQueueElement *elems[VIRTQUEUE_MAX_SIZE];
+    size_t lens[VIRTQUEUE_MAX_SIZE];
     struct iovec mhdr_sg[VIRTQUEUE_MAX_SIZE];
     struct virtio_net_hdr_mrg_rxbuf mhdr;
     unsigned mhdr_cnt = 0;
-    size_t offset, i, guest_offset;
+    size_t offset, i, guest_offset, j;
+    ssize_t err;
 
     if (!virtio_net_can_receive(nc)) {
         return -1;
@@ -1780,6 +1783,12 @@ static ssize_t virtio_net_receive_rcu(NetClientState *nc, const uint8_t *buf,
 
         total = 0;
 
+        if (i == VIRTQUEUE_MAX_SIZE) {
+            virtio_error(vdev, "virtio-net unexpected long buffer chain");
+            err = size;
+            goto err;
+        }
+
         elem = virtqueue_pop(q->rx_vq, sizeof(VirtQueueElement));
         if (!elem) {
             if (i) {
@@ -1791,7 +1800,8 @@ static ssize_t virtio_net_receive_rcu(NetClientState *nc, const uint8_t *buf,
                              n->guest_hdr_len, n->host_hdr_len,
                              vdev->guest_features);
             }
-            return -1;
+            err = -1;
+            goto err;
         }
 
         if (elem->in_num < 1) {
@@ -1799,7 +1809,8 @@ static ssize_t virtio_net_receive_rcu(NetClientState *nc, const uint8_t *buf,
                          "virtio-net receive queue contains no in buffers");
             virtqueue_detach_element(q->rx_vq, elem, 0);
             g_free(elem);
-            return -1;
+            err = -1;
+            goto err;
         }
 
         sg = elem->in_sg;
@@ -1836,12 +1847,13 @@ static ssize_t virtio_net_receive_rcu(NetClientState *nc, const uint8_t *buf,
         if (!n->mergeable_rx_bufs && offset < size) {
             virtqueue_unpop(q->rx_vq, elem, total);
             g_free(elem);
-            return size;
+            err = size;
+            goto err;
         }
 
-        /* signal other side */
-        virtqueue_fill(q->rx_vq, elem, total, i++);
-        g_free(elem);
+        elems[i] = elem;
+        lens[i] = total;
+        i++;
     }
 
     if (mhdr_cnt) {
@@ -1851,10 +1863,23 @@ static ssize_t virtio_net_receive_rcu(NetClientState *nc, const uint8_t *buf,
                      &mhdr.num_buffers, sizeof mhdr.num_buffers);
     }
 
+    for (j = 0; j < i; j++) {
+        /* signal other side */
+        virtqueue_fill(q->rx_vq, elems[j], lens[j], j);
+        g_free(elems[j]);
+    }
+
     virtqueue_flush(q->rx_vq, i);
     virtio_notify(vdev, q->rx_vq);
 
     return size;
+
+err:
+    for (j = 0; j < i; j++) {
+        g_free(elems[j]);
+    }
+
+    return err;
 }
 
 static ssize_t virtio_net_do_receive(NetClientState *nc, const uint8_t *buf,

programming_language: C