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vhost/net: complete zerocopy ubufs only once

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vhost-net initializes one ubuf_info per outstanding zerocopy TX
descriptor and hands it to the backend socket.  The networking stack may
then clone a zerocopy skb before all skb references are released.  For
example, batman-adv fragmentation reaches skb_split(), which calls
skb_zerocopy_clone() and increments the same ubuf_info refcount.

vhost_zerocopy_complete() currently treats every ubuf callback as a
completed vhost descriptor.  It dereferences ubuf->ctx, writes the
descriptor completion state, and drops the vhost_net_ubuf_ref even when
the callback only releases a cloned skb reference.  A backend reset can
therefore wait for and free the vhost_net_ubuf_ref while another cloned
skb still carries the same ubuf_info.  A later completion then
dereferences the freed ubufs pointer.

KASAN reports the stale completion as:

  BUG: KASAN: slab-use-after-free in vhost_zerocopy_complete+0x1d7/0x1f0
  BUG: KASAN: slab-use-after-free in vhost_zerocopy_complete+0x101/0x1f0
  vhost_zerocopy_complete
  skb_copy_ubufs
  __dev_forward_skb2
  veth_xmit

The freed object was allocated from vhost_net_ioctl() while setting the
backend and freed through kfree_rcu()/kvfree_rcu_bulk after backend
removal, while delayed skb completion still reached
vhost_zerocopy_complete().

Honor the generic ubuf_info refcount before touching vhost state, and run
the vhost descriptor completion only for the final ubuf reference.  This
matches the msg_zerocopy_complete() ownership rule for cloned zerocopy
skbs.

Fixes: bab632d69e ("vhost: vhost TX zero-copy support")
Signed-off-by: Qing Ming <a0yami@mailbox.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-ID: <20260601104300.197210-1-a0yami@mailbox.org>
This commit is contained in:
Qing Ming 2026-06-01 18:43:00 +08:00 committed by Michael S. Tsirkin
parent 9c15238034
commit 8f6898fe80
1 changed files with 11 additions and 4 deletions
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15
drivers/vhost/net.c
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@ -390,13 +390,20 @@ static void vhost_zerocopy_signal_used(struct vhost_net *net,
static void vhost_zerocopy_complete(struct sk_buff *skb,
struct ubuf_info *ubuf_base, bool success)
{
struct ubuf_info_msgzc *ubuf = uarg_to_msgzc(ubuf_base);
struct vhost_net_ubuf_ref *ubufs = ubuf->ctx;
struct vhost_virtqueue *vq = ubufs->vq;
struct ubuf_info_msgzc *ubuf;
struct vhost_net_ubuf_ref *ubufs;
struct vhost_virtqueue *vq;
int cnt;
rcu_read_lock_bh();
/* Only the final cloned skb reference completes the vhost descriptor. */
if (!refcount_dec_and_test(&ubuf_base->refcnt))
return;
ubuf = uarg_to_msgzc(ubuf_base);
ubufs = ubuf->ctx;
vq = ubufs->vq;
rcu_read_lock_bh();
/* set len to mark this desc buffers done DMA */
vq->heads[ubuf->desc].len = success ?
VHOST_DMA_DONE_LEN : VHOST_DMA_FAILED_LEN;