Message-ID: <CAMKc4jDpMsk1TtSN-GPLM1M_qp_jpoE1XL1g5qXRUiB-M0BPgQ@mail.gmail.com>
Date: Thu, 11 Dec 2025 14:43:59 +0000
From: Melbin Mathew Antony <mlbnkm1@...il.com>
To: Stefano Garzarella <sgarzare@...hat.com>
Cc: "Michael S. Tsirkin" <mst@...hat.com>, stefanha@...hat.com, kvm@...r.kernel.org, 
	netdev@...r.kernel.org, virtualization@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, jasowang@...hat.com, xuanzhuo@...ux.alibaba.com, 
	eperezma@...hat.com, davem@...emloft.net, edumazet@...gle.com, 
	kuba@...nel.org, pabeni@...hat.com, horms@...nel.org
Subject: Re: [PATCH net v3] vsock/virtio: cap TX credit to local buffer size

Hi Stefano, Michael,

Thanks for the feedback and for pointing out the s64 issue in
virtio_transport_get_credit() and the vsock_test regression.

I can take this up and send a small series:

  1/2 – vsock/virtio: cap TX credit to local buffer size
        - use a helper to bound peer_buf_alloc by buf_alloc
        - compute available credit in s64 like has_space(), and clamp
          negative values to zero before applying the caller’s credit

  2/2 – vsock/test: fix seqpacket message bounds test
        - include your vsock_test.c change so the seqpacket bounds test
          keeps working with the corrected TX credit handling

I’ll roll these into a [PATCH net v4 0/2] series and send it out shortly.

Thanks again for all the guidance,
Melbin


On Thu, Dec 11, 2025 at 1:57 PM Stefano Garzarella <sgarzare@...hat.com> wrote:
>
> On Thu, Dec 11, 2025 at 08:05:11AM -0500, Michael S. Tsirkin wrote:
> >On Thu, Dec 11, 2025 at 01:51:04PM +0100, Melbin K Mathew wrote:
> >> The virtio vsock transport currently derives its TX credit directly from
> >> peer_buf_alloc, which is populated from the remote endpoint's
> >> SO_VM_SOCKETS_BUFFER_SIZE value.
> >>
> >> On the host side, this means the amount of data we are willing to queue
> >> for a given connection is scaled purely by a peer-chosen value, rather
> >> than by the host's own vsock buffer configuration. A guest that
> >> advertises a very large buffer and reads slowly can cause the host to
> >> allocate a correspondingly large amount of sk_buff memory for that
> >> connection.
> >>
> >> In practice, a malicious guest can:
> >>
> >>   - set a large AF_VSOCK buffer size (e.g. 2 GiB) with
> >>     SO_VM_SOCKETS_BUFFER_MAX_SIZE / SO_VM_SOCKETS_BUFFER_SIZE, and
> >>
> >>   - open multiple connections to a host vsock service that sends data
> >>     while the guest drains slowly.
> >>
> >> On an unconstrained host this can drive Slab/SUnreclaim into the tens of
> >> GiB range, causing allocation failures and OOM kills in unrelated host
> >> processes while the offending VM remains running.
> >>
> >> On non-virtio transports and compatibility:
> >>
> >>   - VMCI uses the AF_VSOCK buffer knobs to size its queue pairs per
> >>     socket based on the local vsk->buffer_* values; the remote side
> >>     can’t enlarge those queues beyond what the local endpoint
> >>     configured.
> >>
> >>   - Hyper-V’s vsock transport uses fixed-size VMBus ring buffers and
> >>     an MTU bound; there is no peer-controlled credit field comparable
> >>     to peer_buf_alloc, and the remote endpoint can’t drive in-flight
> >>     kernel memory above those ring sizes.
> >>
> >>   - The loopback path reuses virtio_transport_common.c, so it
> >>     naturally follows the same semantics as the virtio transport.
> >>
> >> Make virtio-vsock consistent with that model by intersecting the peer’s
> >> advertised receive window with the local vsock buffer size when
> >> computing TX credit. We introduce a small helper and use it in
> >> virtio_transport_get_credit(), virtio_transport_has_space() and
> >> virtio_transport_seqpacket_enqueue(), so that:
> >>
> >>     effective_tx_window = min(peer_buf_alloc, buf_alloc)
> >>
> >> This prevents a remote endpoint from forcing us to queue more data than
> >> our own configuration allows, while preserving the existing credit
> >> semantics and keeping virtio-vsock compatible with the other transports.
> >>
> >> On an unpatched Ubuntu 22.04 host (~64 GiB RAM), running a PoC with
> >> 32 guest vsock connections advertising 2 GiB each and reading slowly
> >> drove Slab/SUnreclaim from ~0.5 GiB to ~57 GiB and the system only
> >> recovered after killing the QEMU process.
> >>
> >> With this patch applied, rerunning the same PoC yields:
> >>
> >>   Before:
> >>     MemFree:        ~61.6 GiB
> >>     MemAvailable:   ~62.3 GiB
> >>     Slab:           ~142 MiB
> >>     SUnreclaim:     ~117 MiB
> >>
> >>   After 32 high-credit connections:
> >>     MemFree:        ~61.5 GiB
> >>     MemAvailable:   ~62.3 GiB
> >>     Slab:           ~178 MiB
> >>     SUnreclaim:     ~152 MiB
> >>
> >> i.e. only ~35 MiB increase in Slab/SUnreclaim, no host OOM, and the
> >> guest remains responsive.
> >>
> >> Fixes: 06a8fc78367d ("VSOCK: Introduce virtio_vsock_common.ko")
> >> Suggested-by: Stefano Garzarella <sgarzare@...hat.com>
> >> Signed-off-by: Melbin K Mathew <mlbnkm1@...il.com>
> >> ---
> >>  net/vmw_vsock/virtio_transport_common.c | 27 ++++++++++++++++++++++---
> >>  1 file changed, 24 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> >> index dcc8a1d58..02eeb96dd 100644
> >> --- a/net/vmw_vsock/virtio_transport_common.c
> >> +++ b/net/vmw_vsock/virtio_transport_common.c
> >> @@ -491,6 +491,25 @@ void virtio_transport_consume_skb_sent(struct sk_buff *skb, bool consume)
> >>  }
> >>  EXPORT_SYMBOL_GPL(virtio_transport_consume_skb_sent);
> >>
> >> +/* Return the effective peer buffer size for TX credit computation.
> >> + *
> >> + * The peer advertises its receive buffer via peer_buf_alloc, but we
> >> + * cap that to our local buf_alloc (derived from
> >> + * SO_VM_SOCKETS_BUFFER_SIZE and already clamped to buffer_max_size)
> >> + * so that a remote endpoint cannot force us to queue more data than
> >> + * our own configuration allows.
> >> + */
> >> +static u32 virtio_transport_tx_buf_alloc(struct virtio_vsock_sock *vvs)
> >> +{
> >> +    return min(vvs->peer_buf_alloc, vvs->buf_alloc);
> >> +}
> >> +
> >>  u32 virtio_transport_get_credit(struct virtio_vsock_sock *vvs, u32 credit)
> >>  {
> >>      u32 ret;
> >> @@ -499,7 +518,8 @@ u32 virtio_transport_get_credit(struct virtio_vsock_sock *vvs, u32 credit)
> >>              return 0;
> >>
> >>      spin_lock_bh(&vvs->tx_lock);
> >> -    ret = vvs->peer_buf_alloc - (vvs->tx_cnt - vvs->peer_fwd_cnt);
> >> +    ret = virtio_transport_tx_buf_alloc(vvs) -
> >> +            (vvs->tx_cnt - vvs->peer_fwd_cnt);
> >>      if (ret > credit)
> >>              ret = credit;
> >>      vvs->tx_cnt += ret;
> >> @@ -831,7 +851,7 @@ virtio_transport_seqpacket_enqueue(struct vsock_sock *vsk,
> >>
> >>      spin_lock_bh(&vvs->tx_lock);
> >>
> >> -    if (len > vvs->peer_buf_alloc) {
> >> +    if (len > virtio_transport_tx_buf_alloc(vvs)) {
> >>              spin_unlock_bh(&vvs->tx_lock);
> >>              return -EMSGSIZE;
> >>      }
> >> @@ -882,7 +902,8 @@ static s64 virtio_transport_has_space(struct vsock_sock *vsk)
> >>      struct virtio_vsock_sock *vvs = vsk->trans;
> >>      s64 bytes;
> >>
> >> -    bytes = (s64)vvs->peer_buf_alloc - (vvs->tx_cnt - vvs->peer_fwd_cnt);
> >> +    bytes = (s64)virtio_transport_tx_buf_alloc(vvs) -
> >> +            (vvs->tx_cnt - vvs->peer_fwd_cnt);
> >>      if (bytes < 0)
> >>              bytes = 0;
> >>
> >
> >Acked-by: Michael S. Tsirkin <mst@...hat.com>
> >
> >
> >Looking at this, why is one place casting to s64 the other is not?
>
> Yeah, I pointed out that too in previous interactions. IMO we should fix
> virtio_transport_get_credit() since the peer can reduce `peer_buf_alloc`
> so it will overflow. Fortunately, we are limited by the credit requested
> by the caller, but we are still sending stuff when we shouldn't be.
>
> @Melbin let me know if you will fix it, otherwise I can do that, but I'd
> like to do in a single series (multiple patches), since they depends on
> each other.
>
> So if you prefer, I can pickup this patch and post a series with this +
> the other fix + the fix on the test I posted on the v2.
>
> Stefano
>

Powered by blists - more mailing lists