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[87.11.6.2]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-45055f2203csm20313623f8f.37.2026.05.07.05.59.44 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 07 May 2026 05:59:45 -0700 (PDT) Date: Thu, 7 May 2026 14:59:13 +0200 From: Stefano Garzarella To: "Michael S. Tsirkin" Cc: Eric Dumazet , Arseniy Krasnov , Bobby Eshleman , Stefan Hajnoczi , "David S . Miller" , Jakub Kicinski , Paolo Abeni , Simon Horman , netdev@vger.kernel.org, eric.dumazet@gmail.com, Arseniy Krasnov , Jason Wang , Xuan Zhuo , Eugenio =?utf-8?B?UMOpcmV6?= , kvm@vger.kernel.org, virtualization@lists.linux.dev Subject: Re: [PATCH net] vsock/virtio: fix potential unbounded skb queue Message-ID: References: <20260430122653.554058-1-edumazet@google.com> <20260506113554-mutt-send-email-mst@kernel.org> <20260507074113-mutt-send-email-mst@kernel.org> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20260507074113-mutt-send-email-mst@kernel.org> On Thu, May 07, 2026 at 07:45:10AM -0400, Michael S. Tsirkin wrote: >On Thu, May 07, 2026 at 11:09:47AM +0200, Stefano Garzarella wrote: >> On Wed, May 06, 2026 at 11:37:45AM -0400, Michael S. Tsirkin wrote: >> > On Tue, May 05, 2026 at 06:11:13PM +0200, Stefano Garzarella wrote: >> > > On Tue, May 05, 2026 at 07:14:36AM -0700, Eric Dumazet wrote: >> > > > On Tue, May 5, 2026 at 6:52 AM Stefano Garzarella wrote: >> > > > > >> > > > > On Thu, Apr 30, 2026 at 12:26:52PM +0000, Eric Dumazet wrote: >> > > > > >virtio_transport_inc_rx_pkt() checks vvs->rx_bytes + len > vvs->buf_alloc. >> > > > > > >> > > > > >virtio_transport_recv_enqueue() skips coalescing for packets >> > > > > >with VIRTIO_VSOCK_SEQ_EOM. >> > > > > > >> > > > > >If fed with packets with len == 0 and VIRTIO_VSOCK_SEQ_EOM, >> > > > > >a very large number of packets can be queued >> > > > > >because vvs->rx_bytes stays at 0. >> > > > > > >> > > > > >Fix this by estimating the skb metadata size: >> > > > > > >> > > > > > (Number of skbs in the queue) * SKB_TRUESIZE(0) >> > > > > > >> > > > > >Fixes: 077706165717 ("virtio/vsock: don't use skbuff state to account credit") >> > > > > >Signed-off-by: Eric Dumazet >> > > > > >Cc: Arseniy Krasnov >> > > > > >Cc: Stefan Hajnoczi >> > > > > >Cc: Stefano Garzarella >> > > > > >Cc: "Michael S. Tsirkin" >> > > > > >Cc: Jason Wang >> > > > > >Cc: Xuan Zhuo >> > > > > >Cc: "Eugenio Pérez" >> > > > > >Cc: kvm@vger.kernel.org >> > > > > >Cc: virtualization@lists.linux.dev >> > > > > >--- >> > > > > > net/vmw_vsock/virtio_transport_common.c | 4 +++- >> > > > > > 1 file changed, 3 insertions(+), 1 deletion(-) >> > > > > > >> > > > > >diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c >> > > > > >index 416d533f493d7b07e9c77c43f741d28cfcd0953e..9b8014516f4fb1130ae184635fbba4dfee58bd64 100644 >> > > > > >--- a/net/vmw_vsock/virtio_transport_common.c >> > > > > >+++ b/net/vmw_vsock/virtio_transport_common.c >> > > > > >@@ -447,7 +447,9 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk, >> > > > > > static bool virtio_transport_inc_rx_pkt(struct virtio_vsock_sock *vvs, >> > > > > > u32 len) >> > > > > > { >> > > > > >- if (vvs->buf_used + len > vvs->buf_alloc) >> > > > > >+ u64 skb_overhead = (skb_queue_len(&vvs->rx_queue) + 1) * SKB_TRUESIZE(0); >> > > > > >+ >> > > > > >+ if (skb_overhead + vvs->buf_used + len > vvs->buf_alloc) >> > > > > > return false; >> > > > > >> > > > > I'm not sure about this fix, I mean that maybe this is incomplete. >> > > > > In virtio-vsock, there is a credit mechanism between the two peers: >> > > > > https://docs.oasis-open.org/virtio/virtio/v1.3/csd01/virtio-v1.3-csd01.html#x1-4850003 >> > > > > >> > > > > This takes only the payload into account, so it’s true that this problem >> > > > > exists; however, perhaps we should also inform the other peer of a lower >> > > > > credit balance, otherwise the other peer will believe it has much more >> > > > > credit than it actually does, send a large payload, and then the packet >> > > > > will be discarded and the data lost (there are no retransmissions, >> > > > > etc.). >> > > > >> > > > I dunno, perhaps revert 077706165717 ("virtio/vsock: don't use skbuff >> > > > state to account credit") >> > > > and find a better fix then? >> > > >> > > IIRC the same issue was there before the commit fixed by that one (commit >> > > 71dc9ec9ac7d ("virtio/vsock: replace virtio_vsock_pkt with sk_buff")), so >> > > not sure about reverting it TBH. >> > > >> > > CCing Arseniy and Bobby. >> > > >> > > > >> > > > There is always a discrepancy between skb->len and skb->truesize. >> > > > You will not be able to announce a 1MB window, and accept one milliion >> > > > skb of 1-byte each. >> > > > >> > > > This kind of contract is broken. >> > > > >> > > >> > > Yep, I agree, but before we start discarding data (and losing it), IMHO we >> > > should at least inform the other peer that we're out of space. >> > > >> > > @Stefan, @Michael, do you think we can do something in the spec to avoid >> > > this issue and in some way take into account also the metadata in the >> > > credit. I mean to avoid the 1-byte packets flooding. >> > > >> > > Thanks, >> > > Stefano >> > >> > Why do we need the metadata? Just don't keep it around if you begin >> > running low on memory. >> >> I don't think removing the skuffs will be easy; we added them for ebpf, >> zero-copy, and seqpacket as well. > >You do not need to remove them completely. > >> For now, we're already doing something: >> merging the skuffs if they don't have EOM set. > > >Right that's good. You could go further and merge with EOM too >if you stick the info about message boundaries somewhere else. This adds a lot of complexity IMO, but we can try. Do you have something in mind? > >> As a quick fix, I'm thinking of reducing the `buf_alloc` value to account >> for the overhead and notifying the other peer, at least until we find a >> better solution. >> >> Stefano > >well if you want to support pathological cases such as 1 byte messages >that would mean like 100x reduction no? > Yep, but since this patch is already merged, IMHO that is better than losing data in those pathological cases. Thanks, Stefano