Subject: Re: [PATCH net] vsock/virtio: fix potential unbounded skb queue

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 8 May 2026 05:43:19 -0400

On Fri, May 08, 2026 at 11:41:21AM +0200, Stefano Garzarella wrote:
> On Thu, May 07, 2026 at 06:48:47PM -0400, Michael S. Tsirkin wrote:
> > On Thu, May 07, 2026 at 02:59:13PM +0200, Stefano Garzarella wrote:
> > > 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:
> 
> [...]
> 
> > > > > 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?
> > 
> > BER is clearly overkill but here's a POC that claude made for me,
> > just to give u an idea. It's clearly has a ton of issues,
> > for example I dislike how GFP_ATOMIC is handled.
> 
> Okay, I somewhat understand, but clearly this isn't net material, so for now
> I think the best thing to do is to merge the fixup I sent (or something
> similar):
> https://lore.kernel.org/netdev/[email-masked]/

will respond there.

> This is a major change that should be merged with more caution.
> Could this have too much of an impact on performance?
> 
> Thanks,
> Stefano

Upstream is so broken now, I'd not worry about perf even.

> > Yet it seems to work fine in light testing.
> > 
> > -->
> > 
> > 
> > vsock/virtio: use DWARF ULEB128 to record EOM boundaries, enable cross-EOM skb coalescing
> > 
> > virtio_transport_recv_enqueue() currently refuses to coalesce an
> > incoming skb with the previous one when the previous skb carries
> > VIRTIO_VSOCK_SEQ_EOM.  This forces one skb per seqpacket message.
> > For workloads with many small or zero-byte messages the per-skb
> > overhead (~960 bytes) dominates, causing unbounded memory growth.
> > 
> > Decouple message boundary tracking from the skb structure: store
> > boundary offsets in a compact side buffer using DWARF ULEB128
> > encoding with the EOR flag folded into the low bit, then allow
> > the data of multiple complete messages to be coalesced into a single
> > skb.
> > 
> > Cross-EOM coalescing fires only when:
> > - both the tail skb and the incoming packet carry EOM (complete msgs)
> > - the incoming packet fits in the tail skb's tailroom
> > - no BPF psock is attached (read_skb expects one msg per skb)
> > 
> > On allocation failure the code falls back to separate skbs (existing
> > behaviour).  Credit accounting is unchanged; the boundary buffer is
> > capped at PAGE_SIZE.
> > 
> > Signed-off-by: Michael S. Tsirkin <[email-masked]>
> > Co-Authored-By: Claude Opus 4.6 (1M context) <[email-masked]>
> > 
> > diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
> > index f91704731057..e36b9ab28372 100644
> > --- a/include/linux/virtio_vsock.h
> > +++ b/include/linux/virtio_vsock.h
> > @@ -12,6 +12,7 @@
> > struct virtio_vsock_skb_cb {
> > 	bool reply;
> > 	bool tap_delivered;
> > +	bool has_boundary_entries;
> > 	u32 offset;
> > };
> > 
> > @@ -167,6 +168,12 @@ struct virtio_vsock_sock {
> > 	u32 buf_used;
> > 	struct sk_buff_head rx_queue;
> > 	u32 msg_count;
> > +
> > +	/* ULEB128-encoded seqpacket message boundary buffer */
> > +	u8 *boundary_buf;
> > +	u32 boundary_len;
> > +	u32 boundary_alloc;
> > +	u32 boundary_off;
> > };
> > 
> > struct virtio_vsock_pkt_info {
> > diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> > index 416d533f493d..81654f70f72c 100644
> > --- a/net/vmw_vsock/virtio_transport_common.c
> > +++ b/net/vmw_vsock/virtio_transport_common.c
> > @@ -11,6 +11,7 @@
> > #include <linux/sched/signal.h>
> > #include <linux/ctype.h>
> > #include <linux/list.h>
> > +#include <linux/skmsg.h>
> > #include <linux/virtio_vsock.h>
> > #include <uapi/linux/vsockmon.h>
> > 
> > @@ -26,6 +27,91 @@
> > /* Threshold for detecting small packets to copy */
> > #define GOOD_COPY_LEN  128
> > 
> > +#define VSOCK_BOUNDARY_BUF_INIT	64
> > +#define VSOCK_BOUNDARY_BUF_MAX	PAGE_SIZE
> > +
> > +/* ULEB128 boundary encoding: value = (msg_len << 1) | eor.
> > + * Each byte carries 7 data bits; bit 7 is set on all but the last byte.
> > + * Max 5 bytes for a u32 msg_len (33 bits with eor shift).
> > + */
> > +static int vsock_uleb_encode_boundary(u8 *buf, u32 msg_len, bool eor)
> > +{
> > +	u64 val = ((u64)msg_len << 1) | eor;
> > +	int n = 0;
> > +
> > +	do {
> > +		buf[n] = val & 0x7f;
> > +		val >>= 7;
> > +		if (val)
> > +			buf[n] |= 0x80;
> > +		n++;
> > +	} while (val);
> > +
> > +	return n;
> > +}
> > +
> > +static int vsock_uleb_decode_boundary(const u8 *buf, u32 avail,
> > +				      u32 *msg_len, bool *eor)
> > +{
> > +	u64 val = 0;
> > +	int shift = 0;
> > +	int n = 0;
> > +
> > +	do {
> > +		if (n >= avail || shift >= 35)
> > +			return -EINVAL;
> > +		val |= (u64)(buf[n] & 0x7f) << shift;
> > +		shift += 7;
> > +	} while (buf[n++] & 0x80);
> > +
> > +	*eor = val & 1;
> > +	*msg_len = val >> 1;
> > +	return n;
> > +}
> > +
> > +static void vsock_boundary_buf_compact(struct virtio_vsock_sock *vvs)
> > +{
> > +	if (vvs->boundary_off == 0)
> > +		return;
> > +
> > +	vvs->boundary_len -= vvs->boundary_off;
> > +	memmove(vvs->boundary_buf, vvs->boundary_buf + vvs->boundary_off,
> > +		vvs->boundary_len);
> > +	vvs->boundary_off = 0;
> > +}
> > +
> > +static int vsock_boundary_buf_ensure(struct virtio_vsock_sock *vvs, u32 needed)
> > +{
> > +	u32 new_alloc;
> > +	u8 *new_buf;
> > +
> > +	if (vvs->boundary_alloc >= needed)
> > +		return 0;
> > +
> > +	/* Reclaim consumed space before growing */
> > +	if (vvs->boundary_off) {
> > +		needed -= vvs->boundary_off;
> > +		vsock_boundary_buf_compact(vvs);
> > +		if (vvs->boundary_alloc >= needed)
> > +			return 0;
> > +	}
> > +
> > +	new_alloc = max(needed, vvs->boundary_alloc ? vvs->boundary_alloc * 2
> > +						    : VSOCK_BOUNDARY_BUF_INIT);
> > +	if (new_alloc > VSOCK_BOUNDARY_BUF_MAX)
> > +		new_alloc = VSOCK_BOUNDARY_BUF_MAX;
> > +	if (new_alloc < needed)
> > +		return -ENOMEM;
> > +
> > +	new_buf = krealloc(vvs->boundary_buf, new_alloc, GFP_ATOMIC);
> > +	if (!new_buf)
> > +		return -ENOMEM;
> > +
> > +	vvs->boundary_buf = new_buf;
> > +	vvs->boundary_alloc = new_alloc;
> > +	return 0;
> > +}
> > +
> > static void virtio_transport_cancel_close_work(struct vsock_sock *vsk,
> > 					       bool cancel_timeout);
> > static s64 virtio_transport_has_space(struct virtio_vsock_sock *vvs);
> > @@ -682,41 +768,74 @@ virtio_transport_seqpacket_do_peek(struct vsock_sock *vsk,
> > 	total = 0;
> > 	len = msg_data_left(msg);
> > 
> > -	skb_queue_walk(&vvs->rx_queue, skb) {
> > -		struct virtio_vsock_hdr *hdr;
> > +	skb = skb_peek(&vvs->rx_queue);
> > +	if (skb && VIRTIO_VSOCK_SKB_CB(skb)->has_boundary_entries) {
> > +		u32 msg_len, offset;
> > +		size_t bytes;
> > +		bool eor;
> > +		int ret;
> > 
> > -		if (total < len) {
> > -			size_t bytes;
> > +		ret = vsock_uleb_decode_boundary(
> > +			vvs->boundary_buf + vvs->boundary_off,
> > +			vvs->boundary_len - vvs->boundary_off,
> > +			&msg_len, &eor);
> > +		if (ret < 0)
> > +			goto unlock;
> > +
> > +		offset = VIRTIO_VSOCK_SKB_CB(skb)->offset;
> > +		bytes = min(len, (size_t)msg_len);
> > +
> > +		if (bytes) {
> > 			int err;
> > 
> > -			bytes = len - total;
> > -			if (bytes > skb->len)
> > -				bytes = skb->len;
> > -
> > 			spin_unlock_bh(&vvs->rx_lock);
> > -
> > -			/* sk_lock is held by caller so no one else can dequeue.
> > -			 * Unlock rx_lock since skb_copy_datagram_iter() may sleep.
> > -			 */
> > -			err = skb_copy_datagram_iter(skb, VIRTIO_VSOCK_SKB_CB(skb)->offset,
> > +			err = skb_copy_datagram_iter(skb, offset,
> > 						     &msg->msg_iter, bytes);
> > 			if (err)
> > 				return err;
> > -
> > 			spin_lock_bh(&vvs->rx_lock);
> > 		}
> > 
> > -		total += skb->len;
> > -		hdr = virtio_vsock_hdr(skb);
> > +		total = msg_len;
> > +		if (eor)
> > +			msg->msg_flags |= MSG_EOR;
> > +	} else {
> > +		skb_queue_walk(&vvs->rx_queue, skb) {
> > +			struct virtio_vsock_hdr *hdr;
> > 
> > -		if (le32_to_cpu(hdr->flags) & VIRTIO_VSOCK_SEQ_EOM) {
> > -			if (le32_to_cpu(hdr->flags) & VIRTIO_VSOCK_SEQ_EOR)
> > -				msg->msg_flags |= MSG_EOR;
> > +			if (total < len) {
> > +				size_t bytes;
> > +				int err;
> > 
> > -			break;
> > +				bytes = len - total;
> > +				if (bytes > skb->len)
> > +					bytes = skb->len;
> > +
> > +				spin_unlock_bh(&vvs->rx_lock);
> > +
> > +				err = skb_copy_datagram_iter(
> > +					skb,
> > +					VIRTIO_VSOCK_SKB_CB(skb)->offset,
> > +					&msg->msg_iter, bytes);
> > +				if (err)
> > +					return err;
> > +
> > +				spin_lock_bh(&vvs->rx_lock);
> > +			}
> > +
> > +			total += skb->len;
> > +			hdr = virtio_vsock_hdr(skb);
> > +
> > +			if (le32_to_cpu(hdr->flags) & VIRTIO_VSOCK_SEQ_EOM) {
> > +				if (le32_to_cpu(hdr->flags) &
> > +				    VIRTIO_VSOCK_SEQ_EOR)
> > +					msg->msg_flags |= MSG_EOR;
> > +				break;
> > +			}
> > 		}
> > 	}
> > 
> > +unlock:
> > 	spin_unlock_bh(&vvs->rx_lock);
> > 
> > 	return total;
> > @@ -740,57 +859,105 @@ static int virtio_transport_seqpacket_do_dequeue(struct vsock_sock *vsk,
> > 	}
> > 
> > 	while (!msg_ready) {
> > -		struct virtio_vsock_hdr *hdr;
> > -		size_t pkt_len;
> > -
> > -		skb = __skb_dequeue(&vvs->rx_queue);
> > +		skb = skb_peek(&vvs->rx_queue);
> > 		if (!skb)
> > 			break;
> > -		hdr = virtio_vsock_hdr(skb);
> > -		pkt_len = (size_t)le32_to_cpu(hdr->len);
> > 
> > -		if (dequeued_len >= 0) {
> > +		if (VIRTIO_VSOCK_SKB_CB(skb)->has_boundary_entries) {
> > 			size_t bytes_to_copy;
> > +			u32 msg_len, offset;
> > +			bool eor;
> > +			int ret;
> > 
> > -			bytes_to_copy = min(user_buf_len, pkt_len);
> > +			ret = vsock_uleb_decode_boundary(
> > +				vvs->boundary_buf + vvs->boundary_off,
> > +				vvs->boundary_len - vvs->boundary_off,
> > +				&msg_len, &eor);
> > +			if (ret < 0)
> > +				break;
> > +			vvs->boundary_off += ret;
> > 
> > -			if (bytes_to_copy) {
> > +			offset = VIRTIO_VSOCK_SKB_CB(skb)->offset;
> > +			bytes_to_copy = min(user_buf_len, (size_t)msg_len);
> > +
> > +			if (bytes_to_copy && dequeued_len >= 0) {
> > 				int err;
> > 
> > -				/* sk_lock is held by caller so no one else can dequeue.
> > -				 * Unlock rx_lock since skb_copy_datagram_iter() may sleep.
> > -				 */
> > 				spin_unlock_bh(&vvs->rx_lock);
> > -
> > -				err = skb_copy_datagram_iter(skb, 0,
> > +				err = skb_copy_datagram_iter(skb, offset,
> > 							     &msg->msg_iter,
> > 							     bytes_to_copy);
> > -				if (err) {
> > -					/* Copy of message failed. Rest of
> > -					 * fragments will be freed without copy.
> > -					 */
> > -					dequeued_len = err;
> > -				} else {
> > -					user_buf_len -= bytes_to_copy;
> > -				}
> > -
> > 				spin_lock_bh(&vvs->rx_lock);
> > +				if (err)
> > +					dequeued_len = err;
> > +				else
> > +					user_buf_len -= bytes_to_copy;
> > 			}
> > 
> > 			if (dequeued_len >= 0)
> > -				dequeued_len += pkt_len;
> > -		}
> > +				dequeued_len += msg_len;
> > 
> > -		if (le32_to_cpu(hdr->flags) & VIRTIO_VSOCK_SEQ_EOM) {
> > +			VIRTIO_VSOCK_SKB_CB(skb)->offset += msg_len;
> > 			msg_ready = true;
> > 			vvs->msg_count--;
> > 
> > -			if (le32_to_cpu(hdr->flags) & VIRTIO_VSOCK_SEQ_EOR)
> > +			if (eor)
> > 				msg->msg_flags |= MSG_EOR;
> > -		}
> > 
> > -		virtio_transport_dec_rx_pkt(vvs, pkt_len, pkt_len);
> > -		kfree_skb(skb);
> > +			virtio_transport_dec_rx_pkt(vvs, msg_len, msg_len);
> > +
> > +			if (VIRTIO_VSOCK_SKB_CB(skb)->offset >= skb->len) {
> > +				__skb_unlink(skb, &vvs->rx_queue);
> > +				kfree_skb(skb);
> > +			}
> > +
> > +			if (vvs->boundary_off >= vvs->boundary_len / 2)
> > +				vsock_boundary_buf_compact(vvs);
> > +		} else {
> > +			struct virtio_vsock_hdr *hdr;
> > +			size_t pkt_len;
> > +
> > +			skb = __skb_dequeue(&vvs->rx_queue);
> > +			if (!skb)
> > +				break;
> > +			hdr = virtio_vsock_hdr(skb);
> > +			pkt_len = (size_t)le32_to_cpu(hdr->len);
> > +
> > +			if (dequeued_len >= 0) {
> > +				size_t bytes_to_copy;
> > +
> > +				bytes_to_copy = min(user_buf_len, pkt_len);
> > +
> > +				if (bytes_to_copy) {
> > +					int err;
> > +
> > +					spin_unlock_bh(&vvs->rx_lock);
> > +					err = skb_copy_datagram_iter(
> > +						skb, 0, &msg->msg_iter,
> > +						bytes_to_copy);
> > +					if (err)
> > +						dequeued_len = err;
> > +					else
> > +						user_buf_len -= bytes_to_copy;
> > +					spin_lock_bh(&vvs->rx_lock);
> > +				}
> > +
> > +				if (dequeued_len >= 0)
> > +					dequeued_len += pkt_len;
> > +			}
> > +
> > +			if (le32_to_cpu(hdr->flags) & VIRTIO_VSOCK_SEQ_EOM) {
> > +				msg_ready = true;
> > +				vvs->msg_count--;
> > +
> > +				if (le32_to_cpu(hdr->flags) &
> > +				    VIRTIO_VSOCK_SEQ_EOR)
> > +					msg->msg_flags |= MSG_EOR;
> > +			}
> > +
> > +			virtio_transport_dec_rx_pkt(vvs, pkt_len, pkt_len);
> > +			kfree_skb(skb);
> > +		}
> > 	}
> > 
> > 	spin_unlock_bh(&vvs->rx_lock);
> > @@ -1132,6 +1299,7 @@ void virtio_transport_destruct(struct vsock_sock *vsk)
> > 
> > 	virtio_transport_cancel_close_work(vsk, true);
> > 
> > +	kfree(vvs->boundary_buf);
> > 	kfree(vvs);
> > 	vsk->trans = NULL;
> > }
> > @@ -1224,6 +1392,11 @@ static void virtio_transport_remove_sock(struct vsock_sock *vsk)
> > 	 * removing it.
> > 	 */
> > 	__skb_queue_purge(&vvs->rx_queue);
> > +	kfree(vvs->boundary_buf);
> > +	vvs->boundary_buf = NULL;
> > +	vvs->boundary_len = 0;
> > +	vvs->boundary_alloc = 0;
> > +	vvs->boundary_off = 0;
> > 	vsock_remove_sock(vsk);
> > }
> > 
> > @@ -1395,23 +1568,62 @@ virtio_transport_recv_enqueue(struct vsock_sock *vsk,
> > 	    !skb_is_nonlinear(skb)) {
> > 		struct virtio_vsock_hdr *last_hdr;
> > 		struct sk_buff *last_skb;
> > +		bool last_has_eom;
> > +		bool has_eom;
> > 
> > 		last_skb = skb_peek_tail(&vvs->rx_queue);
> > 		last_hdr = virtio_vsock_hdr(last_skb);
> > +		last_has_eom = le32_to_cpu(last_hdr->flags) & VIRTIO_VSOCK_SEQ_EOM;
> > +		has_eom = le32_to_cpu(hdr->flags) & VIRTIO_VSOCK_SEQ_EOM;
> > 
> > -		/* If there is space in the last packet queued, we copy the
> > -		 * new packet in its buffer. We avoid this if the last packet
> > -		 * queued has VIRTIO_VSOCK_SEQ_EOM set, because this is
> > -		 * delimiter of SEQPACKET message, so 'pkt' is the first packet
> > -		 * of a new message.
> > -		 */
> > -		if (skb->len < skb_tailroom(last_skb) &&
> > -		    !(le32_to_cpu(last_hdr->flags) & VIRTIO_VSOCK_SEQ_EOM)) {
> > -			memcpy(skb_put(last_skb, skb->len), skb->data, skb->len);
> > -			free_pkt = true;
> > -			last_hdr->flags |= hdr->flags;
> > -			le32_add_cpu(&last_hdr->len, len);
> > -			goto out;
> > +		if (skb->len < skb_tailroom(last_skb)) {
> > +			if (!last_has_eom) {
> > +				/* Same-message coalescing (existing path) */
> > +				memcpy(skb_put(last_skb, skb->len),
> > +				       skb->data, skb->len);
> > +				free_pkt = true;
> > +				last_hdr->flags |= hdr->flags;
> > +				le32_add_cpu(&last_hdr->len, len);
> > +				goto out;
> > +			}
> > +
> > +			/* Cross-EOM: coalesce complete messages into one skb,
> > +			 * recording message boundaries in a compact BER buffer.
> > +			 * Only when incoming packet also has EOM (complete msg).
> > +			 */
> > +			if (has_eom && !sk_psock(sk_vsock(vsk))) {
> > +				bool prev_eor, cur_eor;
> > +				u8 tmp[12];
> > +				int n = 0;
> > +
> > +				cur_eor = le32_to_cpu(hdr->flags) &
> > +					  VIRTIO_VSOCK_SEQ_EOR;
> > +
> > +				if (!VIRTIO_VSOCK_SKB_CB(last_skb)->has_boundary_entries) {
> > +					u32 prev_len = le32_to_cpu(last_hdr->len);
> > +
> > +					prev_eor = le32_to_cpu(last_hdr->flags) &
> > +						   VIRTIO_VSOCK_SEQ_EOR;
> > +					n += vsock_uleb_encode_boundary(
> > +						tmp + n, prev_len, prev_eor);
> > +				}
> > +				n += vsock_uleb_encode_boundary(
> > +					tmp + n, len, cur_eor);
> > +
> > +				if (!vsock_boundary_buf_ensure(
> > +					    vvs, vvs->boundary_len + n)) {
> > +					memcpy(vvs->boundary_buf +
> > +					       vvs->boundary_len, tmp, n);
> > +					vvs->boundary_len += n;
> > +					VIRTIO_VSOCK_SKB_CB(last_skb)->has_boundary_entries = true;
> > +					memcpy(skb_put(last_skb, skb->len),
> > +					       skb->data, skb->len);
> > +					free_pkt = true;
> > +					last_hdr->flags |= hdr->flags;
> > +					le32_add_cpu(&last_hdr->len, len);
> > +					goto out;
> > +				}
> > +			}
> > 		}
> > 	}
> > 
> >