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Tsirkin" , Jason Wang , =?UTF-8?q?Eugenio=20P=C3=A9rez?= , Xuan Zhuo , kvm@vger.kernel.org, virtualization@lists.linux.dev, netdev@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH net] vhost/vsock: trim nonlinear SKBs to declared payload length Date: Wed, 30 Sep 2026 13:41:45 +0900 Message-ID: <20260930044147.3818241-1-4ncienth@gmail.com> X-Mailer: git-send-email 2.55.0 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit vhost_vsock_alloc_skb() sizes its skb from the total guest descriptor length, while virtio_vsock_hdr.len independently declares the payload length. Since commit ab9aa2f3afc2 ("vhost/vsock: Allocate nonlinear SKBs for handling large receive buffers"), large descriptors use skb fragments. virtio_vsock_skb_put() currently changes only skb->len for a nonlinear skb, leaving skb->data_len and all fragments attached. The zero-payload fast path skips the helper entirely. A guest can therefore retain the full descriptor allocation while receive credit accounts no payload. On Linux v7.2, 455 zero-payload skbs retained 30,255,680 bytes on a 256 KiB receive buffer while rx_bytes and buf_used remained zero. A full-payload control retained 265,984 bytes in four skbs. The existing SKB_TRUESIZE(0) queue budget caps skb count but does not account for these descriptor-sized fragments. Set skb->len to the fragment length before calling pskb_trim(), then trim to the declared payload length. This releases unused fragments and lets skb_condense() reduce truesize. Move the zero-payload return after the helper so zero-length packets are trimmed too. These skbs are newly allocated, unique, and have no frag_list, so the trim does not enter an allocation-bearing path. Keep a warning for a violated caller contract. The fixed v7.2 image left a one-byte skb with no fragments and reduced the zero-payload queue to one 960-byte skb. The build had no compiler warnings, and the run had no sanitizer, WARN, oops, or panic findings. Fixes: ab9aa2f3afc2 ("vhost/vsock: Allocate nonlinear SKBs for handling large receive buffers") Cc: stable@vger.kernel.org Assisted-by: LLM Signed-off-by: Daehyeon Ko <4ncienth@gmail.com> --- Required configuration is CONFIG_VSOCKETS, CONFIG_VIRTIO_VSOCKETS_COMMON, and CONFIG_VHOST_VSOCK. The source reproducer is available privately to maintainers and is omitted from this public AI-assisted report. It exercises the real allocation helper and VSOCK receive queue from an in-kernel module, not a live guest virtqueue. Guest control is source-confirmed, but live guest end-to-end validation and deliberate host OOM were not performed. No KASAN splat is expected or claimed; the oracle is retained truesize and receive credit state above. drivers/vhost/vsock.c | 6 ++---- include/linux/virtio_vsock.h | 9 ++++++--- 2 files changed, 8 insertions(+), 7 deletions(-) diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c index abed1fbcf66c..fa59456abda9 100644 --- a/drivers/vhost/vsock.c +++ b/drivers/vhost/vsock.c @@ -400,10 +400,6 @@ vhost_vsock_alloc_skb(struct vhost_virtqueue *vq, payload_len = le32_to_cpu(hdr->len); - /* No payload */ - if (!payload_len) - return skb; - /* The pkt is too big or the length in the header is invalid */ if (payload_len + sizeof(*hdr) > len) { kfree_skb(skb); @@ -411,6 +407,8 @@ vhost_vsock_alloc_skb(struct vhost_virtqueue *vq, } virtio_vsock_skb_put(skb, payload_len); + if (!payload_len) + return skb; if (skb_copy_datagram_from_iter(skb, 0, &iov_iter, payload_len)) { vq_err(vq, "Failed to copy %zu byte payload\n", payload_len); diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h index f91704731057..31358683e23e 100644 --- a/include/linux/virtio_vsock.h +++ b/include/linux/virtio_vsock.h @@ -51,10 +51,13 @@ static inline void virtio_vsock_skb_put(struct sk_buff *skb, u32 len) { DEBUG_NET_WARN_ON_ONCE(skb->len); - if (skb_is_nonlinear(skb)) - skb->len = len; - else + if (skb_is_nonlinear(skb)) { + skb->len = skb->data_len; + if (WARN_ON_ONCE(pskb_trim(skb, len))) + return; + } else { skb_put(skb, len); + } } static inline struct sk_buff * base-commit: 54518e0e827f4ca9229ae657022c60bf60f5c1bf -- 2.55.0