Merge branch 'vsock-virtio-fix-tx-credit-handling'

Stefano Garzarella says:

====================
vsock/virtio: fix TX credit handling

The original series was posted by Melbin K Mathew <mlbnkm1@gmail.com> till v4.
Since it's a real issue and the original author seems busy, I'm sending
the new version fixing my comments but keeping the authorship (and restoring
mine on patch 2 as reported on v4).

v5: https://lore.kernel.org/netdev/20260116201517.273302-1-sgarzare@redhat.com/
v4: https://lore.kernel.org/netdev/20251217181206.3681159-1-mlbnkm1@gmail.com/

From Melbin K Mathew <mlbnkm1@gmail.com>:

This series fixes TX credit handling in virtio-vsock:

Patch 1: Fix potential underflow in get_credit() using s64 arithmetic
Patch 2: Fix vsock_test seqpacket bounds test
Patch 3: Cap TX credit to local buffer size (security hardening)
Patch 4: Add stream TX credit bounds regression test

The core issue is that a malicious guest can advertise a huge buffer
size via SO_VM_SOCKETS_BUFFER_SIZE, causing the host to allocate
excessive sk_buff memory when sending data to that guest.

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; the system only
recovered after killing the QEMU process.

With this series applied, the same PoC shows only ~35 MiB increase in
Slab/SUnreclaim, no host OOM, and the guest remains responsive.
====================

Link: https://patch.msgid.link/20260121093628.9941-1-sgarzare@redhat.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: Paolo Abeni

net/vmw_vsock/virtio_transport_common.c

@@ -28,6 +28,7 @@
 
 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);
 
 static const struct virtio_transport *
 virtio_transport_get_ops(struct vsock_sock *vsk)
@@ -499,9 +500,7 @@ 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);
-	if (ret > credit)
-		ret = credit;
+	ret = min_t(u32, credit, virtio_transport_has_space(vvs));
 	vvs->tx_cnt += ret;
 	vvs->bytes_unsent += ret;
 	spin_unlock_bh(&vvs->tx_lock);
@@ -822,6 +821,15 @@ virtio_transport_seqpacket_dequeue(struct vsock_sock *vsk,
 }
 EXPORT_SYMBOL_GPL(virtio_transport_seqpacket_dequeue);
 
+static u32 virtio_transport_tx_buf_size(struct virtio_vsock_sock *vvs)
+{
+	/* The peer advertises its receive buffer via peer_buf_alloc, but we
+	 * cap it to our local buf_alloc so a remote peer cannot force us to
+	 * queue more data than our own buffer configuration allows.
+	 */
+	return min(vvs->peer_buf_alloc, vvs->buf_alloc);
+}
+
 int
 virtio_transport_seqpacket_enqueue(struct vsock_sock *vsk,
 				   struct msghdr *msg,
@@ -831,7 +839,7 @@ 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_size(vvs)) {
 		spin_unlock_bh(&vvs->tx_lock);
 		return -EMSGSIZE;
 	}
@@ -877,12 +885,16 @@ u32 virtio_transport_seqpacket_has_data(struct vsock_sock *vsk)
 }
 EXPORT_SYMBOL_GPL(virtio_transport_seqpacket_has_data);
 
-static s64 virtio_transport_has_space(struct vsock_sock *vsk)
+static s64 virtio_transport_has_space(struct virtio_vsock_sock *vvs)
 {
-	struct virtio_vsock_sock *vvs = vsk->trans;
 	s64 bytes;
 
-	bytes = (s64)vvs->peer_buf_alloc - (vvs->tx_cnt - vvs->peer_fwd_cnt);
+	/* Use s64 arithmetic so if the peer shrinks peer_buf_alloc while
+	 * we have bytes in flight (tx_cnt - peer_fwd_cnt), the subtraction
+	 * does not underflow.
+	 */
+	bytes = (s64)virtio_transport_tx_buf_size(vvs) -
+		(vvs->tx_cnt - vvs->peer_fwd_cnt);
 	if (bytes < 0)
 		bytes = 0;
 
@@ -895,7 +907,7 @@ s64 virtio_transport_stream_has_space(struct vsock_sock *vsk)
 	s64 bytes;
 
 	spin_lock_bh(&vvs->tx_lock);
-	bytes = virtio_transport_has_space(vsk);
+	bytes = virtio_transport_has_space(vvs);
 	spin_unlock_bh(&vvs->tx_lock);
 
 	return bytes;
@@ -1492,7 +1504,7 @@ static bool virtio_transport_space_update(struct sock *sk,
 	spin_lock_bh(&vvs->tx_lock);
 	vvs->peer_buf_alloc = le32_to_cpu(hdr->buf_alloc);
 	vvs->peer_fwd_cnt = le32_to_cpu(hdr->fwd_cnt);
-	space_available = virtio_transport_has_space(vsk);
+	space_available = virtio_transport_has_space(vvs);
 	spin_unlock_bh(&vvs->tx_lock);
 	return space_available;
 }

tools/testing/vsock/vsock_test.c

@@ -347,10 +347,12 @@ static void test_stream_msg_peek_server(const struct test_opts *opts)
 }
 
 #define SOCK_BUF_SIZE (2 * 1024 * 1024)
+#define SOCK_BUF_SIZE_SMALL (64 * 1024)
 #define MAX_MSG_PAGES 4
 
 static void test_seqpacket_msg_bounds_client(const struct test_opts *opts)
 {
+	unsigned long long sock_buf_size;
 	unsigned long curr_hash;
 	size_t max_msg_size;
 	int page_size;
@@ -363,6 +365,16 @@ static void test_seqpacket_msg_bounds_client(const struct test_opts *opts)
 		exit(EXIT_FAILURE);
 	}
 
+	sock_buf_size = SOCK_BUF_SIZE;
+
+	setsockopt_ull_check(fd, AF_VSOCK, SO_VM_SOCKETS_BUFFER_MAX_SIZE,
+			     sock_buf_size,
+			     "setsockopt(SO_VM_SOCKETS_BUFFER_MAX_SIZE)");
+
+	setsockopt_ull_check(fd, AF_VSOCK, SO_VM_SOCKETS_BUFFER_SIZE,
+			     sock_buf_size,
+			     "setsockopt(SO_VM_SOCKETS_BUFFER_SIZE)");
+
 	/* Wait, until receiver sets buffer size. */
 	control_expectln("SRVREADY");
 
@@ -2219,6 +2231,101 @@ static void test_stream_accepted_setsockopt_server(const struct test_opts *opts)
 	close(fd);
 }
 
+static void test_stream_tx_credit_bounds_client(const struct test_opts *opts)
+{
+	unsigned long long sock_buf_size;
+	size_t total = 0;
+	char buf[4096];
+	int fd;
+
+	memset(buf, 'A', sizeof(buf));
+
+	fd = vsock_stream_connect(opts->peer_cid, opts->peer_port);
+	if (fd < 0) {
+		perror("connect");
+		exit(EXIT_FAILURE);
+	}
+
+	sock_buf_size = SOCK_BUF_SIZE_SMALL;
+
+	setsockopt_ull_check(fd, AF_VSOCK, SO_VM_SOCKETS_BUFFER_MAX_SIZE,
+			     sock_buf_size,
+			     "setsockopt(SO_VM_SOCKETS_BUFFER_MAX_SIZE)");
+
+	setsockopt_ull_check(fd, AF_VSOCK, SO_VM_SOCKETS_BUFFER_SIZE,
+			     sock_buf_size,
+			     "setsockopt(SO_VM_SOCKETS_BUFFER_SIZE)");
+
+	if (fcntl(fd, F_SETFL, fcntl(fd, F_GETFL, 0) | O_NONBLOCK) < 0) {
+		perror("fcntl(F_SETFL)");
+		exit(EXIT_FAILURE);
+	}
+
+	control_expectln("SRVREADY");
+
+	for (;;) {
+		ssize_t sent = send(fd, buf, sizeof(buf), 0);
+
+		if (sent == 0) {
+			fprintf(stderr, "unexpected EOF while sending bytes\n");
+			exit(EXIT_FAILURE);
+		}
+
+		if (sent < 0) {
+			if (errno == EINTR)
+				continue;
+
+			if (errno == EAGAIN || errno == EWOULDBLOCK)
+				break;
+
+			perror("send");
+			exit(EXIT_FAILURE);
+		}
+
+		total += sent;
+	}
+
+	control_writeln("CLIDONE");
+	close(fd);
+
+	/* We should not be able to send more bytes than the value set as
+	 * local buffer size.
+	 */
+	if (total > sock_buf_size) {
+		fprintf(stderr,
+			"TX credit too large: queued %zu bytes (expected <= %llu)\n",
+			total, sock_buf_size);
+		exit(EXIT_FAILURE);
+	}
+}
+
+static void test_stream_tx_credit_bounds_server(const struct test_opts *opts)
+{
+	unsigned long long sock_buf_size;
+	int fd;
+
+	fd = vsock_stream_accept(VMADDR_CID_ANY, opts->peer_port, NULL);
+	if (fd < 0) {
+		perror("accept");
+		exit(EXIT_FAILURE);
+	}
+
+	sock_buf_size = SOCK_BUF_SIZE;
+
+	setsockopt_ull_check(fd, AF_VSOCK, SO_VM_SOCKETS_BUFFER_MAX_SIZE,
+			     sock_buf_size,
+			     "setsockopt(SO_VM_SOCKETS_BUFFER_MAX_SIZE)");
+
+	setsockopt_ull_check(fd, AF_VSOCK, SO_VM_SOCKETS_BUFFER_SIZE,
+			     sock_buf_size,
+			     "setsockopt(SO_VM_SOCKETS_BUFFER_SIZE)");
+
+	control_writeln("SRVREADY");
+	control_expectln("CLIDONE");
+
+	close(fd);
+}
+
 static struct test_case test_cases[] = {
 	{
 		.name = "SOCK_STREAM connection reset",
@@ -2408,6 +2515,11 @@ static struct test_case test_cases[] = {
 		.run_client = test_stream_msgzcopy_mangle_client,
 		.run_server = test_stream_msgzcopy_mangle_server,
 	},
+	{
+		.name = "SOCK_STREAM TX credit bounds",
+		.run_client = test_stream_tx_credit_bounds_client,
+		.run_server = test_stream_tx_credit_bounds_server,
+	},
 	{},
 };