linux-kernel - Re: [PATCH net 3/3] vsock: Fix transport

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Message-ID: <qvdeycblu6lsk7me77wsgoi3b5fyspz4gnrvl3m5lrqobveqwv@fhuhssggsxtk>
Date: Fri, 20 Jun 2025 10:37:49 +0200
From: Stefano Garzarella <sgarzare@...hat.com>
To: Michal Luczaj <mhal@...x.co>
Cc: "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, virtualization@...ts.linux.dev, 
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net 3/3] vsock: Fix transport_* TOCTOU

On Wed, Jun 18, 2025 at 02:34:02PM +0200, Michal Luczaj wrote:
>Transport assignment may race with module unload. Protect new_transport
>from becoming a stale pointer.
>
>This also takes care of an insecure call in vsock_use_local_transport();
>add a lockdep assert.
>
>BUG: unable to handle page fault for address: fffffbfff8056000
>Oops: Oops: 0000 [#1] SMP KASAN
>RIP: 0010:vsock_assign_transport+0x366/0x600
>Call Trace:
> vsock_connect+0x59c/0xc40
> __sys_connect+0xe8/0x100
> __x64_sys_connect+0x6e/0xc0
> do_syscall_64+0x92/0x1c0
> entry_SYSCALL_64_after_hwframe+0x4b/0x53
>
>Fixes: c0cfa2d8a788 ("vsock: add multi-transports support")
>Signed-off-by: Michal Luczaj <mhal@...x.co>
>---
> net/vmw_vsock/af_vsock.c | 24 +++++++++++++++++++-----
> 1 file changed, 19 insertions(+), 5 deletions(-)
>
>diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
>index 337540efc237c8bc482a6730948fc773c00854f1..133d7c8d2231e5c2e5e6a697de3b104fe05d8020 100644
>--- a/net/vmw_vsock/af_vsock.c
>+++ b/net/vmw_vsock/af_vsock.c
>@@ -407,6 +407,8 @@ EXPORT_SYMBOL_GPL(vsock_enqueue_accept);
>
> static bool vsock_use_local_transport(unsigned int remote_cid)
> {
>+	lockdep_assert_held(&vsock_register_mutex);
>+
> 	if (!transport_local)
> 		return false;
>
>@@ -464,6 +466,8 @@ int vsock_assign_transport(struct vsock_sock *vsk, struct vsock_sock *psk)
>
> 	remote_flags = vsk->remote_addr.svm_flags;
>
>+	mutex_lock(&vsock_register_mutex);
>+
> 	switch (sk->sk_type) {
> 	case SOCK_DGRAM:
> 		new_transport = transport_dgram;
>@@ -479,12 +483,15 @@ int vsock_assign_transport(struct vsock_sock *vsk, struct vsock_sock *psk)
> 			new_transport = transport_h2g;
> 		break;
> 	default:
>-		return -ESOCKTNOSUPPORT;
>+		ret = -ESOCKTNOSUPPORT;
>+		goto unlock;
> 	}
>
> 	if (vsk->transport) {
>-		if (vsk->transport == new_transport)
>-			return 0;
>+		if (vsk->transport == new_transport) {
>+			ret = 0;
>+			goto unlock;
>+		}
>
> 		/* transport->release() must be called with sock lock acquired.
> 		 * This path can only be taken during vsock_connect(), where we
>@@ -508,8 +515,12 @@ int vsock_assign_transport(struct vsock_sock *vsk, struct vsock_sock *psk)
> 	/* We increase the module refcnt to prevent the transport unloading
> 	 * while there are open sockets assigned to it.
> 	 */
>-	if (!new_transport || !try_module_get(new_transport->module))
>-		return -ENODEV;
>+	if (!new_transport || !try_module_get(new_transport->module)) {
>+		ret = -ENODEV;
>+		goto unlock;
>+	}
>+

I'd add a comment here to explain that we can release it since we
successfully increased the `new_transport` refcnt.

>+	mutex_unlock(&vsock_register_mutex);
>
> 	if (sk->sk_type == SOCK_SEQPACKET) {
> 		if (!new_transport->seqpacket_allow ||
>@@ -528,6 +539,9 @@ int vsock_assign_transport(struct vsock_sock *vsk, struct vsock_sock *psk)
> 	vsk->transport = new_transport;
>
> 	return 0;
>+unlock:

I'd call it `err:` so it's clear is the error path.

Thanks,
Stefano

>+	mutex_unlock(&vsock_register_mutex);
>+	return ret;
> }
> EXPORT_SYMBOL_GPL(vsock_assign_transport);
>
>
>-- 
>2.49.0
>