linux-kernel - Re: [PATCH RFC net v2 1/3] vsock: Fix transport

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Message-ID: <zdiqu6pszqwb4y5o7oqzdovfvzkbrvc6ijuxoef2iloklahyoy@njsnvn7hfwye>
Date: Wed, 25 Jun 2025 10:43:12 +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>, 
	Stefan Hajnoczi <stefanha@...hat.com>, virtualization@...ts.linux.dev, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC net v2 1/3] vsock: Fix transport_{g2h,h2g} TOCTOU

On Fri, Jun 20, 2025 at 09:52:43PM +0200, Michal Luczaj wrote:
>vsock_find_cid() and vsock_dev_do_ioctl() may race with module unload.
>transport_{g2h,h2g} may become NULL after the NULL check.
>
>Introduce vsock_transport_local_cid() to protect from a potential
>null-ptr-deref.
>
>KASAN: null-ptr-deref in range [0x0000000000000118-0x000000000000011f]
>RIP: 0010:vsock_find_cid+0x47/0x90
>Call Trace:
> __vsock_bind+0x4b2/0x720
> vsock_bind+0x90/0xe0
> __sys_bind+0x14d/0x1e0
> __x64_sys_bind+0x6e/0xc0
> do_syscall_64+0x92/0x1c0
> entry_SYSCALL_64_after_hwframe+0x4b/0x53
>
>KASAN: null-ptr-deref in range [0x0000000000000118-0x000000000000011f]
>RIP: 0010:vsock_dev_do_ioctl.isra.0+0x58/0xf0
>Call Trace:
> __x64_sys_ioctl+0x12d/0x190
> do_syscall_64+0x92/0x1c0
> entry_SYSCALL_64_after_hwframe+0x4b/0x53
>
>Fixes: c0cfa2d8a788 ("vsock: add multi-transports support")
>Suggested-by: Stefano Garzarella <sgarzare@...hat.com>
>Signed-off-by: Michal Luczaj <mhal@...x.co>
>---
> net/vmw_vsock/af_vsock.c | 23 +++++++++++++++++------
> 1 file changed, 17 insertions(+), 6 deletions(-)
>
>diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
>index 2e7a3034e965db30b6ee295370d866e6d8b1c341..63a920af5bfe6960306a3e5eeae0cbf30648985e 100644
>--- a/net/vmw_vsock/af_vsock.c
>+++ b/net/vmw_vsock/af_vsock.c
>@@ -531,9 +531,21 @@ int vsock_assign_transport(struct vsock_sock *vsk, struct vsock_sock *psk)
> }
> EXPORT_SYMBOL_GPL(vsock_assign_transport);
>
>+static u32 vsock_transport_local_cid(const struct vsock_transport **transport)

Why we need double pointer?

>+{
>+	u32 cid = VMADDR_CID_ANY;
>+
>+	mutex_lock(&vsock_register_mutex);
>+	if (*transport)
>+		cid = (*transport)->get_local_cid();
>+	mutex_unlock(&vsock_register_mutex);
>+
>+	return cid;
>+}
>+
> bool vsock_find_cid(unsigned int cid)
> {
>-	if (transport_g2h && cid == transport_g2h->get_local_cid())
>+	if (cid == vsock_transport_local_cid(&transport_g2h))
> 		return true;
>
> 	if (transport_h2g && cid == VMADDR_CID_HOST)
>@@ -2536,18 +2548,17 @@ static long vsock_dev_do_ioctl(struct file *filp,
> 			       unsigned int cmd, void __user *ptr)
> {
> 	u32 __user *p = ptr;
>-	u32 cid = VMADDR_CID_ANY;
> 	int retval = 0;
>+	u32 cid;
>
> 	switch (cmd) {
> 	case IOCTL_VM_SOCKETS_GET_LOCAL_CID:
> 		/* To be compatible with the VMCI behavior, we prioritize the
> 		 * guest CID instead of well-know host CID (VMADDR_CID_HOST).
> 		 */
>-		if (transport_g2h)
>-			cid = transport_g2h->get_local_cid();
>-		else if (transport_h2g)
>-			cid = transport_h2g->get_local_cid();
>+		cid = vsock_transport_local_cid(&transport_g2h);
>+		if (cid == VMADDR_CID_ANY)
>+			cid = vsock_transport_local_cid(&transport_h2g);

I still prefer the old `if ... else if ...`, what is the reason of this
change? I may miss the point.

But overall LGTM!

Thanks,
Stefano

>
> 		if (put_user(cid, p) != 0)
> 			retval = -EFAULT;
>
>-- 
>2.49.0
>