vsock: Fix transport_{g2h,h2g} TOCTOU

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@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Link: https://patch.msgid.link/20250703-vsock-transports-toctou-v4-1-98f0eb530747@rbox.co
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
index 2e7a3034e965db30b6ee295370d866e6d8b1c341..39473b9e0829f240045262aef00cbae82a425dcc 100644 (file)
--- a/net/vmw_vsock/af_vsock.c
+++ b/net/vmw_vsock/af_vsock.c
@@ -531,9 +531,25 @@ int vsock_assign_transport(struct vsock_sock *vsk, struct vsock_sock *psk)
 }
 EXPORT_SYMBOL_GPL(vsock_assign_transport);
 
+/*
+ * Provide safe access to static transport_{h2g,g2h,dgram,local} callbacks.
+ * Otherwise we may race with module removal. Do not use on `vsk->transport`.
+ */
+static u32 vsock_registered_transport_cid(const struct vsock_transport **transport)
+{
+       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_registered_transport_cid(&transport_g2h))
                return true;
 
        if (transport_h2g && cid == VMADDR_CID_HOST)
@@ -2536,18 +2552,17 @@ 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_registered_transport_cid(&transport_g2h);
+               if (cid == VMADDR_CID_ANY)
+                       cid = vsock_registered_transport_cid(&transport_h2g);
 
                if (put_user(cid, p) != 0)
                        retval = -EFAULT;