[PATCH RFC net v2 1/3] vsock: Fix transport_{g2h,h2g} TOCTOU

[Michal Luczaj]

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 <sgarz...@redhat.com>
Signed-off-by: Michal Luczaj <m...@rbox.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)
+{
+       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);
 
                if (put_user(cid, p) != 0)
                        retval = -EFAULT;

-- 
2.49.0