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

On Fri, 20 Jun 2025 at 16:23, Michal Luczaj [off-list ref] wrote:

On 6/20/25 15:20, Stefano Garzarella wrote:

quoted

On Fri, Jun 20, 2025 at 02:58:49PM +0200, Michal Luczaj wrote:

quoted

On 6/20/25 10:32, Stefano Garzarella wrote:

quoted

On Wed, Jun 18, 2025 at 02:34:00PM +0200, Michal Luczaj wrote:

quoted

Checking transport_{h2g,g2h} != NULL may race with vsock_core_unregister().
Make sure pointers remain valid.

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")
Signed-off-by: Michal Luczaj <redacted>
---
net/vmw_vsock/af_vsock.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
index 2e7a3034e965db30b6ee295370d866e6d8b1c341..047d1bc773fab9c315a6ccd383a451fa11fb703e 100644
--- a/net/vmw_vsock/af_vsock.c
+++ b/net/vmw_vsock/af_vsock.c

@@ -2541,6 +2541,8 @@ static long vsock_dev_do_ioctl(struct file *filp,

   switch (cmd) {
   case IOCTL_VM_SOCKETS_GET_LOCAL_CID:
+          mutex_lock(&vsock_register_mutex);
+
           /* To be compatible with the VMCI behavior, we prioritize the
            * guest CID instead of well-know host CID (VMADDR_CID_HOST).
            */

@@ -2549,6 +2551,8 @@ static long vsock_dev_do_ioctl(struct file *filp,
           else if (transport_h2g)
                   cid = transport_h2g->get_local_cid();

+          mutex_unlock(&vsock_register_mutex);

What about if we introduce a new `vsock_get_local_cid`:

u32 vsock_get_local_cid() {
    u32 cid = VMADDR_CID_ANY;

    mutex_lock(&vsock_register_mutex);
    /* 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();
    mutex_lock(&vsock_register_mutex);

    return cid;
}


And we use it here, and in the place fixed by next patch?

I think we can fix all in a single patch, the problem here is to call
transport_*->get_local_cid() without the lock IIUC.

Do you mean:

bool vsock_find_cid(unsigned int cid)
{
-       if (transport_g2h && cid == transport_g2h->get_local_cid())
+       if (transport_g2h && cid == vsock_get_local_cid())
               return true;

?

Nope, I meant:

  bool vsock_find_cid(unsigned int cid)
  {
-       if (transport_g2h && cid == transport_g2h->get_local_cid())
-               return true;
-
-       if (transport_h2g && cid == VMADDR_CID_HOST)
+       if (cid == vsock_get_local_cid())
                 return true;

         if (transport_local && cid == VMADDR_CID_LOCAL)

But it does change the behaviour, doesn't it? With this patch, (with g2h
loaded) if cid fails to match g2h->get_local_cid(), we don't fall back to
h2g case any more, i.e. no more comparing cid with VMADDR_CID_HOST.

It's friday... yep, you're right!

quoted

But now I'm thinking if we should also include `transport_local` in the
new `vsock_get_local_cid()`.

I think that will fix an issue when calling
IOCTL_VM_SOCKETS_GET_LOCAL_CID and only vsock-loopback kernel module is
loaded, so maybe we can do 2 patches:

1. fix IOCTL_VM_SOCKETS_GET_LOCAL_CID to check also `transport_local`
    Fixes: 0e12190578d0 ("vsock: add local transport support in the vsock core")

What would be the transport priority with transport_local thrown in? E.g.
if we have both local and g2h, ioctl should return VMADDR_CID_LOCAL or
transport_g2h->get_local_cid()?

Should return the G2H, LOCAL is more for debug/test, so I'd return it
only if anything else is loaded.

quoted

2. move that code in vsock_get_local_cid() with proper locking and use
it also in vsock_find_cid()

WDYT?

Yeah, sure about 1, I'll add it to the series. I'm just still not certain
how useful vsock_get_local_cid() would be for vsock_find_cid().

Feel free to drop 1 too, we can send it later if it's not really
related to this issue.
About the series, maybe it is better to have a single patch that fixes
the access to ->get_local_cid() with proper locking.
But I don't have a strong opinion on that. I see it like a single
problem to fix, but up to you.

Thanks,
Stefano