[PATCH net] hv_sock: Report no unread data before a channel is available

[Andrea Parri <parri.andrea@gmail.com>]

The Hyper-V transport is assigned before the asynchronous VMBus channel
offer completes.  Its zero-initialized hvsock therefore has a NULL chan
until hvs_open_connection() associates the channel with the socket.

SIOCINQ can query the assigned transport while the socket is connecting,
or after connect() fails once the transport has been assigned.  Unlike
poll(), that path does not check stream_is_active() before calling the
transport's stream_has_data callback.  hvs_stream_has_data() then passes
the NULL channel to hvs_channel_readable_payload(), and any user in the
guest can trigger an oops:

  BUG: kernel NULL pointer dereference, address: 0000000000000178
  RIP: 0010:hvs_stream_has_data+0x23/0x120
  Call Trace:
   <TASK>
   ? vsock_ioctl+0x74/0xf0
   sock_do_ioctl+0x42/0xf0
   sock_ioctl+0x27d/0x310
   do_vfs_ioctl+0x700/0x810
   __x64_sys_ioctl+0x7d/0xf0

Return zero unread bytes when no channel is available, before accessing
the receive state or ring.  Leave the existing channel-backed receive
handling unchanged.

The oops was reproduced on an unmodified v7.4-rc1 kernel in a WSL2
guest with a small user-space program, available on request.  With this
patch, the program reports zero unread bytes and the kernel logs nothing.
Other vsock_test cases were not run, as most of them need a host-side
peer.

AI assistance was used to identify the issue and develop the fix.

Fixes: f7c722659275 ("vsock: Add support for SIOCINQ ioctl")
Cc: stable@vger.kernel.org
Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
---
An alternative is to check stream_is_active() in vsock_do_ioctl(), as
vsock_poll() does.  That would also change SIOCINQ on VMCI sockets that
are not connected yet, from -ENOENT to 0.  I kept this fix in hv_sock to
limit it to the crash, and can send the generic check separately for
net-next.

The reproducer is a new vsock_test case.  I have not posted it, as
Documentation/process/security-bugs.rst asks for reproducers of bugs
found with AI assistance.  I can send it privately, and will post it
once this fix is merged.

 net/vmw_vsock/hyperv_transport.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/vmw_vsock/hyperv_transport.c b/net/vmw_vsock/hyperv_transport.c
index 855b15971c359..551eabe690144 100644
--- a/net/vmw_vsock/hyperv_transport.c
+++ b/net/vmw_vsock/hyperv_transport.c
@@ -697,6 +697,9 @@ static s64 hvs_stream_has_data(struct vsock_sock *vsk)
 	struct hvsock *hvs = vsk->trans;
 	s64 ret;
 
+	if (!hvs->chan)
+		return 0;
+
 	if (hvs->recv_data_len > 0)
 		return hvs->recv_data_len;
 

base-commit: 475ff14ba558bb8857bca878f1d593784ccc4f0f
-- 
2.53.0