CVE-2024-55916

In the Linux kernel, the following vulnerability has been resolved:

Drivers: hv: util: Avoid accessing a ringbuffer not initialized yet

If the KVP (or VSS) daemon starts before the VMBus channels ringbuffer is fully initialized, we can hit the panic below:

hv_utils: Registering HyperV Utility Driver hv_vmbus: registering driver hv_utils … BUG: kernel NULL pointer dereference, address: 0000000000000000 CPU: 44 UID: 0 PID: 2552 Comm: hv_kvp_daemon Tainted: G E 6.11.0-rc3+ #1 RIP: 0010:hv_pkt_iter_first+0x12/0xd0 Call Trace: … vmbus_recvpacket hv_kvp_onchannelcallback vmbus_on_event tasklet_action_common tasklet_action handle_softirqs irq_exit_rcu sysvec_hyperv_stimer0 asm_sysvec_hyperv_stimer0 … kvp_register_done hvt_op_read vfs_read ksys_read __x64_sys_read

This can happen because the KVP/VSS channel callback can be invoked even before the channel is fully opened:

1. as soon as hv_kvp_init() -> hvutil_transport_init() creates /dev/vmbus/hv_kvp, the kvp daemon can open the device file immediately and register itself to the driver by writing a message KVP_OP_REGISTER1 to the file (which is handled by kvp_on_msg() ->kvp_handle_handshake()) and reading the file for the drivers response, which is handled by hvt_op_read(), which calls hvt->on_read(), i.e. kvp_register_done().

2. the problem with kvp_register_done() is that it can cause the channel callback to be called even before the channel is fully opened, and when the channel callback is starting to run, util_probe()-> vmbus_open() may have not initialized the ringbuffer yet, so the callback can hit the panic of NULL pointer dereference.

To reproduce the panic consistently, we can add a ssleep(10) for KVP in __vmbus_open(), just before the first hv_ringbuffer_init(), and then we unload and reload the driver hv_utils, and run the daemon manually within the 10 seconds.

Fix the panic by reordering the steps in util_probe() so the char dev entry used by the KVP or VSS daemon is not created until after vmbus_open() has completed. This reordering prevents the race condition from happening.

Weakness

A NULL pointer dereference occurs when the application dereferences a pointer that it expects to be valid, but is NULL, typically causing a crash or exit.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/042253c57be901bfd19f15b68267442b70f510d5
• https://git.kernel.org/stable/c/07a756a49f4b4290b49ea46e089cbe6f79ff8d26
• https://git.kernel.org/stable/c/3dd7a30c6d7f90afcf19e9b072f572ba524d7ec6
• https://git.kernel.org/stable/c/718fe694a334be9d1a89eed22602369ac18d6583
• https://git.kernel.org/stable/c/89fcec5e466b3ac9b376e0d621c71effa1a7983f
• https://git.kernel.org/stable/c/d81f4e73aff9b861671df60e5100ad25cc16fbf8
• https://git.kernel.org/stable/c/f091a224a2c82f1e302b1768d73bb6332f687321