CVE-2024-26996

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

usb: gadget: f_ncm: Fix UAF ncm object at re-bind after usb ep transport error

When ncm function is working and then stop usb0 interface for link down, eth_stop() is called. At this piont, accidentally if usb transport error should happen in usb_ep_enable(), in_ep and/or out_ep may not be enabled.

After that, ncm_disable() is called to disable for ncm unbind but gether_disconnect() is never called since in_ep is not enabled.

As the result, ncm object is released in ncm unbind but dev->port_usb associated to ncm->port is not NULL.

And when ncm bind again to recover netdev, ncm object is reallocated but usb0 interface is already associated to previous released ncm object.

Therefore, once usb0 interface is up and eth_start_xmit() is called, released ncm object is dereferrenced and it might cause use-after-free memory.

[function unlink via configfs] usb0: eth_stop dev->port_usb=ffffff9b179c3200 –> error happens in usb_ep_enable(). NCM: ncm_disable: ncm=ffffff9b179c3200 –> no gether_disconnect() since ncm->port.in_ep->enabled is false. NCM: ncm_unbind: ncm unbind ncm=ffffff9b179c3200 NCM: ncm_free: ncm free ncm=ffffff9b179c3200 <– released ncm

[function link via configfs] NCM: ncm_alloc: ncm alloc ncm=ffffff9ac4f8a000 NCM: ncm_bind: ncm bind ncm=ffffff9ac4f8a000 NCM: ncm_set_alt: ncm=ffffff9ac4f8a000 alt=0 usb0: eth_open dev->port_usb=ffffff9b179c3200 <– previous released ncm usb0: eth_start dev->port_usb=ffffff9b179c3200 <– eth_start_xmit() –> dev->wrap() Unable to handle kernel paging request at virtual address dead00000000014f

This patch addresses the issue by checking if ncm->netdev is not NULL at ncm_disable() to call gether_disconnect() to deassociate dev->port_usb. Its more reasonable to check ncm->netdev to call gether_connect/disconnect rather than check ncm->port.in_ep->enabled since it might not be enabled but the gether connection might be established.

Weakness

Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code.

Affected Software

Extended Description

The use of previously-freed memory can have any number of adverse consequences, ranging from the corruption of valid data to the execution of arbitrary code, depending on the instantiation and timing of the flaw. The simplest way data corruption may occur involves the system’s reuse of the freed memory. Use-after-free errors have two common and sometimes overlapping causes:

In this scenario, the memory in question is allocated to another pointer validly at some point after it has been freed. The original pointer to the freed memory is used again and points to somewhere within the new allocation. As the data is changed, it corrupts the validly used memory; this induces undefined behavior in the process. If the newly allocated data happens to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.

Potential Mitigations

References

• https://git.kernel.org/stable/c/0588bbbd718a8130b98c54518f1e0b569ce60a93
• https://git.kernel.org/stable/c/6334b8e4553cc69f51e383c9de545082213d785e
• https://git.kernel.org/stable/c/7250326cbb1f4f90391ac511a126b936cefb5bb7
• https://git.kernel.org/stable/c/7f67c2020cb08499c400abf0fc32c65e4d9a09ca
• https://git.kernel.org/stable/c/f356fd0cbd9c9cbd0854657a80d1608d0d732db3
• https://git.kernel.org/stable/c/0588bbbd718a8130b98c54518f1e0b569ce60a93
• https://git.kernel.org/stable/c/6334b8e4553cc69f51e383c9de545082213d785e
• https://git.kernel.org/stable/c/7250326cbb1f4f90391ac511a126b936cefb5bb7
• https://git.kernel.org/stable/c/7f67c2020cb08499c400abf0fc32c65e4d9a09ca
• https://git.kernel.org/stable/c/f356fd0cbd9c9cbd0854657a80d1608d0d732db3