xenbus: Use kref to track req lifetime

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: xenbus: Use kref to track req lifetime Marek reported seeing a NULL pointer fault in the xenbus_thread callstack: BUG: kernel NULL pointer dereference, address: 0000000000000000 RIP: e030:__wake_up_common+0x4c/0x180 Call Trace: <TASK> __wake_up_common_lock+0x82/0xd0 process_msg+0x18e/0x2f0 xenbus_thread+0x165/0x1c0 process_msg+0x18e is req->cb(req). req->cb is set to xs_wake_up(), a thin wrapper around wake_up(), or xenbus_dev_queue_reply(). It seems like it was xs_wake_up() in this case. It seems like req may have woken up the xs_wait_for_reply(), which kfree()ed the req. When xenbus_thread resumes, it faults on the zero-ed data. Linux Device Drivers 2nd edition states: "Normally, a wake_up call can cause an immediate reschedule to happen, meaning that other processes might run before wake_up returns." ... which would match the behaviour observed. Change to keeping two krefs on each request. One for the caller, and one for xenbus_thread. Each will kref_put() when finished, and the last will free it. This use of kref matches the description in Documentation/core-api/kref.rst

References

• https://git.kernel.org/stable/c/0e94a246bb6d9538010b6c02d2b1d4717a97b2e5
• https://git.kernel.org/stable/c/f1bcac367bc95631afbb918348f30dec887d0e1b
• https://git.kernel.org/stable/c/4d260a5558df4650eb87bc41b2c9ac2d6b2ba447
• https://git.kernel.org/stable/c/8b02f85e84dc6f7c150cef40ddb69af5a25659e5
• https://git.kernel.org/stable/c/cbfaf46b88a4c01b64c4186cdccd766c19ae644c
• https://git.kernel.org/stable/c/8e9c8a0393b5f85f1820c565ab8105660f4e8f92
• https://git.kernel.org/stable/c/2466b0f66795c3c426cacc8998499f38031dbb59
• https://git.kernel.org/stable/c/1f0304dfd9d217c2f8b04a9ef4b3258a66eedd27