CVE-2026-63803

Summary

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

hdlc_ppp: sync per-proto timers before freeing hdlc state

Each PPP control protocol (LCP/IPCP/IPV6CP) embedded in struct ppp registers a timer via timer_setup(). That struct ppp is the hdlc->state allocation, which detach_hdlc_protocol() frees with kfree() in both teardown paths: unregister_hdlc_device() and the re-attach inside attach_hdlc_protocol().

The ppp proto never registered a .detach callback, so detach_hdlc_protocol() performs no timer synchronization before the kfree(). The only cancel, timer_delete(&proto->timer) in ppp_cp_event(), is partial (it does not wait for a running callback) and only runs on the ->CLOSED transition; ppp_stop()/ppp_close() do not sync either. A ppp_timer callback already executing (blocked on ppp->lock) survives the kfree and then dereferences proto->state / ppp->lock in freed memory, leading to a use-after-free.

Fix this by adding a .detach helper that calls timer_shutdown_sync() on every per-proto timer. detach_hdlc_protocol() invokes proto->detach(dev) before kfree(hdlc->state), so timer_shutdown_sync() now runs on both free paths. timer_shutdown_sync() is used instead of timer_delete_sync() because the keepalive path re-arms the timer through add_timer()/mod_timer() and shutdown blocks any re-activation during teardown.

Initialize the per-protocol timers in ppp_ioctl() when the protocol is attached, and remove the now-redundant timer_setup() from ppp_start(), so that the timers are initialized exactly once at attach time and ppp_timer_release() never operates on uninitialized timer_list structures. attach_hdlc_protocol() uses kmalloc() (not kzalloc), so struct ppp's protos[i].timer is uninitialized garbage until the first timer_setup(); without this init-at-attach, attaching the PPP protocol without ever bringing the device up would leave timer_shutdown_sync() operating on uninitialized memory in .detach. Moving the init out of ppp_start() (which only runs on NETDEV_UP) into the attach path makes the initialization unconditional and avoids initializing the same timer_list twice.

This bug was found by static analysis.

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/8308122bc9c065b1f376e081ed300129a2ac9545
• https://git.kernel.org/stable/c/ce8f9ddca0c9f217342a8b49efd309aa35b81a36
• https://git.kernel.org/stable/c/508a0139d3bf60f6a03d2fbfb63a89a9463d983a
• https://git.kernel.org/stable/c/c64dbef1c0fbd36f9530aa75112acdf6a6d3cfd8
• https://git.kernel.org/stable/c/5a84398101bf9f11e84b176343e4e3ba83e668c0
• https://git.kernel.org/stable/c/a594debfd4e7ec39413647458907f689ef57fd2f
• https://git.kernel.org/stable/c/c78a4e41ab5ead6193ad8a2dd92e8906bae659fa

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