CVE-2026-97938reboot: fix cad_pid use-after-free race

Official description

In the Linux kernel, the following vulnerability has been resolved: reboot: fix cad_pid use-after-free race cad_pid is a single kernel-wide struct pid pointer. proc_do_cad_pid() reads it and passes it to pid_vnr() without protecting the lifetime of the referenced struct pid. A concurrent writer can replace cad_pid and drop the final reference to the old struct pid after the reader has loaded the pointer but before pid_vnr() has finished dereferencing it, causing a use-after-free. kill_cad_pid() has the same lifetime race when it passes cad_pid to kill_pid(). At the time this issue was reported, an unprivileged user could reach the sysctl through user and PID namespaces because cad_pid was registered in pid_table[]. Moving cad_pid back to the global reboot sysctl table corrected that namespace and permission mismatch, but did not fix the underlying lifetime race. Fix this by treating cad_pid as an RCU-protected pointer at both read sites and by waiting for a grace period before dropping the old reference on the write side. call_rcu(&old_pid->rcu, ...) cannot be used here because free_pid() also queues pid->rcu; queueing the same rcu_head twice can corrupt the RCU callback list. Original KASAN crash stack: kernel/pid.c:545 pid_nr_ns() # reads freed pid->level kernel/pid.c:556 pid_vnr() # calls pid_nr_ns() kernel/pid.c:775 proc_do_cad_pid() # calls pid_vnr(cad_pid)

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/0c1a966bc047af2801a70b8eb13f9df5d7ab925f
• https://git.kernel.org/stable/c/ae3e53fe3c1241855e3a88f74877738349948c78
• https://git.kernel.org/stable/c/ad72e2566643fff7f01666d845fb026898155e1f
• https://git.kernel.org/stable/c/9a17b0e053197a6a42cdc75e75a35b17aa662088
• https://git.kernel.org/stable/c/5a88f78df753993469dab4d1831f8fb4256a9468