In the Linux kernel, the following vulnerability has been resolved: uprobes: Fix NULL pointer...

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

uprobes: Fix NULL pointer dereference in hprobe_expire()

Forking a task that has a pending uretprobe can oops the kernel with a NULL pointer dereference in the clone() path:

BUG: kernel NULL pointer dereference, address: 0000000000000018 Oops: 0002 [#1] SMP NOPTI RIP: 0010:hprobe_expire CR2: 0000000000000018 Call Trace: uprobe_copy_process copy_process kernel_clone __x64_sys_clone do_syscall_64 entry_SYSCALL_64_after_hwframe

This was found on real hosts on Meta fleet.

I've got the impression that this is what is happening:

CPU 1 CPU 2 (traced task)

                             hit uprobe, prepare_uretprobe():
                               hprobe LEASED, refcount >= 1

uprobe_unregister() put_uprobe(): refcount -> 0 fork() -> dup_utask() hprobe_expire(hprobe, true) try_get_uprobe() -> NULL get_uprobe(NULL) <-- Oops

Only take the extra reference when the uprobe is non-NULL; a NULL means it is gone and is the correct value to return.