On Thu, 2 Nov 2023 11:14:33 +0900 Masami Hiramatsu (Google) <mhira...@kernel.org> wrote: > > > > What happens here is that the kprobe event creates a trace_event_file > > "file" descriptor that represents the file in tracefs to the event. It > > maintains state of the event (is it enabled for the given instance?). > > Opening the "enable" file gets a reference to the event "file" descriptor > > via the open file descriptor. When the kprobe event is deleted, the file is > > also deleted from the tracefs system which also frees the event "file" > > descriptor. > > Ouch! I thought the file descriptor has been hold by the opened process.
Well, the struct *filp is, but not the filp->private that points to the struct trace_event_file *file. > > > > > But as the tracefs file is still opened by user space, it will not be > > totally removed until the final dput() is called on it. But this is not > > true with the event "file" descriptor that is already freed. If the user > > does a write to or simply closes the file descriptor it will reference the > > event "file" descriptor that was just freed, causing a use-after-free bug. > > > > To solve this, add a ref count to the event "file" descriptor as well as a > > new flag called "FREED". The "file" will not be freed until the last > > reference is released. But the FREE flag will be set when the event is > > removed to prevent any more modifications to that event from happening, > > even if there's still a reference to the event "file" descriptor. > > > > Link: > > https://lore.kernel.org/linux-trace-kernel/20231031000031.1e705...@gandalf.local.home/ > > > > Looks good to me. > > Reviewed-by: Masami Hiramatsu (Google) <mhira...@kernel.org> > > BTW, can we add some tracefs selftests to ftracetest or independent test? Yes, I was thinking about this, but that will have to wait till after this gets in mainline. I'm already way behind schedule. Thanks, -- Steve