Re: [PATCH v2 7/7] rust: file: add abstraction for `poll_table`

[Boqun Feng <boqun.feng@gmail.com>]

On Tue, Dec 12, 2023 at 05:01:28PM +0000, Benno Lossin wrote:
> On 12/12/23 10:59, Alice Ryhl wrote:
> > On Fri, Dec 8, 2023 at 6:53 PM Benno Lossin <benno.lossin@proton.me> wrote:
> >> On 12/6/23 12:59, Alice Ryhl wrote:
> >>> +    fn get_qproc(&self) -> bindings::poll_queue_proc {
> >>> +        let ptr = self.0.get();
> >>> +        // SAFETY: The `ptr` is valid because it originates from a reference, and the `_qproc`
> >>> +        // field is not modified concurrently with this call since we have an immutable reference.
> >>
> >> This needs an invariant on `PollTable` (i.e. `self.0` is valid).
> > 
> > How would you phrase it?
> 
> - `self.0` contains a valid `bindings::poll_table`.
> - `self.0` is only modified via references to `Self`.
> 
> >>> +        unsafe { (*ptr)._qproc }
> >>> +    }
> >>> +
> >>> +    /// Register this [`PollTable`] with the provided [`PollCondVar`], so that it can be notified
> >>> +    /// using the condition variable.
> >>> +    pub fn register_wait(&mut self, file: &File, cv: &PollCondVar) {
> >>> +        if let Some(qproc) = self.get_qproc() {
> >>> +            // SAFETY: The pointers to `self` and `file` are valid because they are references.
> >>
> >> What about cv.wait_list...
> > 
> > I can add it to the list of things that are valid due to references.
> 

Actually, there is an implied safety requirement here, it's about how
qproc is implemented. As we can see, PollCondVar::drop() will wait for a
RCU grace period, that means the waiter (a file or something) has to use
RCU to access the cv.wait_list, otherwise, the synchronize_rcu() in
PollCondVar::drop() won't help.

To phrase it, it's more like:

(in the safety requirement of `PollTable::from_ptr` and the type
invariant of `PollTable`):

", further, if the qproc function in poll_table publishs the pointer of
the wait_queue_head, it must publish it in a way that reads on the
published pointer have to be in an RCU read-side critical section."

and here we can said,

"per type invariant, `qproc` cannot publish `cv.wait_list` without
proper RCU protection, so it's safe to use `cv.wait_list` here, and with
the synchronize_rcu() in PollCondVar::drop(), free of the wait_list will
be delayed until all usages are done."

I know, this is quite verbose, but just imagine some one removes the
rcu_read_lock() and rcu_read_unlock() in ep_remove_wait_queue(), the
poll table from epoll (using ep_ptable_queue_proc()) is still valid one
according to the current safety requirement, but now there is a
use-after-free in the following case:

	CPU 0                        CPU1
	                             ep_remove_wait_queue():
				       struct wait_queue_head *whead;
	                               whead = smp_load_acquire(...);
	                               if (whead) { // not null
	PollCondVar::drop():
	  __wake_pollfree();
	  synchronize_rcu(); // no current RCU readers, yay.
	  <free the wait_queue_head>
	                                 remove_wait_queue(whead, ...); // BOOM, use-after-free

Regards,
Boqun