Re: [PATCH v5 8/9] rust: file: add `DeferredFdCloser`

[Trevor Gross]

On Fri, Feb 9, 2024 at 5:22 AM Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote:
>
> To close an fd from kernel space, we could call `ksys_close`. However,
> if we do this to an fd that is held using `fdget`, then we may trigger a
> use-after-free. Introduce a helper that can be used to close an fd even
> if the fd is currently held with `fdget`. This is done by grabbing an
> extra refcount to the file and dropping it in a task work once we return
> to userspace.
>
> This is necessary for Rust Binder because otherwise the user might try
> to have Binder close its fd for /dev/binder, which would cause problems
> as this happens inside an ioctl on /dev/binder, and ioctls hold the fd
> using `fdget`.
>
> Additional motivation can be found in commit 80cd795630d6 ("binder: fix
> use-after-free due to ksys_close() during fdget()") and in the comments
> on `binder_do_fd_close`.
>
> If there is some way to detect whether an fd is currently held with
> `fdget`, then this could be optimized to skip the allocation and task
> work when this is not the case. Another possible optimization would be
> to combine several fds into a single task work, since this is used with
> fd arrays that might hold several fds.
>
> That said, it might not be necessary to optimize it, because Rust Binder
> has two ways to send fds: BINDER_TYPE_FD and BINDER_TYPE_FDA. With
> BINDER_TYPE_FD, it is userspace's responsibility to close the fd, so
> this mechanism is used only by BINDER_TYPE_FDA, but fd arrays are used
> rarely these days.
>
> Reviewed-by: Benno Lossin <benno.lossin@xxxxxxxxx>
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@xxxxxxxxx>
> Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> ---

Reviewed-by: Trevor Gross <tmgross@xxxxxxxxx>

> +    /// Schedule a task work that closes the file descriptor when this task returns to userspace.
> +    ///
> +    /// Fails if this is called from a context where we cannot run work when returning to
> +    /// userspace. (E.g., from a kthread.)
> +    pub fn close_fd(self, fd: u32) -> Result<(), DeferredFdCloseError> {
> +        use bindings::task_work_notify_mode_TWA_RESUME as TWA_RESUME;
> +
> +        // In this method, we schedule the task work before closing the file. This is because
> +        // scheduling a task work is fallible, and we need to know whether it will fail before we
> +        // attempt to close the file.
> +
> +        // Task works are not available on kthreads.
> +        let current = crate::current!();
> +        if current.is_kthread() {
> +            return Err(DeferredFdCloseError::TaskWorkUnavailable);
> +        }
> +
> +        // Transfer ownership of the box's allocation to a raw pointer. This disables the
> +        // destructor, so we must manually convert it back to a Box to drop it.
> +        //
> +        // Until we convert it back to a `Box`, there are no aliasing requirements on this
> +        // pointer.
> +        let inner = Box::into_raw(self.inner);
> +
> +        // The `callback_head` field is first in the struct, so this cast correctly gives us a
> +        // pointer to the field.
> +        let callback_head = inner.cast::<bindings::callback_head>();
> +        // SAFETY: This pointer offset operation does not go out-of-bounds.
> +        let file_field = unsafe { core::ptr::addr_of_mut!((*inner).file) };
> +
> +        let current = current.as_raw();
> +
> +        // SAFETY: This function currently has exclusive access to the `DeferredFdCloserInner`, so
> +        // it is okay for us to perform unsynchronized writes to its `callback_head` field.
> +        unsafe { bindings::init_task_work(callback_head, Some(Self::do_close_fd)) };
> +
> +        // SAFETY: This inserts the `DeferredFdCloserInner` into the task workqueue for the current
> +        // task. If this operation is successful, then this transfers exclusive ownership of the
> +        // `callback_head` field to the C side until it calls `do_close_fd`, and we don't touch or
> +        // invalidate the field during that time.
> +        //
> +        // When the C side calls `do_close_fd`, the safety requirements of that method are
> +        // satisfied because when a task work is executed, the callback is given ownership of the
> +        // pointer.
> +        //
> +        // The file pointer is currently null. If it is changed to be non-null before `do_close_fd`
> +        // is called, then that change happens due to the write at the end of this function, and
> +        // that write has a safety comment that explains why the refcount can be dropped when
> +        // `do_close_fd` runs.
> +        let res = unsafe { bindings::task_work_add(current, callback_head, TWA_RESUME) };
> +
> +        if res != 0 {
> +            // SAFETY: Scheduling the task work failed, so we still have ownership of the box, so
> +            // we may destroy it.
> +            unsafe { drop(Box::from_raw(inner)) };
> +
> +            return Err(DeferredFdCloseError::TaskWorkUnavailable);
> +        }
> +
> +        // This removes the fd from the fd table in `current`. The file is not fully closed until
> +        // `filp_close` is called. We are given ownership of one refcount to the file.
> +        //
> +        // SAFETY: This is safe no matter what `fd` is. If the `fd` is valid (that is, if the
> +        // pointer is non-null), then we call `filp_close` on the returned pointer as required by
> +        // `file_close_fd`.
> +        let file = unsafe { bindings::file_close_fd(fd) };
> +        if file.is_null() {
> +            // We don't clean up the task work since that might be expensive if the task work queue
> +            // is long. Just let it execute and let it clean up for itself.
> +            return Err(DeferredFdCloseError::BadFd);
> +        }
> +
> +        // Acquire a second refcount to the file.
> +        //
> +        // SAFETY: The `file` pointer points at a file with a non-zero refcount.
> +        unsafe { bindings::get_file(file) };
> +
> +        // This method closes the fd, consuming one of our two refcounts There could be active
> +        // light refcounts created from that fd, so we must ensure that the file has a positive
> +        // refcount for the duration of those active light refcounts. We do that by holding on to
> +        // the second refcount until the current task returns to userspace.
> +        //
> +        // SAFETY: The `file` pointer is valid. Passing `current->files` as the file table to close
> +        // it in is correct, since we just got the `fd` from `file_close_fd` which also uses
> +        // `current->files`.
> +        //
> +        // Note: fl_owner_t is currently a void pointer.
> +        unsafe { bindings::filp_close(file, (*current).files as bindings::fl_owner_t) };
> +
> +        // We update the file pointer that the task work is supposed to fput. This transfers
> +        // ownership of our last refcount.
> +        //
> +        // INVARIANT: This changes the `file` field of a `DeferredFdCloserInner` from null to
> +        // non-null. This doesn't break the type invariant for `DeferredFdCloserInner` because we
> +        // still own a refcount to the file, so we can pass ownership of that refcount to the
> +        // `DeferredFdCloserInner`.
> +        //
> +        // When `do_close_fd` runs, it must be safe for it to `fput` the refcount. However, this is
> +        // the case because all light refcounts that are associated with the fd we closed
> +        // previously must be dropped when `do_close_fd`, since light refcounts must be dropped
> +        // before returning to userspace.
> +        //
> +        // SAFETY: Task works are executed on the current thread right before we return to
> +        // userspace, so this write is guaranteed to happen before `do_close_fd` is called, which
> +        // means that a race is not possible here.
> +        unsafe { *file_field = file };
> +
> +        Ok(())
> +    }

This documentation is quite a work of art :)