Re: [PATCH v2 1/2] rust: serdev: Fix race condition on driver unbind

["Gary Guo" <gary@garyguo.net>]

On Sat Sep 5, 2026 at 2:30 PM BST, Markus Probst wrote:
> On device unbind, the pointer to the driver data (`PrivateData`) will first
> be set to NULL by `drvdata_obtain` and only after that the serdev device
> will be closed by Drop. Thus there is a small window in which the serdev
> device is still open, but the pointer to the driver data is NULL. Therefore
> it is possible that `receive_buf_callback` might try to access the `active`
> mutex on a null pointer.
>
> Add function `drvdata_drop` that leaves the pointer to the driver data
> valid until the Drop has completed. Use it in the post unbind callback.
>
> Fixes: 99f59aa82341 ("rust: add basic serial device bus abstractions")
> Reported-by: Sashiko Bot <sashiko-bot@kernel.org>
> Closes: https://lore.kernel.org/linux-serial/20260905000836.C8FC91F00A3D@smtp.kernel.org/
> Signed-off-by: Markus Probst <markus.probst@posteo.de>
> ---
>  rust/kernel/device.rs | 27 +++++++++++++++++++++++++++
>  rust/kernel/driver.rs |  2 +-
>  2 files changed, 28 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/device.rs b/rust/kernel/device.rs
> index 2291d85b6849..3886cc713c28 100644
> --- a/rust/kernel/device.rs
> +++ b/rust/kernel/device.rs
> @@ -219,6 +219,7 @@ pub fn set_drvdata<T>(&self, data: impl PinInit<T, Error>) -> Result {
>      ///
>      /// - The type `T` must match the type of the `ForeignOwnable` previously stored by
>      ///   [`Device::set_drvdata`].
> +    /// - Must only be called before the device is fully unbound.
>      pub(crate) unsafe fn drvdata_obtain<T>(&self) -> Option<Pin<KBox<T>>> {
>          // SAFETY: By the type invariants, `self.as_raw()` is a valid pointer to a `struct device`.
>          let ptr = unsafe { bindings::dev_get_drvdata(self.as_raw()) };
> @@ -236,6 +237,32 @@ pub(crate) unsafe fn drvdata_obtain<T>(&self) -> Option<Pin<KBox<T>>> {
>          //   in `into_foreign()`.
>          Some(unsafe { Pin::<KBox<T>>::from_foreign(ptr.cast()) })
>      }
> +
> +    /// Drop the private data stored in this [`Device`].
> +    ///
> +    /// The pointer to the private data remains valid until the drop is complete.
> +    ///
> +    /// # Safety
> +    ///
> +    /// - The type `T` must match the type of the `ForeignOwnable` previously stored by
> +    ///   [`Device::set_drvdata`].
> +    pub(crate) unsafe fn drvdata_drop<T>(&self) {
> +        // SAFETY: By the type invariants, `self.as_raw()` is a valid pointer to a `struct device`.
> +        let ptr = unsafe { bindings::dev_get_drvdata(self.as_raw()) };
> +
> +        if ptr.is_null() {
> +            return;
> +        }

How does this help the problem? While drop is running, other code should not
attempt to obtain a reference to the data anymore. Otherwise this still have UB
potential by accessing fields that are just destroyed (not to mention that Rust
alias model also forbid it).

I think the existing actually catches it better, because *if* NULL pointer can
be observed by callbacks, a synchronization is missing in the subsystem. The bus
should first perform a synchronization to ensure callbacks are no longer fired,
and then proceed to clean up resources.

Best,
Gary

> +
> +        // SAFETY:
> +        // - If `ptr` is not NULL, it comes from a previous call to `into_foreign()`.
> +        // - `dev_get_drvdata()` guarantees to return the same pointer given to `dev_set_drvdata()`
> +        //   in `into_foreign()`.
> +        drop(unsafe { Pin::<KBox<T>>::from_foreign(ptr.cast()) });
> +
> +        // SAFETY: By the type invariants, `self.as_raw()` is a valid pointer to a `struct device`.
> +        unsafe { bindings::dev_set_drvdata(self.as_raw(), core::ptr::null_mut()) };
> +    }
>  }
>  
>  impl<Ctx: InternalBoundContext> Device<Ctx> {
> diff --git a/rust/kernel/driver.rs b/rust/kernel/driver.rs
> index c9c74c4dde8f..83410141ef1c 100644
> --- a/rust/kernel/driver.rs
> +++ b/rust/kernel/driver.rs
> @@ -204,7 +204,7 @@ extern "C" fn post_unbind_callback(dev: *mut bindings::device) {
>          //
>          // SAFETY: By the safety requirements of the `Driver` trait, `T::DriverData` is the
>          // driver's bus device private data type.
> -        drop(unsafe { dev.drvdata_obtain::<T::DriverData<'_>>() });
> +        unsafe { dev.drvdata_drop::<T::DriverData<'_>>() };
>      }
>  
>      /// Attach generic `struct device_driver` callbacks.