Message-ID: <20250612145145.12143-4-dakr@kernel.org>
Date: Thu, 12 Jun 2025 16:51:40 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: gregkh@...uxfoundation.org,
	rafael@...nel.org,
	ojeda@...nel.org,
	alex.gaynor@...il.com,
	boqun.feng@...il.com,
	gary@...yguo.net,
	bjorn3_gh@...tonmail.com,
	benno.lossin@...ton.me,
	a.hindborg@...nel.org,
	aliceryhl@...gle.com,
	tmgross@...ch.edu
Cc: rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Danilo Krummrich <dakr@...nel.org>
Subject: [PATCH 3/4] rust: devres: get rid of Devres' inner Arc

So far Devres uses an inner memory allocation and reference count, i.e.
an inner Arc, in order to ensure that the devres callback can't run into
a use-after-free in case where the Devres object is dropped while the
devres callback runs concurrently.

Instead, use a completion in order to avoid a potential UAF: In
Devres::drop(), if we detect that we can't remove the devres action
anymore, we wait for the completion that is completed from the devres
callback. If, in turn, we were able to successfully remove the devres
action, we can just go ahead.

This, again, allows us to get rid of the internal Arc, and instead let
Devres consume an `impl PinInit<T, E>` in order to return an
`impl PinInit<Devres<T>, E>`, which enables us to get away with less
memory allocations.

Additionally, having the resulting explicit synchronization in
Devres::drop() prevents potential subtle undesired side effects of the
devres callback dropping the final Arc reference asynchronously within
the devres callback.

Signed-off-by: Danilo Krummrich <dakr@...nel.org>
---
 drivers/gpu/nova-core/driver.rs |   7 +-
 drivers/gpu/nova-core/gpu.rs    |   6 +-
 rust/kernel/devres.rs           | 187 +++++++++++++++-----------------
 rust/kernel/pci.rs              |  20 ++--
 samples/rust/rust_driver_pci.rs |  19 ++--
 5 files changed, 117 insertions(+), 122 deletions(-)

diff --git a/drivers/gpu/nova-core/driver.rs b/drivers/gpu/nova-core/driver.rs
index 8c86101c26cb..110f2b355db4 100644
--- a/drivers/gpu/nova-core/driver.rs
+++ b/drivers/gpu/nova-core/driver.rs
@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 
-use kernel::{auxiliary, bindings, c_str, device::Core, pci, prelude::*};
+use kernel::{auxiliary, bindings, c_str, device::Core, pci, prelude::*, sync::Arc};
 
 use crate::gpu::Gpu;
 
@@ -34,7 +34,10 @@ fn probe(pdev: &pci::Device<Core>, _info: &Self::IdInfo) -> Result<Pin<KBox<Self
         pdev.enable_device_mem()?;
         pdev.set_master();
 
-        let bar = pdev.iomap_region_sized::<BAR0_SIZE>(0, c_str!("nova-core/bar0"))?;
+        let bar = Arc::pin_init(
+            pdev.iomap_region_sized::<BAR0_SIZE>(0, c_str!("nova-core/bar0")),
+            GFP_KERNEL,
+        )?;
 
         let this = KBox::pin_init(
             try_pin_init!(Self {
diff --git a/drivers/gpu/nova-core/gpu.rs b/drivers/gpu/nova-core/gpu.rs
index 60b86f370284..47653c14838b 100644
--- a/drivers/gpu/nova-core/gpu.rs
+++ b/drivers/gpu/nova-core/gpu.rs
@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 
-use kernel::{device, devres::Devres, error::code::*, pci, prelude::*};
+use kernel::{device, devres::Devres, error::code::*, pci, prelude::*, sync::Arc};
 
 use crate::driver::Bar0;
 use crate::firmware::{Firmware, FIRMWARE_VERSION};
@@ -161,14 +161,14 @@ fn new(bar: &Bar0) -> Result<Spec> {
 pub(crate) struct Gpu {
     spec: Spec,
     /// MMIO mapping of PCI BAR 0
-    bar: Devres<Bar0>,
+    bar: Arc<Devres<Bar0>>,
     fw: Firmware,
 }
 
 impl Gpu {
     pub(crate) fn new(
         pdev: &pci::Device<device::Bound>,
-        devres_bar: Devres<Bar0>,
+        devres_bar: Arc<Devres<Bar0>>,
     ) -> Result<impl PinInit<Self>> {
         let bar = devres_bar.access(pdev.as_ref())?;
         let spec = Spec::new(bar)?;
diff --git a/rust/kernel/devres.rs b/rust/kernel/devres.rs
index 04435e810249..4ee9037f4ad4 100644
--- a/rust/kernel/devres.rs
+++ b/rust/kernel/devres.rs
@@ -13,20 +13,10 @@
     ffi::c_void,
     prelude::*,
     revocable::{Revocable, RevocableGuard},
-    sync::{rcu, Arc, Completion},
+    sync::{rcu, Completion},
     types::{ARef, ForeignOwnable},
 };
 
-#[pin_data]
-struct DevresInner<T> {
-    dev: ARef<Device>,
-    callback: unsafe extern "C" fn(*mut c_void),
-    #[pin]
-    data: Revocable<T>,
-    #[pin]
-    revoke: Completion,
-}
-
 /// This abstraction is meant to be used by subsystems to containerize [`Device`] bound resources to
 /// manage their lifetime.
 ///
@@ -86,100 +76,93 @@ struct DevresInner<T> {
 /// # fn no_run(dev: &Device<Bound>) -> Result<(), Error> {
 /// // SAFETY: Invalid usage for example purposes.
 /// let iomem = unsafe { IoMem::<{ core::mem::size_of::<u32>() }>::new(0xBAAAAAAD)? };
-/// let devres = Devres::new(dev, iomem, GFP_KERNEL)?;
+/// let devres = KBox::pin_init(Devres::new(dev, iomem), GFP_KERNEL)?;
 ///
 /// let res = devres.try_access().ok_or(ENXIO)?;
 /// res.write8(0x42, 0x0);
 /// # Ok(())
 /// # }
 /// ```
-pub struct Devres<T>(Arc<DevresInner<T>>);
-
-impl<T> DevresInner<T> {
-    fn new(dev: &Device<Bound>, data: T, flags: Flags) -> Result<Arc<DevresInner<T>>> {
-        let inner = Arc::pin_init(
-            try_pin_init!( DevresInner {
-                dev: dev.into(),
-                callback: Self::devres_callback,
-                data <- Revocable::new(data),
-                revoke <- Completion::new(),
-            }),
-            flags,
-        )?;
-
-        // Convert `Arc<DevresInner>` into a raw pointer and make devres own this reference until
-        // `Self::devres_callback` is called.
-        let data = inner.clone().into_raw();
+#[pin_data(PinnedDrop)]
+pub struct Devres<T> {
+    dev: ARef<Device>,
+    callback: unsafe extern "C" fn(*mut c_void),
+    #[pin]
+    data: Revocable<T>,
+    #[pin]
+    devm: Completion,
+    #[pin]
+    revoke: Completion,
+}
 
-        // SAFETY: `devm_add_action` guarantees to call `Self::devres_callback` once `dev` is
-        // detached.
-        let ret =
-            unsafe { bindings::devm_add_action(dev.as_raw(), Some(inner.callback), data as _) };
+impl<T> Devres<T> {
+    /// Creates a new [`Devres`] instance of the given `data`. The `data` encapsulated within the
+    /// returned `Devres` instance' `data` will be revoked once the device is detached.
+    pub fn new<'a, E>(
+        dev: &'a Device<Bound>,
+        data: impl PinInit<T, E> + 'a,
+    ) -> impl PinInit<Self, Error> + 'a
+    where
+        T: 'a,
+        Error: From<E>,
+    {
+        let callback = Self::devres_callback;
 
-        if ret != 0 {
-            // SAFETY: We just created another reference to `inner` in order to pass it to
-            // `bindings::devm_add_action`. If `bindings::devm_add_action` fails, we have to drop
-            // this reference accordingly.
-            let _ = unsafe { Arc::from_raw(data) };
-            return Err(Error::from_errno(ret));
-        }
+        try_pin_init!(&this in Self {
+            data <- Revocable::new(data),
+            devm <- Completion::new(),
+            revoke <- Completion::new(),
+            callback,
+            dev: {
+                // SAFETY:
+                // - `dev.as_raw()` is a pointer to a valid bound device.
+                // - `this` is guaranteed to be a valid for the duration of the lifetime of `Self`.
+                // - `devm_add_action()` is guaranteed not to call `callback` until `this` has been
+                //    properly initialized, because we require `dev` (i.e. the *bound* device) to
+                //    live at least as long as the returned `impl PinInit<Self, Error>`.
+                to_result(unsafe {
+                    bindings::devm_add_action(dev.as_raw(), Some(callback), this.as_ptr().cast())
+                })?;
 
-        Ok(inner)
+                dev.into()
+            },
+        })
     }
 
     fn as_ptr(&self) -> *const Self {
-        self as _
-    }
-
-    fn remove_action(this: &Arc<Self>) -> bool {
-        // SAFETY:
-        // - `self.inner.dev` is a valid `Device`,
-        // - the `action` and `data` pointers are the exact same ones as given to devm_add_action()
-        //   previously,
-        // - `self` is always valid, even if the action has been released already.
-        let success = unsafe {
-            bindings::devm_remove_action_nowarn(
-                this.dev.as_raw(),
-                Some(this.callback),
-                this.as_ptr() as _,
-            )
-        } == 0;
-
-        if success {
-            // SAFETY: We leaked an `Arc` reference to devm_add_action() in `DevresInner::new`; if
-            // devm_remove_action_nowarn() was successful we can (and have to) claim back ownership
-            // of this reference.
-            let _ = unsafe { Arc::from_raw(this.as_ptr()) };
-        }
-
-        success
+        self
     }
 
     #[allow(clippy::missing_safety_doc)]
     unsafe extern "C" fn devres_callback(ptr: *mut kernel::ffi::c_void) {
-        let ptr = ptr as *mut DevresInner<T>;
-        // Devres owned this memory; now that we received the callback, drop the `Arc` and hence the
-        // reference.
-        // SAFETY: Safe, since we leaked an `Arc` reference to devm_add_action() in
-        //         `DevresInner::new`.
-        let inner = unsafe { Arc::from_raw(ptr) };
+        // SAFETY: In `Self::new` we've passed a valid pointer to `Self` to `devm_add_action()`,
+        // hence `ptr` must be a valid pointer to `Self`.
+        let this = unsafe { &*ptr.cast::<Self>() };
 
-        if !inner.data.revoke() {
+        if !this.data.revoke() {
             // If `revoke()` returns false, it means that `Devres::drop` already started revoking
-            // `inner.data` for us. Hence we have to wait until `Devres::drop()` signals that it
-            // completed revoking `inner.data`.
-            inner.revoke.wait_for_completion();
+            // `data` for us. Hence we have to wait until `Devres::drop` signals that it
+            // completed revoking `data`.
+            this.revoke.wait_for_completion();
         }
-    }
-}
 
-impl<T> Devres<T> {
-    /// Creates a new [`Devres`] instance of the given `data`. The `data` encapsulated within the
-    /// returned `Devres` instance' `data` will be revoked once the device is detached.
-    pub fn new(dev: &Device<Bound>, data: T, flags: Flags) -> Result<Self> {
-        let inner = DevresInner::new(dev, data, flags)?;
+        // Signal that we're done using `this`.
+        this.devm.complete_all();
+    }
 
-        Ok(Devres(inner))
+    fn remove_action(&self) -> bool {
+        // SAFETY:
+        // - `self.dev` is a valid `Device`,
+        // - the `action` and `data` pointers are the exact same ones as given to devm_add_action()
+        //   previously,
+        // - `self` is always valid, even if the action has been released already.
+        (unsafe {
+            bindings::devm_remove_action_nowarn(
+                self.dev.as_raw(),
+                Some(self.callback),
+                self.as_ptr().cast_mut().cast(),
+            )
+        } == 0)
     }
 
     /// Obtain `&'a T`, bypassing the [`Revocable`].
@@ -211,43 +194,51 @@ pub fn new(dev: &Device<Bound>, data: T, flags: Flags) -> Result<Self> {
     /// }
     /// ```
     pub fn access<'a>(&'a self, dev: &'a Device<Bound>) -> Result<&'a T> {
-        if self.0.dev.as_raw() != dev.as_raw() {
+        if self.dev.as_raw() != dev.as_raw() {
             return Err(EINVAL);
         }
 
         // SAFETY: `dev` being the same device as the device this `Devres` has been created for
-        // proves that `self.0.data` hasn't been revoked and is guaranteed to not be revoked as
-        // long as `dev` lives; `dev` lives at least as long as `self`.
-        Ok(unsafe { self.0.data.access() })
+        // proves that `self.data` hasn't been revoked and is guaranteed to not be revoked as long
+        // as `dev` lives; `dev` lives at least as long as `self`.
+        Ok(unsafe { self.data.access() })
     }
 
     /// [`Devres`] accessor for [`Revocable::try_access`].
     pub fn try_access(&self) -> Option<RevocableGuard<'_, T>> {
-        self.0.data.try_access()
+        self.data.try_access()
     }
 
     /// [`Devres`] accessor for [`Revocable::try_access_with`].
     pub fn try_access_with<R, F: FnOnce(&T) -> R>(&self, f: F) -> Option<R> {
-        self.0.data.try_access_with(f)
+        self.data.try_access_with(f)
     }
 
     /// [`Devres`] accessor for [`Revocable::try_access_with_guard`].
     pub fn try_access_with_guard<'a>(&'a self, guard: &'a rcu::Guard) -> Option<&'a T> {
-        self.0.data.try_access_with_guard(guard)
+        self.data.try_access_with_guard(guard)
     }
 }
 
-impl<T> Drop for Devres<T> {
-    fn drop(&mut self) {
+#[pinned_drop]
+impl<T> PinnedDrop for Devres<T> {
+    fn drop(self: Pin<&mut Self>) {
         // SAFETY: When `drop` runs, it is guaranteed that nobody is accessing the revocable data
         // anymore, hence it is safe not to wait for the grace period to finish.
-        if unsafe { self.0.data.revoke_nosync() } {
-            // We revoked `self.0.data` before the devres action did, hence try to remove it.
-            if !DevresInner::remove_action(&self.0) {
+        if unsafe { self.data.revoke_nosync() } {
+            // We revoked `self.data` before the devres action did, hence try to remove it.
+            if !self.remove_action() {
                 // We could not remove the devres action, which means that it now runs concurrently,
-                // hence signal that `self.0.data` has been revoked successfully.
-                self.0.revoke.complete_all();
+                // hence signal that `self.data` has been revoked by us successfully.
+                self.revoke.complete_all();
+
+                // Wait for `Self::devres_callback` to be done using this object.
+                self.devm.wait_for_completion();
             }
+        } else {
+            // `Self::devres_callback` revokes `self.data` for us, hence wait for it to be done
+            // using this object.
+            self.devm.wait_for_completion();
         }
     }
 }
diff --git a/rust/kernel/pci.rs b/rust/kernel/pci.rs
index 8435f8132e38..db0eb7eaf9b1 100644
--- a/rust/kernel/pci.rs
+++ b/rust/kernel/pci.rs
@@ -5,7 +5,6 @@
 //! C header: [`include/linux/pci.h`](srctree/include/linux/pci.h)
 
 use crate::{
-    alloc::flags::*,
     bindings, container_of, device,
     device_id::RawDeviceId,
     devres::Devres,
@@ -398,19 +397,20 @@ pub fn resource_len(&self, bar: u32) -> Result<bindings::resource_size_t> {
 impl Device<device::Bound> {
     /// Mapps an entire PCI-BAR after performing a region-request on it. I/O operation bound checks
     /// can be performed on compile time for offsets (plus the requested type size) < SIZE.
-    pub fn iomap_region_sized<const SIZE: usize>(
-        &self,
+    pub fn iomap_region_sized<'a, const SIZE: usize>(
+        &'a self,
         bar: u32,
-        name: &CStr,
-    ) -> Result<Devres<Bar<SIZE>>> {
-        let bar = Bar::<SIZE>::new(self, bar, name)?;
-        let devres = Devres::new(self.as_ref(), bar, GFP_KERNEL)?;
-
-        Ok(devres)
+        name: &'a CStr,
+    ) -> impl PinInit<Devres<Bar<SIZE>>, Error> + 'a {
+        Devres::new(self.as_ref(), Bar::<SIZE>::new(self, bar, name))
     }
 
     /// Mapps an entire PCI-BAR after performing a region-request on it.
-    pub fn iomap_region(&self, bar: u32, name: &CStr) -> Result<Devres<Bar>> {
+    pub fn iomap_region<'a>(
+        &'a self,
+        bar: u32,
+        name: &'a CStr,
+    ) -> impl PinInit<Devres<Bar>, Error> + 'a {
         self.iomap_region_sized::<0>(bar, name)
     }
 }
diff --git a/samples/rust/rust_driver_pci.rs b/samples/rust/rust_driver_pci.rs
index 15147e4401b2..5c35f1414172 100644
--- a/samples/rust/rust_driver_pci.rs
+++ b/samples/rust/rust_driver_pci.rs
@@ -25,8 +25,10 @@ impl TestIndex {
     const NO_EVENTFD: Self = Self(0);
 }
 
+#[pin_data(PinnedDrop)]
 struct SampleDriver {
     pdev: ARef<pci::Device>,
+    #[pin]
     bar: Devres<Bar0>,
 }
 
@@ -73,13 +75,11 @@ fn probe(pdev: &pci::Device<Core>, info: &Self::IdInfo) -> Result<Pin<KBox<Self>
         pdev.enable_device_mem()?;
         pdev.set_master();
 
-        let bar = pdev.iomap_region_sized::<{ Regs::END }>(0, c_str!("rust_driver_pci"))?;
-
-        let drvdata = KBox::new(
-            Self {
+        let drvdata = KBox::pin_init(
+            try_pin_init!(Self {
                 pdev: pdev.into(),
-                bar,
-            },
+                bar <- pdev.iomap_region_sized::<{ Regs::END }>(0, c_str!("rust_driver_pci")),
+            }),
             GFP_KERNEL,
         )?;
 
@@ -90,12 +90,13 @@ fn probe(pdev: &pci::Device<Core>, info: &Self::IdInfo) -> Result<Pin<KBox<Self>
             Self::testdev(info, bar)?
         );
 
-        Ok(drvdata.into())
+        Ok(drvdata)
     }
 }
 
-impl Drop for SampleDriver {
-    fn drop(&mut self) {
+#[pinned_drop]
+impl PinnedDrop for SampleDriver {
+    fn drop(self: Pin<&mut Self>) {
         dev_dbg!(self.pdev.as_ref(), "Remove Rust PCI driver sample.\n");
     }
 }
-- 
2.49.0

Powered by blists - more mailing lists