6.15-stable review patch. If anyone has any objections, please let me know.
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From: Danilo Krummrich dakr@kernel.org
commit f744201c6159fc7323c40936fd079525f7063598 upstream.
In Devres::drop() we first remove the devres action and then drop the wrapped device resource.
The design goal is to give the owner of a Devres object control over when the device resource is dropped, but limit the overall scope to the corresponding device being bound to a driver.
However, there's a race that was introduced with commit 8ff656643d30 ("rust: devres: remove action in `Devres::drop`"), but also has been (partially) present from the initial version on.
In Devres::drop(), the devres action is removed successfully and subsequently the destructor of the wrapped device resource runs. However, there is no guarantee that the destructor of the wrapped device resource completes before the driver core is done unbinding the corresponding device.
If in Devres::drop(), the devres action can't be removed, it means that the devres callback has been executed already, or is still running concurrently. In case of the latter, either Devres::drop() wins revoking the Revocable or the devres callback wins revoking the Revocable. If Devres::drop() wins, we (again) have no guarantee that the destructor of the wrapped device resource completes before the driver core is done unbinding the corresponding device.
CPU0 CPU1 ------------------------------------------------------------------------ Devres::drop() { Devres::devres_callback() { self.data.revoke() { this.data.revoke() { is_available.swap() == true is_available.swap == false } }
// [...] // device fully unbound drop_in_place() { // release device resource } } }
Depending on the specific device resource, this can potentially lead to user-after-free bugs.
In order to fix this, implement the following logic.
In the devres callback, we're always good when we get to revoke the device resource ourselves, i.e. Revocable::revoke() returns true.
If Revocable::revoke() returns false, it means that Devres::drop(), concurrently, already drops the device resource and we have to wait for Devres::drop() to signal that it finished dropping the device resource.
Note that if we hit the case where we need to wait for the completion of Devres::drop() in the devres callback, it means that we're actually racing with a concurrent Devres::drop() call, which already started revoking the device resource for us. This is rather unlikely and means that the concurrent Devres::drop() already started doing our work and we just need to wait for it to complete it for us. Hence, there should not be any additional overhead from that.
(Actually, for now it's even better if Devres::drop() does the work for us, since it can bypass the synchronize_rcu() call implied by Revocable::revoke(), but this goes away anyways once I get to implement the split devres callback approach, which allows us to first flip the atomics of all registered Devres objects of a certain device, execute a single synchronize_rcu() and then drop all revocable objects.)
In Devres::drop() we try to revoke the device resource. If that is *not* successful, it means that the devres callback already did and we're good.
Otherwise, we try to remove the devres action, which, if successful, means that we're good, since the device resource has just been revoked by us *before* we removed the devres action successfully.
If the devres action could not be removed, it means that the devres callback must be running concurrently, hence we signal that the device resource has been revoked by us, using the completion.
This makes it safe to drop a Devres object from any task and at any point of time, which is one of the design goals.
Fixes: 76c01ded724b ("rust: add devres abstraction") Reported-by: Alice Ryhl aliceryhl@google.com Closes: https://lore.kernel.org/lkml/aD64YNuqbPPZHAa5@google.com/ Reviewed-by: Benno Lossin lossin@kernel.org Link: https://lore.kernel.org/r/20250612121817.1621-4-dakr@kernel.org Signed-off-by: Danilo Krummrich dakr@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- rust/kernel/devres.rs | 37 +++++++++++++++++++++++++++++-------- 1 file changed, 29 insertions(+), 8 deletions(-)
--- a/rust/kernel/devres.rs +++ b/rust/kernel/devres.rs @@ -13,7 +13,7 @@ use crate::{ ffi::c_void, prelude::*, revocable::Revocable, - sync::Arc, + sync::{Arc, Completion}, types::ARef, };
@@ -25,13 +25,17 @@ struct DevresInner<T> { 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. /// /// [`Device`] bound resources should be freed when either the resource goes out of scope or the -/// [`Device`] is unbound respectively, depending on what happens first. +/// [`Device`] is unbound respectively, depending on what happens first. In any case, it is always +/// guaranteed that revoking the device resource is completed before the corresponding [`Device`] +/// is unbound. /// /// To achieve that [`Devres`] registers a devres callback on creation, which is called once the /// [`Device`] is unbound, revoking access to the encapsulated resource (see also [`Revocable`]). @@ -105,6 +109,7 @@ impl<T> DevresInner<T> { dev: dev.into(), callback: Self::devres_callback, data <- Revocable::new(data), + revoke <- Completion::new(), }), flags, )?; @@ -133,26 +138,28 @@ impl<T> DevresInner<T> { self as _ }
- fn remove_action(this: &Arc<Self>) { + 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 ret = unsafe { + let success = unsafe { bindings::devm_remove_action_nowarn( this.dev.as_raw(), Some(this.callback), this.as_ptr() as _, ) - }; + } == 0;
- if ret == 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 }
#[allow(clippy::missing_safety_doc)] @@ -164,7 +171,12 @@ impl<T> DevresInner<T> { // `DevresInner::new`. let inner = unsafe { Arc::from_raw(ptr) };
- inner.data.revoke(); + if !inner.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(); + } } }
@@ -196,6 +208,15 @@ impl<T> Deref for Devres<T> {
impl<T> Drop for Devres<T> { fn drop(&mut self) { - DevresInner::remove_action(&self.0); + // 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.revoke_nosync() } { + // We revoked `self.0.data` before the devres action did, hence try to remove it. + if !DevresInner::remove_action(&self.0) { + // 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(); + } + } } }