[PATCH v4] rust: cpufreq: reject NULL from cpufreq_cpu_get()

[Mehmet Koseoglu <mehmet.mkoseoglu@gmail.com>]

cpufreq_cpu_get() returns either a referenced policy or NULL.
PolicyCpu::from_cpu() passed its return value to from_err_ptr(), which
rejects ERR_PTR values but accepts NULL.

If the lookup fails, Policy::from_raw_mut() therefore constructs a mutable
reference from NULL. Dropping the resulting PolicyCpu then passes the
invalid pointer to cpufreq_cpu_put(), causing an oops in kobject_put().

Reject NULL with NonNull before constructing the Policy reference. Return
ENODEV instead.

A KUnit negative-control run reproduced the oops with the original
conversion. The same test passed with this change. The reproducer is
available on request.

Fixes: 6ebdd7c93177 ("rust: cpufreq: Extend abstractions for policy and driver ops")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Mehmet Koseoglu <mehmet.mkoseoglu@gmail.com>
Reviewed-by: Onur Özkan <work@onurozkan.dev>
Reviewed-by: spidermana <xuyiwen14@gmail.com>
---
No worries at all, here is a v4 update with the comment update and
review tags.

Changes in v4:
- Clarify that the policy pointer is non-NULL and cpufreq_cpu_get() holds
  a reference for the lifetime of the returned reference.
- Add Reviewed-by tags from Onur Özkan and spidermana (using the corrected
  tag from spidermana's follow-up).
- Resend at Viresh's request to replace the previously applied v3.

Changes in v3:
- Add Cc: stable@vger.kernel.org.

Changes in v2:
- Format modified imports using the kernel's vertical import style.

v3: https://lore.kernel.org/r/20260829161013.102967-1-mehmet.mkoseoglu@gmail.com
v2: https://lore.kernel.org/r/20260828161834.29539-1-mehmet.mkoseoglu@gmail.com

 rust/kernel/cpufreq.rs | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/rust/kernel/cpufreq.rs b/rust/kernel/cpufreq.rs
index affa2b9490ef..b75d692846a2 100644
--- a/rust/kernel/cpufreq.rs
+++ b/rust/kernel/cpufreq.rs
@@ -14,7 +14,13 @@
     cpumask,
     device::{Bound, Device},
     devres,
-    error::{code::*, from_err_ptr, from_result, to_result, Result, VTABLE_DEFAULT_ERROR},
+    error::{
+        code::*,
+        from_result,
+        to_result,
+        Result,
+        VTABLE_DEFAULT_ERROR, //
+    },
     ffi::{c_char, c_ulong},
     prelude::*,
     types::ForeignOwnable,
@@ -29,7 +35,10 @@
     marker::PhantomData,
     ops::{Deref, DerefMut},
     pin::Pin,
-    ptr,
+    ptr::{
+        self,
+        NonNull, //
+    },
 };
 
 use macros::vtable;
@@ -687,12 +696,13 @@ fn clear_data<T: ForeignOwnable>(&mut self) -> Option<T> {
 impl<'a> PolicyCpu<'a> {
     fn from_cpu(cpu: CpuId) -> Result<Self> {
         // SAFETY: It is safe to call `cpufreq_cpu_get` for any valid CPU.
-        let ptr = from_err_ptr(unsafe { bindings::cpufreq_cpu_get(u32::from(cpu)) })?;
+        let ptr =
+            NonNull::new(unsafe { bindings::cpufreq_cpu_get(u32::from(cpu)) }).ok_or(ENODEV)?;
 
         Ok(Self(
-            // SAFETY: The `ptr` is guaranteed to be valid and remains valid for the lifetime of
-            // the returned reference.
+            // SAFETY: `ptr` is non-NULL and `cpufreq_cpu_get()` took a reference on it, so it is
+            // valid for writing and remains valid for the lifetime of the returned reference.
-            unsafe { Policy::from_raw_mut(ptr) },
+            unsafe { Policy::from_raw_mut(ptr.as_ptr()) },
         ))
     }
 }
-- 
2.55.0