CVE-2026-72069: locking/rt: Fix the incorrect RCU protection in rt_spin_unlock()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

locking/rt: Fix the incorrect RCU protection in rt_spin_unlock()

rt_spin_unlock() releases the RCU protection before unlocking the
lock. That opens the door for the following UAF scenario:

 T1					T2
 spin_lock(&p->lock);		rcu_read_lock();
 invalidate(p);			p = rcu_dereference(ptr);
 rcu_assign_pointer(ptr, NULL);	if (!p) return;
 spin_unlock(&p->lock);		spin_lock(&p->lock)
 				   lock(&lock->lock);
				   rcu_read_lock();
 kfree_rcu(p);			rcu_read_unlock();
				....
				spin_unlock(&p->lock)
				  rcu_read_unlock(); // Ends grace period
 rcu_do_batch()
   kfree(p);
			    UAF ->	  rt_mutex_cmpxchg_release(&lock->lock...)

Regular spinlocks keep preemption disabled accross the unlock operation,
which provides full RCU protection, but the RT substitution fails to
resemble that. Same applies for the rwlock substitution.

Move the rcu_read_unlock() invocation past the unlock operations to match
the non-RT semantics. This makes it asymmetric vs. rt_xxx_lock(), but
that's harmless as the caller needs to hold RCU read lock across the lock
operation. The migrate_enable() call stays before the unlock operation
because there is no per CPU operation in the unlock path which would
require migration to be kept disabled.

The Linux kernel CVE team has assigned CVE-2026-72069 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 5.15 with commit 0f383b6dc96e976dfbf2721b0bf10bd96103b341 and fixed in 6.6.148 with commit 3cfaac77b3c32ac3940df28866de263c3f45d24c
	Issue introduced in 5.15 with commit 0f383b6dc96e976dfbf2721b0bf10bd96103b341 and fixed in 6.12.101 with commit 1f0d56d3f1e88f20f6e46109402f8c15d59bac37
	Issue introduced in 5.15 with commit 0f383b6dc96e976dfbf2721b0bf10bd96103b341 and fixed in 6.18.40 with commit 633cadbc0b8323f5cc140a285d2432089dbb534e
	Issue introduced in 5.15 with commit 0f383b6dc96e976dfbf2721b0bf10bd96103b341 and fixed in 7.1.5 with commit 83f9fb561c1c3917e19f95523dd933c7d30291aa
	Issue introduced in 5.15 with commit 0f383b6dc96e976dfbf2721b0bf10bd96103b341 and fixed in 7.2-rc1 with commit 89038cc87d80c77e7aa6f42a64b2573b74af339f

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-72069
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	kernel/locking/spinlock_rt.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/3cfaac77b3c32ac3940df28866de263c3f45d24c
	https://git.kernel.org/stable/c/1f0d56d3f1e88f20f6e46109402f8c15d59bac37
	https://git.kernel.org/stable/c/633cadbc0b8323f5cc140a285d2432089dbb534e
	https://git.kernel.org/stable/c/83f9fb561c1c3917e19f95523dd933c7d30291aa
	https://git.kernel.org/stable/c/89038cc87d80c77e7aa6f42a64b2573b74af339f