FAILED: patch "[PATCH] sched/core: Fix use-after-free bug in dup_user_cpus_ptr()" failed to apply to 5.15-stable tree

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

Possible dependencies:

87ca4f9efbd7 ("sched/core: Fix use-after-free bug in dup_user_cpus_ptr()")
8f9ea86fdf99 ("sched: Always preserve the user requested cpumask")
713a2e21a513 ("sched: Introduce affinity_context")
d664e399128b ("sched: Fix missing prototype warnings")
e81daa7b6489 ("sched/headers: Reorganize, clean up and optimize kernel/sched/build_utility.c dependencies") 0dda4eeb4849 ("sched/headers: Reorganize, clean up and optimize kernel/sched/build_policy.c dependencies") c4ad6fcb67c4 ("sched/headers: Reorganize, clean up and optimize kernel/sched/fair.c dependencies") e66f6481a8c7 ("sched/headers: Reorganize, clean up and optimize kernel/sched/core.c dependencies") b9e9c6ca6e54 ("sched/headers: Standardize kernel/sched/sched.h header dependencies") f96eca432015 ("sched/headers: Introduce kernel/sched/build_policy.c and build multiple .c files there") 801c14195510 ("sched/headers: Introduce kernel/sched/build_utility.c and build multiple .c files there") d90a2f160a1c ("sched/headers: Add header guard to kernel/sched/stats.h and kernel/sched/autogroup.h") 99cf983cc8bc ("sched/preempt: Add PREEMPT_DYNAMIC using static keys")
33c64734be34 ("sched/preempt: Decouple HAVE_PREEMPT_DYNAMIC from GENERIC_ENTRY") 4624a14f4daa ("sched/preempt: Simplify irqentry_exit_cond_resched() callers")
8a69fe0be143 ("sched/preempt: Refactor sched_dynamic_update()")
4c7485584d48 ("sched/preempt: Move PREEMPT_DYNAMIC logic later")
6ae71436cda7 ("Merge tag 'sched_core_for_v5.17_rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip")

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 87ca4f9efbd7cc649ff43b87970888f2812945b8 Mon Sep 17 00:00:00 2001
From: Waiman Long <[email protected]>
Date: Fri, 30 Dec 2022 23:11:19 -0500
Subject: [PATCH] sched/core: Fix use-after-free bug in dup_user_cpus_ptr()
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Since commit 07ec77a1d4e8 ("sched: Allow task CPU affinity to be restricted on asymmetric systems"), the setting and clearing of user_cpus_ptr are done under pi_lock for arm64 architecture. However, dup_user_cpus_ptr() accesses user_cpus_ptr without any lock protection. Since sched_setaffinity() can be invoked from another process, the process being modified may be undergoing fork() at the same time. When racing with the clearing of user_cpus_ptr in __set_cpus_allowed_ptr_locked(), it can lead to user-after-free and possibly double-free in arm64 kernel.

Commit 8f9ea86fdf99 ("sched: Always preserve the user requested cpumask") fixes this problem as user_cpus_ptr, once set, will never be cleared in a task's lifetime. However, this bug was re-introduced in commit 851a723e45d1 ("sched: Always clear user_cpus_ptr in do_set_cpus_allowed()") which allows the clearing of user_cpus_ptr in do_set_cpus_allowed(). This time, it will affect all arches.

Fix this bug by always clearing the user_cpus_ptr of the newly cloned/forked task before the copying process starts and check the user_cpus_ptr state of the source task under pi_lock.

Note to stable, this patch won't be applicable to stable releases. Just copy the new dup_user_cpus_ptr() function over.

index 965d813c28ad..f9f6e5413dcf 100644
@@ -2612,19 +2612,43 @@ void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
 int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src,
 		      int node)
 {
+	cpumask_t *user_mask;
 	unsigned long flags;
 
-	if (!src->user_cpus_ptr)
+	/*
+	 * Always clear dst->user_cpus_ptr first as their user_cpus_ptr's
+	 * may differ by now due to racing.
+	 */
+	dst->user_cpus_ptr = NULL;
+
+	/*
+	 * This check is racy and losing the race is a valid situation.
+	 * It is not worth the extra overhead of taking the pi_lock on
+	 * every fork/clone.
+	 */
+	if (data_race(!src->user_cpus_ptr))
 		return 0;
 
-	dst->user_cpus_ptr = kmalloc_node(cpumask_size(), GFP_KERNEL, node);
-	if (!dst->user_cpus_ptr)
+	user_mask = kmalloc_node(cpumask_size(), GFP_KERNEL, node);
+	if (!user_mask)
 		return -ENOMEM;
 
-	/* Use pi_lock to protect content of user_cpus_ptr */
+	/*
+	 * Use pi_lock to protect content of user_cpus_ptr
+	 *
+	 * Though unlikely, user_cpus_ptr can be reset to NULL by a concurrent
+	 * do_set_cpus_allowed().
+	 */
 	raw_spin_lock_irqsave(&src->pi_lock, flags);
-	cpumask_copy(dst->user_cpus_ptr, src->user_cpus_ptr);
+	if (src->user_cpus_ptr) {
+		swap(dst->user_cpus_ptr, user_mask);
+		cpumask_copy(dst->user_cpus_ptr, src->user_cpus_ptr);
+	}
 	raw_spin_unlock_irqrestore(&src->pi_lock, flags);
+
+	if (unlikely(user_mask))
+		kfree(user_mask);
+
 	return 0;
 }