drm/amdkfd: Ensure mm remain valid in svm deferred_list work

svm_deferred_list work should continue to handle deferred_range_list
which maybe split to child range to avoid child range leak, and remove
ranges mmu interval notifier to avoid mm mm_count leak. So taking mm
reference when adding range to deferred list, to ensure mm is valid in
the scheduled deferred_list_work, and drop the mm referrence after range
is handled.

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reported-by: Ruili Ji <ruili.ji@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdkfd/kfd_svm.c

@@ -1985,10 +1985,9 @@ svm_range_update_notifier_and_interval_tree(struct mm_struct *mm,
 }
 
 static void
-svm_range_handle_list_op(struct svm_range_list *svms, struct svm_range *prange)
+svm_range_handle_list_op(struct svm_range_list *svms, struct svm_range *prange,
+			 struct mm_struct *mm)
 {
-	struct mm_struct *mm = prange->work_item.mm;
-
 	switch (prange->work_item.op) {
 	case SVM_OP_NULL:
 		pr_debug("NULL OP 0x%p prange 0x%p [0x%lx 0x%lx]\n",
@@ -2071,34 +2070,41 @@ static void svm_range_deferred_list_work(struct work_struct *work)
 	pr_debug("enter svms 0x%p\n", svms);
 
 	p = container_of(svms, struct kfd_process, svms);
-	/* Avoid mm is gone when inserting mmu notifier */
-	mm = get_task_mm(p->lead_thread);
-	if (!mm) {
-		pr_debug("svms 0x%p process mm gone\n", svms);
-		return;
-	}
-retry:
-	mmap_write_lock(mm);
-
-	/* Checking for the need to drain retry faults must be inside
-	 * mmap write lock to serialize with munmap notifiers.
-	 */
-	if (unlikely(atomic_read(&svms->drain_pagefaults))) {
-		mmap_write_unlock(mm);
-		svm_range_drain_retry_fault(svms);
-		goto retry;
-	}
 
 	spin_lock(&svms->deferred_list_lock);
 	while (!list_empty(&svms->deferred_range_list)) {
 		prange = list_first_entry(&svms->deferred_range_list,
 					  struct svm_range, deferred_list);
-		list_del_init(&prange->deferred_list);
 		spin_unlock(&svms->deferred_list_lock);
 
 		pr_debug("prange 0x%p [0x%lx 0x%lx] op %d\n", prange,
 			 prange->start, prange->last, prange->work_item.op);
 
+		mm = prange->work_item.mm;
+retry:
+		mmap_write_lock(mm);
+
+		/* Checking for the need to drain retry faults must be inside
+		 * mmap write lock to serialize with munmap notifiers.
+		 */
+		if (unlikely(atomic_read(&svms->drain_pagefaults))) {
+			mmap_write_unlock(mm);
+			svm_range_drain_retry_fault(svms);
+			goto retry;
+		}
+
+		/* Remove from deferred_list must be inside mmap write lock, for
+		 * two race cases:
+		 * 1. unmap_from_cpu may change work_item.op and add the range
+		 *    to deferred_list again, cause use after free bug.
+		 * 2. svm_range_list_lock_and_flush_work may hold mmap write
+		 *    lock and continue because deferred_list is empty, but
+		 *    deferred_list work is actually waiting for mmap lock.
+		 */
+		spin_lock(&svms->deferred_list_lock);
+		list_del_init(&prange->deferred_list);
+		spin_unlock(&svms->deferred_list_lock);
+
 		mutex_lock(&svms->lock);
 		mutex_lock(&prange->migrate_mutex);
 		while (!list_empty(&prange->child_list)) {
@@ -2109,19 +2115,20 @@ retry:
 			pr_debug("child prange 0x%p op %d\n", pchild,
 				 pchild->work_item.op);
 			list_del_init(&pchild->child_list);
-			svm_range_handle_list_op(svms, pchild);
+			svm_range_handle_list_op(svms, pchild, mm);
 		}
 		mutex_unlock(&prange->migrate_mutex);
 
-		svm_range_handle_list_op(svms, prange);
+		svm_range_handle_list_op(svms, prange, mm);
 		mutex_unlock(&svms->lock);
+		mmap_write_unlock(mm);
+
+		/* Pairs with mmget in svm_range_add_list_work */
+		mmput(mm);
 
 		spin_lock(&svms->deferred_list_lock);
 	}
 	spin_unlock(&svms->deferred_list_lock);
-
-	mmap_write_unlock(mm);
-	mmput(mm);
 	pr_debug("exit svms 0x%p\n", svms);
 }
 
@@ -2139,6 +2146,9 @@ svm_range_add_list_work(struct svm_range_list *svms, struct svm_range *prange,
 			prange->work_item.op = op;
 	} else {
 		prange->work_item.op = op;
+
+		/* Pairs with mmput in deferred_list_work */
+		mmget(mm);
 		prange->work_item.mm = mm;
 		list_add_tail(&prange->deferred_list,
 			      &prange->svms->deferred_range_list);