staging: erofs: fix race when the managed cache is enabled

When the managed cache is enabled, the last reference count
of a workgroup must be used for its workstation.

Otherwise, it could lead to incorrect (un)freezes in
the reclaim path, and it would be harmful.

A typical race as follows:

Thread 1 (In the reclaim path)  Thread 2
workgroup_freeze(grp, 1)                                refcnt = 1
...
workgroup_unfreeze(grp, 1)                              refcnt = 1
                                workgroup_get(grp)      refcnt = 2 (x)
workgroup_put(grp)                                      refcnt = 1 (x)
                                ...unexpected behaviors

* grp is detached but still used, which violates cache-managed
  freeze constraint.
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/erofs/internal.h

@@ -250,6 +250,7 @@ static inline bool erofs_workgroup_get(struct erofs_workgroup *grp, int *ocnt)
 }
 
 #define __erofs_workgroup_get(grp)	atomic_inc(&(grp)->refcount)
+#define __erofs_workgroup_put(grp)	atomic_dec(&(grp)->refcount)
 
 extern int erofs_workgroup_put(struct erofs_workgroup *grp);
 

drivers/staging/erofs/utils.c

@@ -83,12 +83,21 @@ int erofs_register_workgroup(struct super_block *sb,
 
 	grp = xa_tag_pointer(grp, tag);
 
+	/*
+	 * Bump up reference count before making this workgroup
+	 * visible to other users in order to avoid potential UAF
+	 * without serialized by erofs_workstn_lock.
+	 */
+	__erofs_workgroup_get(grp);
+
 	err = radix_tree_insert(&sbi->workstn_tree,
 				grp->index, grp);
-
-	if (!err) {
-		__erofs_workgroup_get(grp);
-	}
+	if (unlikely(err))
+		/*
+		 * it's safe to decrease since the workgroup isn't visible
+		 * and refcount >= 2 (cannot be freezed).
+		 */
+		__erofs_workgroup_put(grp);
 
 	erofs_workstn_unlock(sbi);
 	radix_tree_preload_end();
@@ -97,19 +106,94 @@ int erofs_register_workgroup(struct super_block *sb,
 
 extern void erofs_workgroup_free_rcu(struct erofs_workgroup *grp);
 
+static void  __erofs_workgroup_free(struct erofs_workgroup *grp)
+{
+	atomic_long_dec(&erofs_global_shrink_cnt);
+	erofs_workgroup_free_rcu(grp);
+}
+
 int erofs_workgroup_put(struct erofs_workgroup *grp)
 {
 	int count = atomic_dec_return(&grp->refcount);
 
 	if (count == 1)
 		atomic_long_inc(&erofs_global_shrink_cnt);
-	else if (!count) {
-		atomic_long_dec(&erofs_global_shrink_cnt);
-		erofs_workgroup_free_rcu(grp);
-	}
+	else if (!count)
+		__erofs_workgroup_free(grp);
 	return count;
 }
 
+#ifdef EROFS_FS_HAS_MANAGED_CACHE
+/* for cache-managed case, customized reclaim paths exist */
+static void erofs_workgroup_unfreeze_final(struct erofs_workgroup *grp)
+{
+	erofs_workgroup_unfreeze(grp, 0);
+	__erofs_workgroup_free(grp);
+}
+
+bool erofs_try_to_release_workgroup(struct erofs_sb_info *sbi,
+				    struct erofs_workgroup *grp,
+				    bool cleanup)
+{
+	/*
+	 * for managed cache enabled, the refcount of workgroups
+	 * themselves could be < 0 (freezed). So there is no guarantee
+	 * that all refcount > 0 if managed cache is enabled.
+	 */
+	if (!erofs_workgroup_try_to_freeze(grp, 1))
+		return false;
+
+	/*
+	 * note that all cached pages should be unlinked
+	 * before delete it from the radix tree.
+	 * Otherwise some cached pages of an orphan old workgroup
+	 * could be still linked after the new one is available.
+	 */
+	if (erofs_try_to_free_all_cached_pages(sbi, grp)) {
+		erofs_workgroup_unfreeze(grp, 1);
+		return false;
+	}
+
+	/*
+	 * it is impossible to fail after the workgroup is freezed,
+	 * however in order to avoid some race conditions, add a
+	 * DBG_BUGON to observe this in advance.
+	 */
+	DBG_BUGON(xa_untag_pointer(radix_tree_delete(&sbi->workstn_tree,
+						     grp->index)) != grp);
+
+	/*
+	 * if managed cache is enable, the last refcount
+	 * should indicate the related workstation.
+	 */
+	erofs_workgroup_unfreeze_final(grp);
+	return true;
+}
+
+#else
+/* for nocache case, no customized reclaim path at all */
+bool erofs_try_to_release_workgroup(struct erofs_sb_info *sbi,
+				    struct erofs_workgroup *grp,
+				    bool cleanup)
+{
+	int cnt = atomic_read(&grp->refcount);
+
+	DBG_BUGON(cnt <= 0);
+	DBG_BUGON(cleanup && cnt != 1);
+
+	if (cnt > 1)
+		return false;
+
+	DBG_BUGON(xa_untag_pointer(radix_tree_delete(&sbi->workstn_tree,
+						     grp->index)) != grp);
+
+	/* (rarely) could be grabbed again when freeing */
+	erofs_workgroup_put(grp);
+	return true;
+}
+
+#endif
+
 unsigned long erofs_shrink_workstation(struct erofs_sb_info *sbi,
 				       unsigned long nr_shrink,
 				       bool cleanup)
@@ -126,42 +210,14 @@ unsigned long erofs_shrink_workstation(struct erofs_sb_info *sbi,
 		batch, first_index, PAGEVEC_SIZE);
 
 	for (i = 0; i < found; ++i) {
-		int cnt;
 		struct erofs_workgroup *grp = xa_untag_pointer(batch[i]);
 
 		first_index = grp->index + 1;
 
-		cnt = atomic_read(&grp->refcount);
-		BUG_ON(cnt <= 0);
-
-		if (cleanup)
-			BUG_ON(cnt != 1);
-
-#ifndef EROFS_FS_HAS_MANAGED_CACHE
-		else if (cnt > 1)
-#else
-		if (!erofs_workgroup_try_to_freeze(grp, 1))
-#endif
+		/* try to shrink each valid workgroup */
+		if (!erofs_try_to_release_workgroup(sbi, grp, cleanup))
 			continue;
 
-		if (xa_untag_pointer(radix_tree_delete(&sbi->workstn_tree,
-			grp->index)) != grp) {
-#ifdef EROFS_FS_HAS_MANAGED_CACHE
-skip:
-			erofs_workgroup_unfreeze(grp, 1);
-#endif
-			continue;
-		}
-
-#ifdef EROFS_FS_HAS_MANAGED_CACHE
-		if (erofs_try_to_free_all_cached_pages(sbi, grp))
-			goto skip;
-
-		erofs_workgroup_unfreeze(grp, 1);
-#endif
-		/* (rarely) grabbed again when freeing */
-		erofs_workgroup_put(grp);
-
 		++freed;
 		if (unlikely(!--nr_shrink))
 			break;