ANDROID: sdcardfs: Protect set_top

If the top is changed while we're attempting to use it, it's
possible that the reference will be put while we are in the
process of grabbing a reference.

Now we grab a spinlock to protect grabbing our reference count.

Additionally, we now set the inode_info's top value to point to
it's own data when initializing, which makes tracking changes
easier.

Change-Id: If15748c786ce4c0480ab8c5051a92523aff284d2
Signed-off-by: Daniel Rosenberg <drosen@google.com>

fs/sdcardfs/derived_perm.c

@@ -32,23 +32,20 @@ static void inherit_derived_state(struct inode *parent, struct inode *child)
 	ci->data->under_android = pi->data->under_android;
 	ci->data->under_cache = pi->data->under_cache;
 	ci->data->under_obb = pi->data->under_obb;
-	set_top(ci, pi->top_data);
 }
 
 /* helper function for derived state */
 void setup_derived_state(struct inode *inode, perm_t perm, userid_t userid,
-					uid_t uid, bool under_android,
+					uid_t uid)
-					struct sdcardfs_inode_data *top)
 {
 	struct sdcardfs_inode_info *info = SDCARDFS_I(inode);
 
 	info->data->perm = perm;
 	info->data->userid = userid;
 	info->data->d_uid = uid;
-	info->data->under_android = under_android;
+	info->data->under_android = false;
 	info->data->under_cache = false;
 	info->data->under_obb = false;
-	set_top(info, top);
 }
 
 /* While renaming, there is a point where we want the path from dentry,
@@ -58,8 +55,8 @@ void get_derived_permission_new(struct dentry *parent, struct dentry *dentry,
 				const struct qstr *name)
 {
 	struct sdcardfs_inode_info *info = SDCARDFS_I(d_inode(dentry));
-	struct sdcardfs_inode_data *parent_data =
+	struct sdcardfs_inode_info *parent_info = SDCARDFS_I(d_inode(parent));
-			SDCARDFS_I(d_inode(parent))->data;
+	struct sdcardfs_inode_data *parent_data = parent_info->data;
 	appid_t appid;
 	unsigned long user_num;
 	int err;
@@ -80,13 +77,15 @@ void get_derived_permission_new(struct dentry *parent, struct dentry *dentry,
 	inherit_derived_state(d_inode(parent), d_inode(dentry));
 
 	/* Files don't get special labels */
-	if (!S_ISDIR(d_inode(dentry)->i_mode))
+	if (!S_ISDIR(d_inode(dentry)->i_mode)) {
+		set_top(info, parent_info);
 		return;
+	}
 	/* Derive custom permissions based on parent and current node */
 	switch (parent_data->perm) {
 	case PERM_INHERIT:
 	case PERM_ANDROID_PACKAGE_CACHE:
-		/* Already inherited above */
+		set_top(info, parent_info);
 		break;
 	case PERM_PRE_ROOT:
 		/* Legacy internal layout places users at top level */
@@ -96,7 +95,6 @@ void get_derived_permission_new(struct dentry *parent, struct dentry *dentry,
 			info->data->userid = 0;
 		else
 			info->data->userid = user_num;
-		set_top(info, info->data);
 		break;
 	case PERM_ROOT:
 		/* Assume masked off by default. */
@@ -104,24 +102,24 @@ void get_derived_permission_new(struct dentry *parent, struct dentry *dentry,
 			/* App-specific directories inside; let anyone traverse */
 			info->data->perm = PERM_ANDROID;
 			info->data->under_android = true;
-			set_top(info, info->data);
+		} else {
+			set_top(info, parent_info);
 		}
 		break;
 	case PERM_ANDROID:
 		if (qstr_case_eq(name, &q_data)) {
 			/* App-specific directories inside; let anyone traverse */
 			info->data->perm = PERM_ANDROID_DATA;
-			set_top(info, info->data);
 		} else if (qstr_case_eq(name, &q_obb)) {
 			/* App-specific directories inside; let anyone traverse */
 			info->data->perm = PERM_ANDROID_OBB;
 			info->data->under_obb = true;
-			set_top(info, info->data);
 			/* Single OBB directory is always shared */
 		} else if (qstr_case_eq(name, &q_media)) {
 			/* App-specific directories inside; let anyone traverse */
 			info->data->perm = PERM_ANDROID_MEDIA;
-			set_top(info, info->data);
+		} else {
+			set_top(info, parent_info);
 		}
 		break;
 	case PERM_ANDROID_OBB:
@@ -132,13 +130,13 @@ void get_derived_permission_new(struct dentry *parent, struct dentry *dentry,
 		if (appid != 0 && !is_excluded(name->name, parent_data->userid))
 			info->data->d_uid =
 				multiuser_get_uid(parent_data->userid, appid);
-		set_top(info, info->data);
 		break;
 	case PERM_ANDROID_PACKAGE:
 		if (qstr_case_eq(name, &q_cache)) {
 			info->data->perm = PERM_ANDROID_PACKAGE_CACHE;
 			info->data->under_cache = true;
 		}
+		set_top(info, parent_info);
 		break;
 	}
 }

fs/sdcardfs/main.c

@@ -341,13 +341,11 @@ static int sdcardfs_read_super(struct vfsmount *mnt, struct super_block *sb,
 	mutex_lock(&sdcardfs_super_list_lock);
 	if (sb_info->options.multiuser) {
 		setup_derived_state(d_inode(sb->s_root), PERM_PRE_ROOT,
-				sb_info->options.fs_user_id, AID_ROOT,
+				sb_info->options.fs_user_id, AID_ROOT);
-				false, SDCARDFS_I(d_inode(sb->s_root))->data);
 		snprintf(sb_info->obbpath_s, PATH_MAX, "%s/obb", dev_name);
 	} else {
 		setup_derived_state(d_inode(sb->s_root), PERM_ROOT,
-				sb_info->options.fs_user_id, AID_ROOT,
+				sb_info->options.fs_user_id, AID_ROOT);
-				false, SDCARDFS_I(d_inode(sb->s_root))->data);
 		snprintf(sb_info->obbpath_s, PATH_MAX, "%s/Android/obb", dev_name);
 	}
 	fixup_tmp_permissions(d_inode(sb->s_root));

fs/sdcardfs/sdcardfs.h

@@ -201,6 +201,7 @@ struct sdcardfs_inode_info {
 	struct sdcardfs_inode_data *data;
 
 	/* top folder for ownership */
+	spinlock_t top_lock;
 	struct sdcardfs_inode_data *top_data;
 
 	struct inode vfs_inode;
@@ -380,7 +381,12 @@ static inline struct sdcardfs_inode_data *data_get(
 static inline struct sdcardfs_inode_data *top_data_get(
 		struct sdcardfs_inode_info *info)
 {
-	return data_get(info->top_data);
+	struct sdcardfs_inode_data *top_data;
+
+	spin_lock(&info->top_lock);
+	top_data = data_get(info->top_data);
+	spin_unlock(&info->top_lock);
+	return top_data;
 }
 
 extern void data_release(struct kref *ref);
@@ -402,15 +408,20 @@ static inline void release_own_data(struct sdcardfs_inode_info *info)
 }
 
 static inline void set_top(struct sdcardfs_inode_info *info,
-			struct sdcardfs_inode_data *top)
+			struct sdcardfs_inode_info *top_owner)
 {
-	struct sdcardfs_inode_data *old_top = info->top_data;
+	struct sdcardfs_inode_data *old_top;
+	struct sdcardfs_inode_data *new_top = NULL;
 
-	if (top)
+	if (top_owner)
-		data_get(top);
+		new_top = top_data_get(top_owner);
-	info->top_data = top;
+
+	spin_lock(&info->top_lock);
+	old_top = info->top_data;
+	info->top_data = new_top;
 	if (old_top)
 		data_put(old_top);
+	spin_unlock(&info->top_lock);
 }
 
 static inline int get_gid(struct vfsmount *mnt,
@@ -516,8 +527,7 @@ struct limit_search {
 };
 
 extern void setup_derived_state(struct inode *inode, perm_t perm,
-		userid_t userid, uid_t uid, bool under_android,
+			userid_t userid, uid_t uid);
-		struct sdcardfs_inode_data *top);
 extern void get_derived_permission(struct dentry *parent, struct dentry *dentry);
 extern void get_derived_permission_new(struct dentry *parent, struct dentry *dentry, const struct qstr *name);
 extern void fixup_perms_recursive(struct dentry *dentry, struct limit_search *limit);

fs/sdcardfs/super.c

@@ -215,6 +215,9 @@ static struct inode *sdcardfs_alloc_inode(struct super_block *sb)
 
 	i->data = d;
 	kref_init(&d->refcount);
+	i->top_data = d;
+	spin_lock_init(&i->top_lock);
+	kref_get(&d->refcount);
 
 	i->vfs_inode.i_version = 1;
 	return &i->vfs_inode;