ANDROID: incremental-fs: fix mount_fs issue

Syzbot recently found a number of issues related to incremental-fs
(see bug numbers below). All have to do with the fact that incr-fs
allows mounts of the same source and target multiple times.
The correct behavior for a file system is to allow only one such
mount, and then every subsequent attempt should fail with a -EBUSY
error code. In case of the issues listed below the common pattern
is that the reproducer calls:

mount("./file0", "./file0", "incremental-fs", 0, NULL)

many times and then invokes a file operation like chmod, setxattr,
or open on the ./file0. This causes a recursive call for all the
mounted instances, which eventually causes a stack overflow and
a kernel crash:

BUG: stack guard page was hit at ffffc90000c0fff8
kernel stack overflow (double-fault): 0000 [#1] PREEMPT SMP KASAN

The reason why many mounts with the same source and target are
possible is because the incfs_mount_fs() as it is allocates a new
super_block for every call, regardless of whether a given mount already
exists or not. This happens every time the sget() function is called
with a test param equal to NULL.
The correct behavior for an FS mount implementation is to call
appropriate mount vfs call for it's type, i.e. mount_bdev() for
a block device backed FS, mount_single() for a pseudo file system,
like sysfs that is mounted in a single, well know location, or
mount_nodev() for other special purpose FS like overlayfs.
In case of incremental-fs the open coded mount logic doesn't check
for abusive mount attempts such as overlays.
To fix this issue the logic needs to be changed to pass a proper
test function to sget() call, which then checks if a super_block
for a mount instance has already been allocated and also allows
the VFS to properly verify invalid mount attempts.

Bug: 211066171
Bug: 213140206
Bug: 213215835
Bug: 211914587
Bug: 211213635
Bug: 213137376
Bug: 211161296

Signed-off-by: Tadeusz Struk <tadeusz.struk@linaro.org>
Change-Id: I66cfc3f1b5aaffb32b0845b2dad3ff26fe952e27

fs/incfs/vfs.c

@@ -321,7 +321,6 @@ static struct mount_info *get_mount_info(struct super_block *sb)
 {
 	struct mount_info *result = sb->s_fs_info;
 
-	WARN_ON(!result);
 	return result;
 }
 
@@ -683,7 +682,7 @@ static int iterate_incfs_dir(struct file *file, struct dir_context *ctx)
 	struct mount_info *mi = get_mount_info(file_superblock(file));
 	bool root;
 
-	if (!dir) {
+	if (!dir || !mi) {
 		error = -EBADF;
 		goto out;
 	}
@@ -1847,6 +1846,9 @@ static int dir_rename(struct inode *old_dir, struct dentry *old_dentry,
 	struct dentry *trap;
 	int error = 0;
 
+	if (!mi)
+		return -EBADF;
+
 	error = mutex_lock_interruptible(&mi->mi_dir_struct_mutex);
 	if (error)
 		return error;
@@ -2094,6 +2096,9 @@ static ssize_t incfs_getxattr(struct dentry *d, const char *name,
 	char *stored_value;
 	size_t stored_size;
 
+	if (!mi)
+		return -EBADF;
+
 	if (di && di->backing_path.dentry)
 		return vfs_getxattr(di->backing_path.dentry, name, value, size);
 
@@ -2130,6 +2135,9 @@ static ssize_t incfs_setxattr(struct dentry *d, const char *name,
 	void **stored_value;
 	size_t *stored_size;
 
+	if (!mi)
+		return -EBADF;
+
 	if (di && di->backing_path.dentry)
 		return vfs_setxattr(di->backing_path.dentry, name, value, size,
 				    flags);
@@ -2170,6 +2178,11 @@ static ssize_t incfs_listxattr(struct dentry *d, char *list, size_t size)
 	return vfs_listxattr(di->backing_path.dentry, list, size);
 }
 
+static int incfs_test_super(struct super_block *s, void *p)
+{
+	return s->s_fs_info != NULL;
+}
+
 struct dentry *incfs_mount_fs(struct file_system_type *type, int flags,
 			      const char *dev_name, void *data)
 {
@@ -2179,7 +2192,8 @@ struct dentry *incfs_mount_fs(struct file_system_type *type, int flags,
 	struct dentry *index_dir;
 	struct super_block *src_fs_sb = NULL;
 	struct inode *root_inode = NULL;
-	struct super_block *sb = sget(type, NULL, set_anon_super, flags, NULL);
+	struct super_block *sb = sget(type, incfs_test_super, set_anon_super,
+				      flags, NULL);
 	int error = 0;
 
 	if (IS_ERR(sb))
@@ -2220,13 +2234,19 @@ struct dentry *incfs_mount_fs(struct file_system_type *type, int flags,
 	src_fs_sb = backing_dir_path.dentry->d_sb;
 	sb->s_maxbytes = src_fs_sb->s_maxbytes;
 
-	mi = incfs_alloc_mount_info(sb, &options, &backing_dir_path);
+	if (!sb->s_fs_info) {
+		mi = incfs_alloc_mount_info(sb, &options, &backing_dir_path);
 
-	if (IS_ERR_OR_NULL(mi)) {
-		error = PTR_ERR(mi);
-		pr_err("incfs: Error allocating mount info. %d\n", error);
-		mi = NULL;
-		goto err;
+		if (IS_ERR_OR_NULL(mi)) {
+			error = PTR_ERR(mi);
+			pr_err("incfs: Error allocating mount info. %d\n",
+			       error);
+			mi = NULL;
+			goto err;
+		}
+		sb->s_fs_info = mi;
+	} else {
+		mi = sb->s_fs_info;
 	}
 
 	index_dir = open_or_create_index_dir(backing_dir_path.dentry);
@@ -2238,21 +2258,22 @@ struct dentry *incfs_mount_fs(struct file_system_type *type, int flags,
 	}
 	mi->mi_index_dir = index_dir;
 
-	sb->s_fs_info = mi;
 	root_inode = fetch_regular_inode(sb, backing_dir_path.dentry);
 	if (IS_ERR(root_inode)) {
 		error = PTR_ERR(root_inode);
 		goto err;
 	}
 
-	sb->s_root = d_make_root(root_inode);
 	if (!sb->s_root) {
-		error = -ENOMEM;
-		goto err;
+		sb->s_root = d_make_root(root_inode);
+		if (!sb->s_root) {
+			error = -ENOMEM;
+			goto err;
+		}
+		error = incfs_init_dentry(sb->s_root, &backing_dir_path);
+		if (error)
+			goto err;
 	}
-	error = incfs_init_dentry(sb->s_root, &backing_dir_path);
-	if (error)
-		goto err;
 
 	path_put(&backing_dir_path);
 	sb->s_flags |= SB_ACTIVE;
@@ -2273,6 +2294,9 @@ static int incfs_remount_fs(struct super_block *sb, int *flags, char *data)
 	struct mount_info *mi = get_mount_info(sb);
 	int err = 0;
 
+	if (!mi)
+		return err;
+
 	sync_filesystem(sb);
 	err = parse_options(&options, (char *)data);
 	if (err)
@@ -2293,12 +2317,16 @@ void incfs_kill_sb(struct super_block *sb)
 	pr_debug("incfs: unmount\n");
 	incfs_free_mount_info(mi);
 	generic_shutdown_super(sb);
+	sb->s_fs_info = NULL;
 }
 
 static int show_options(struct seq_file *m, struct dentry *root)
 {
 	struct mount_info *mi = get_mount_info(root->d_sb);
 
+	if (!mi)
+		return -EBADF;
+
 	seq_printf(m, ",read_timeout_ms=%u", mi->mi_options.read_timeout_ms);
 	seq_printf(m, ",readahead=%u", mi->mi_options.readahead_pages);
 	if (mi->mi_options.read_log_pages != 0) {