cachefiles: Fix NULL pointer dereference in object->file

At present, the object->file has the NULL pointer dereference problem in
ondemand-mode. The root cause is that the allocated fd and object->file
lifetime are inconsistent, and the user-space invocation to anon_fd uses
object->file. Following is the process that triggers the issue:

	  [write fd]				[umount]
cachefiles_ondemand_fd_write_iter
				       fscache_cookie_state_machine
					 cachefiles_withdraw_cookie
  if (!file) return -ENOBUFS
					   cachefiles_clean_up_object
					     cachefiles_unmark_inode_in_use
					     fput(object->file)
					     object->file = NULL
  // file NULL pointer dereference!
  __cachefiles_write(..., file, ...)

Fix this issue by add an additional reference count to the object->file
before write/llseek, and decrement after it finished.

Fixes: c8383054506c ("cachefiles: notify the user daemon when looking up cookie")
Signed-off-by: Zizhi Wo <wozizhi@huawei.com>
Link: https://lore.kernel.org/r/20241107110649.3980193-5-wozizhi@huawei.com
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 24 insertions, 6 deletions

diff --git a/fs/cachefiles/ondemand.c b/fs/cachefiles/ondemand.c
index 38ca6dce8ef2..fe3de9ad57bf 100644
--- a/fs/cachefiles/ondemand.c
+++ b/fs/cachefiles/ondemand.c
@@ -60,20 +60,26 @@ static ssize_t cachefiles_ondemand_fd_write_iter(struct kiocb *kiocb,
 {
 	struct cachefiles_object *object = kiocb->ki_filp->private_data;
 	struct cachefiles_cache *cache = object->volume->cache;
-	struct file *file = object->file;
+	struct file *file;
 	size_t len = iter->count, aligned_len = len;
 	loff_t pos = kiocb->ki_pos;
 	const struct cred *saved_cred;
 	int ret;
 
-	if (!file)
+	spin_lock(&object->lock);
+	file = object->file;
+	if (!file) {
+		spin_unlock(&object->lock);
 		return -ENOBUFS;
+	}
+	get_file(file);
+	spin_unlock(&object->lock);
 
 	cachefiles_begin_secure(cache, &saved_cred);
 	ret = __cachefiles_prepare_write(object, file, &pos, &aligned_len, len, true);
 	cachefiles_end_secure(cache, saved_cred);
 	if (ret < 0)
-		return ret;
+		goto out;
 
 	trace_cachefiles_ondemand_fd_write(object, file_inode(file), pos, len);
 	ret = __cachefiles_write(object, file, pos, iter, NULL, NULL);
@@ -82,6 +88,8 @@ static ssize_t cachefiles_ondemand_fd_write_iter(struct kiocb *kiocb,
 		kiocb->ki_pos += ret;
 	}
 
+out:
+	fput(file);
 	return ret;
 }
 
@@ -89,12 +97,22 @@ static loff_t cachefiles_ondemand_fd_llseek(struct file *filp, loff_t pos,
 					    int whence)
 {
 	struct cachefiles_object *object = filp->private_data;
-	struct file *file = object->file;
+	struct file *file;
+	loff_t ret;
 
-	if (!file)
+	spin_lock(&object->lock);
+	file = object->file;
+	if (!file) {
+		spin_unlock(&object->lock);
 		return -ENOBUFS;
+	}
+	get_file(file);
+	spin_unlock(&object->lock);
 
-	return vfs_llseek(file, pos, whence);
+	ret = vfs_llseek(file, pos, whence);
+	fput(file);
+
+	return ret;
 }
 
 static long cachefiles_ondemand_fd_ioctl(struct file *filp, unsigned int ioctl,