Re: [syzbot] [fuse?] general protection fault in fuse_do_readpage

[Lizhi Xu]

about this case, calltrace is:
erofs_read_superblock()->
	erofs_read_metabuf()->
		erofs_bread()->
			read_mapping_folio()
	
41                  folio = read_mapping_folio(buf->mapping, index, NULL);
file is NULL in read_mapping_folio() at fs/erofs/data.c, and in fuse_do_readpage(),
it need file pass node id and file handle(in userspace), so need to add a
check for file in fuse_read_folio().

#syz test

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index f33fbce86ae0..fe6df701da24 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -902,6 +902,9 @@ static int fuse_read_folio(struct file *file, struct folio *folio)
 	if (fuse_is_bad(inode))
 		goto out;
 
+	if (!file)
+		goto out;
+
 	err = fuse_do_readpage(file, page);
 	fuse_invalidate_atime(inode);
  out: