erofs: fix file-backed mounts over FUSE

syzbot reported a null-ptr-deref in fuse_read_args_fill:
 fuse_read_folio+0xb0/0x100 fs/fuse/file.c:905
 filemap_read_folio+0xc6/0x2a0 mm/filemap.c:2367
 do_read_cache_folio+0x263/0x5c0 mm/filemap.c:3825
 read_mapping_folio include/linux/pagemap.h:1011 [inline]
 erofs_bread+0x34d/0x7e0 fs/erofs/data.c:41
 erofs_read_superblock fs/erofs/super.c:281 [inline]
 erofs_fc_fill_super+0x2b9/0x2500 fs/erofs/super.c:625

Unlike most filesystems, some network filesystems and FUSE need
unavoidable valid `file` pointers for their read I/Os [1].
Anyway, those use cases need to be supported too.

[1] https://docs.kernel.org/filesystems/vfs.html

Reported-by: syzbot+0b1279812c46e48bb0c1@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/r/6727bbdf.050a0220.3c8d68.0a7e.GAE@google.com
Fixes: fb17675 ("erofs: add file-backed mount support")
Tested-by: syzbot+0b1279812c46e48bb0c1@syzkaller.appspotmail.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20241114234905.1873723-1-hsiangkao@linux.alibaba.com

Author: hsiangkao

fs/erofs/data.c

@@ -38,7 +38,7 @@ void *erofs_bread(struct erofs_buf *buf, erofs_off_t offset,
 	}
 	if (!folio || !folio_contains(folio, index)) {
 		erofs_put_metabuf(buf);
-		folio = read_mapping_folio(buf->mapping, index, NULL);
+		folio = read_mapping_folio(buf->mapping, index, buf->file);
 		if (IS_ERR(folio))
 			return folio;
 	}
@@ -61,9 +61,11 @@ void erofs_init_metabuf(struct erofs_buf *buf, struct super_block *sb)
 {
 	struct erofs_sb_info *sbi = EROFS_SB(sb);
 
-	if (erofs_is_fileio_mode(sbi))
-		buf->mapping = file_inode(sbi->fdev)->i_mapping;
-	else if (erofs_is_fscache_mode(sb))
+	buf->file = NULL;
+	if (erofs_is_fileio_mode(sbi)) {
+		buf->file = sbi->fdev;		/* some fs like FUSE needs it */
+		buf->mapping = buf->file->f_mapping;
+	} else if (erofs_is_fscache_mode(sb))
 		buf->mapping = sbi->s_fscache->inode->i_mapping;
 	else
 		buf->mapping = sb->s_bdev->bd_mapping;

fs/erofs/internal.h

@@ -209,6 +209,7 @@ enum erofs_kmap_type {
 
 struct erofs_buf {
 	struct address_space *mapping;
+	struct file *file;
 	struct page *page;
 	void *base;
 	enum erofs_kmap_type kmap_type;