fat: fix uninit-memory access for partial initialized inode

When get an error in the middle of reading an inode, some fields in the
inode might be still not initialized.  And then the evict_inode path may
access those fields via iput().

To fix, this makes sure that inode fields are initialized.

Reported-by: syzbot+9d82b8de2992579da5d0@syzkaller.appspotmail.com
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: <stable@vger.kernel.org>
Link: http://lkml.kernel.org/r/871rqnreqx.fsf@mail.parknet.co.jpSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/fat/inode.c

@@ -750,6 +750,13 @@ static struct inode *fat_alloc_inode(struct super_block *sb)
 		return NULL;
 
 	init_rwsem(&ei->truncate_lock);
+	/* Zeroing to allow iput() even if partial initialized inode. */
+	ei->mmu_private = 0;
+	ei->i_start = 0;
+	ei->i_logstart = 0;
+	ei->i_attrs = 0;
+	ei->i_pos = 0;
+
 	return &ei->vfs_inode;
 }
 
@@ -1374,16 +1381,6 @@ static int parse_options(struct super_block *sb, char *options, int is_vfat,
 	return 0;
 }
 
-static void fat_dummy_inode_init(struct inode *inode)
-{
-	/* Initialize this dummy inode to work as no-op. */
-	MSDOS_I(inode)->mmu_private = 0;
-	MSDOS_I(inode)->i_start = 0;
-	MSDOS_I(inode)->i_logstart = 0;
-	MSDOS_I(inode)->i_attrs = 0;
-	MSDOS_I(inode)->i_pos = 0;
-}
-
 static int fat_read_root(struct inode *inode)
 {
 	struct msdos_sb_info *sbi = MSDOS_SB(inode->i_sb);
@@ -1844,13 +1841,11 @@ int fat_fill_super(struct super_block *sb, void *data, int silent, int isvfat,
 	fat_inode = new_inode(sb);
 	if (!fat_inode)
 		goto out_fail;
-	fat_dummy_inode_init(fat_inode);
 	sbi->fat_inode = fat_inode;
 
 	fsinfo_inode = new_inode(sb);
 	if (!fsinfo_inode)
 		goto out_fail;
-	fat_dummy_inode_init(fsinfo_inode);
 	fsinfo_inode->i_ino = MSDOS_FSINFO_INO;
 	sbi->fsinfo_inode = fsinfo_inode;
 	insert_inode_hash(fsinfo_inode);