fat: Fix uninit-memory access for partial initialized inode

[OGAWA Hirofumi]

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

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

Reported-by: syzbot+9d82b8de2992579da5d0@xxxxxxxxxxxxxxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>
---
 fs/fat/inode.c |   19 +++++++------------
 1 file changed, 7 insertions(+), 12 deletions(-)

diff --git a/fs/fat/inode.c b/fs/fat/inode.c
index 5f04c5c..686130f 100644
--- a/fs/fat/inode.c	2020-02-09 21:29:07.541357068 +0900
+++ b/fs/fat/inode.c	2020-02-22 02:15:20.451313122 +0900
@@ -749,6 +749,13 @@ static struct inode *fat_alloc_inode(str
 		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;
 }
 
@@ -1373,16 +1380,6 @@ out:
 	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);
@@ -1843,13 +1840,11 @@ int fat_fill_super(struct super_block *s
 	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);
_