nilfs2: prevent use of deleted inode

syzbot reported a WARNING in nilfs_rmdir. [1]

Because the inode bitmap is corrupted, an inode with an inode number that
should exist as a ".nilfs" file was reassigned by nilfs_mkdir for "file0",
causing an inode duplication during execution.  And this causes an
underflow of i_nlink in rmdir operations.

The inode is used twice by the same task to unmount and remove directories
".nilfs" and "file0", it trigger warning in nilfs_rmdir.

Avoid to this issue, check i_nlink in nilfs_iget(), if it is 0, it means
that this inode has been deleted, and iput is executed to reclaim it.

[1]
WARNING: CPU: 1 PID: 5824 at fs/inode.c:407 drop_nlink+0xc4/0x110 fs/inode.c:407
...
Call Trace:
 <TASK>
 nilfs_rmdir+0x1b0/0x250 fs/nilfs2/namei.c:342
 vfs_rmdir+0x3a3/0x510 fs/namei.c:4394
 do_rmdir+0x3b5/0x580 fs/namei.c:4453
 __do_sys_rmdir fs/namei.c:4472 [inline]
 __se_sys_rmdir fs/namei.c:4470 [inline]
 __x64_sys_rmdir+0x47/0x50 fs/namei.c:4470
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Link: https://lkml.kernel.org/r/[email protected]
Fixes: d250065 ("nilfs2: pathname operations")
Signed-off-by: Ryusuke Konishi <[email protected]>
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=9260555647a5132edd48
Tested-by: [email protected]
Signed-off-by: Edward Adam Davis <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: ea1davis

fs/nilfs2/inode.c

@@ -544,8 +544,14 @@ struct inode *nilfs_iget(struct super_block *sb, struct nilfs_root *root,
 	inode = nilfs_iget_locked(sb, root, ino);
 	if (unlikely(!inode))
 		return ERR_PTR(-ENOMEM);
-	if (!(inode->i_state & I_NEW))
+
+	if (!(inode->i_state & I_NEW)) {
+		if (!inode->i_nlink) {
+			iput(inode);
+			return ERR_PTR(-ESTALE);
+		}
 		return inode;
+	}
 
 	err = __nilfs_read_inode(sb, root, ino, inode);
 	if (unlikely(err)) {

fs/nilfs2/namei.c

@@ -67,6 +67,11 @@ nilfs_lookup(struct inode *dir, struct dentry *dentry, unsigned int flags)
 		inode = NULL;
 	} else {
 		inode = nilfs_iget(dir->i_sb, NILFS_I(dir)->i_root, ino);
+		if (inode == ERR_PTR(-ESTALE)) {
+			nilfs_error(dir->i_sb,
+					"deleted inode referenced: %lu", ino);
+			return ERR_PTR(-EIO);
+		}
 	}
 
 	return d_splice_alias(inode, dentry);