[PATCH] nilfs2: prevent use of deleted inode

[Ryusuke Konishi <konishi.ryusuke@gmail.com>]

From: Edward Adam Davis <eadavis@qq.com>

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

Reported-by: syzbot+9260555647a5132edd48@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=9260555647a5132edd48
Tested-by: syzbot+9260555647a5132edd48@syzkaller.appspotmail.com
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
Fixes: d25006523d0b ("nilfs2: pathname operations")
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
---
Hi Andrew, please apply this as a bug fix.

This prevents the link count underflow that can occur when deleting
directories in an inconsistent file system, which was recently
reported by syzbot.

Thanks,
Ryusuke Konishi

 fs/nilfs2/inode.c | 8 +++++++-
 fs/nilfs2/namei.c | 5 +++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/fs/nilfs2/inode.c b/fs/nilfs2/inode.c
index cf9ba481ae37..b7d4105f37bf 100644
--- a/fs/nilfs2/inode.c
+++ b/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)) {
diff --git a/fs/nilfs2/namei.c b/fs/nilfs2/namei.c
index 9b108052d9f7..1d836a5540f3 100644
--- a/fs/nilfs2/namei.c
+++ b/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);
-- 
2.43.0