linux-kernel - Re: [PATCH V3] fs/ntfs3: check if the inode is bad before creating symlink

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Message-ID: <20241121031329.354341-1-lizhi.xu@windriver.com>
Date: Thu, 21 Nov 2024 11:13:29 +0800
From: Lizhi Xu <lizhi.xu@...driver.com>
To: <viro@...iv.linux.org.uk>
CC: <almaz.alexandrovich@...agon-software.com>, <brauner@...nel.org>,
        <jack@...e.cz>, <linux-fsdevel@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <lizhi.xu@...driver.com>,
        <ntfs3@...ts.linux.dev>,
        <syzbot+73d8fc29ec7cba8286fa@...kaller.appspotmail.com>,
        <syzkaller-bugs@...glegroups.com>
Subject: Re: [PATCH V3] fs/ntfs3: check if the inode is bad before creating symlink

On Wed, 20 Nov 2024 16:10:45 +0000, Al Viro wrote:
> On Wed, Nov 20, 2024 at 11:04:43AM +0800, Lizhi Xu wrote:
> > syzbot reported a null-ptr-deref in pick_link. [1]
> >
> > First, i_link and i_dir_seq are in the same union, they share the same memory
> > address, and i_dir_seq will be updated during the execution of walk_component,
> > which makes the value of i_link equal to i_dir_seq.
> >
> > Secondly, the chmod execution failed, which resulted in setting the mode value
> > of file0's inode to REG when executing ntfs_bad_inode.
> >
> > Third, when creating a symbolic link using the file0 whose inode has been marked
> > as bad, it is not determined whether its inode is bad, which ultimately leads to
> > null-ptr-deref when performing a mount operation on the symbolic link bus because
> > the i_link value is equal to i_dir_seq=2.
> >
> > Note: ("file0, bus" are defined in reproducer [2])
> >
> > To avoid null-ptr-deref in pick_link, when creating a symbolic link, first check
> > whether the inode of file is already bad.
> 
> I would really like to understand how the hell did that bad inode end up passed
> to d_splice_alias()/d_instantiate()/whatever it had been.
1. In the move_mount() process, the inode is created by ntfs_alloc_inode() and enters d_splice_alias() by ntfs_lookup(), at this time inode is good, as shown below:
move_mount()->
  user_path_at()->
    filename_lookup()->
      path_lookupat()->
        lookup_last()->
          walk_component()->
            __lookup_slow()->
              ntfs_lookup()->
                d_splice_alias()->

2. The subsequent chmod fails, causing the inode to be set to bad.
3. During the link operation, d_instantiate() is executed in ntfs_link() to associate the bad inode with the dentry.
4. During the mount operation, walk_component executes pick_link, triggering null-ptr-deref.

Reproducer:
move_mount(0xffffffffffffff9c, &(0x7f00000003c0)='./file0\x00', 0xffffffffffffff9c, &(0x7f0000000400)='./file0/file0\x00', 0x140)
chmod(&(0x7f0000000080)='./file0\x00', 0x0)
link(&(0x7f0000000200)='./file0\x00', &(0x7f0000000240)='./bus\x00')
mount$overlay(0x0, &(0x7f00000000c0)='./bus\x00', 0x0, 0x0, 0x0)
> 
> That's the root cause - and it looks like ntfs is too free with make_bad_inode()
> in general, which might cause other problems.