Re: [PATCH] ext4: fix null-ptr-deref in ext4_write_info

[Baokun Li]

在 2022/8/5 21:07, Lukas Czerner 写道:
> On Fri, Aug 05, 2022 at 08:39:47PM +0800, Baokun Li wrote:
> > I caught a null-ptr-deref bug as follows:
> > ==================================================================
> > KASAN: null-ptr-deref in range [0x0000000000000068-0x000000000000006f]
> > CPU: 1 PID: 1589 Comm: umount Not tainted 5.10.0-02219-dirty #339
> > RIP: 0010:ext4_write_info+0x53/0x1b0
> > [...]
> > Call Trace:
> >   dquot_writeback_dquots+0x341/0x9a0
> >   ext4_sync_fs+0x19e/0x800
> >   __sync_filesystem+0x83/0x100
> >   sync_filesystem+0x89/0xf0
> >   generic_shutdown_super+0x79/0x3e0
> >   kill_block_super+0xa1/0x110
> >   deactivate_locked_super+0xac/0x130
> >   deactivate_super+0xb6/0xd0
> >   cleanup_mnt+0x289/0x400
> >   __cleanup_mnt+0x16/0x20
> >   task_work_run+0x11c/0x1c0
> >   exit_to_user_mode_prepare+0x203/0x210
> >   syscall_exit_to_user_mode+0x5b/0x3a0
> >   do_syscall_64+0x59/0x70
> >   entry_SYSCALL_64_after_hwframe+0x44/0xa9
> >   ==================================================================
> >
> > Above issue may happen as follows:
> > -------------------------------------
> > exit_to_user_mode_prepare
> >   task_work_run
> >    __cleanup_mnt
> >     cleanup_mnt
> >      deactivate_super
> >       deactivate_locked_super
> >        kill_block_super
> >         generic_shutdown_super
> >          shrink_dcache_for_umount
> >           dentry = sb->s_root
> >           sb->s_root = NULL              <--- Here set NULL
> >          sync_filesystem
> >           __sync_filesystem
> >            sb->s_op->sync_fs > ext4_sync_fs
> >             dquot_writeback_dquots
> >              sb->dq_op->write_info > ext4_write_info
> >               ext4_journal_start(d_inode(sb->s_root), EXT4_HT_QUOTA, 2)
> >                d_inode(sb->s_root)
> >                 s_root->d_inode          <--- Null pointer dereference
> >
> > To solve this problem, we use ext4_journal_start_sb directly
> > to avoid s_root being used.
> Are we syncing the file system after the superblock shutdown and getting
> away with it? This does not look good. Do you have a reproducer?
>
> Thanks!
> -Lukas

Hi, Lukas!

This problem is triggered by a pressure test when I reproduce another 
problem.

So I didn't have the reproducer.

I looked at the error stack code and found that it seems there is 
something wrong.

Moreover, it's really weird to write code like 
"ext4_journal_start(d_inode(sb->s_root), ...)".

> > Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
> > ---
> >   fs/ext4/super.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> > index 9a66abcca1a8..0ce4565422f6 100644
> > --- a/fs/ext4/super.c
> > +++ b/fs/ext4/super.c
> > @@ -6653,7 +6653,7 @@ static int ext4_write_info(struct super_block *sb, int type)
> >   	handle_t *handle;
> >   
> >   	/* Data block + inode block */
> > -	handle = ext4_journal_start(d_inode(sb->s_root), EXT4_HT_QUOTA, 2);
> > +	handle = ext4_journal_start_sb(sb, EXT4_HT_QUOTA, 2);
> >   	if (IS_ERR(handle))
> >   		return PTR_ERR(handle);
> >   	ret = dquot_commit_info(sb, type);
> > --
> > 2.31.1
> .


Thanks!

--
With Best Regards,
Baokun Li