linux-kernel - Re: [PATCH] ext4: fix null-ptr-deref in ext4_write

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Date:   Mon, 15 Aug 2022 17:34:10 +0200
From:   Jan Kara <jack@...e.cz>
To:     Baokun Li <libaokun1@...wei.com>
Cc:     Lukas Czerner <lczerner@...hat.com>, linux-ext4@...r.kernel.org,
        tytso@....edu, adilger.kernel@...ger.ca, jack@...e.cz,
        ritesh.list@...il.com, enwlinux@...il.com,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com,
        yebin10@...wei.com, yukuai3@...wei.com
Subject: Re: [PATCH] ext4: fix null-ptr-deref in ext4_write_info

On Wed 10-08-22 15:20:48, Baokun Li wrote:
> 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), ...)".

I agree. That looks like some leftover from the past. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> 
> > > Signed-off-by: Baokun Li <libaokun1@...wei.com>
> > > ---
> > >   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
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR