linux-kernel - Re: [PATCH 1/3] quota: Check next/prev free block number after reading from quota file

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Date:   Wed, 21 Sep 2022 15:37:15 +0200
From:   Jan Kara <jack@...e.cz>
To:     Zhihao Cheng <chengzhihao1@...wei.com>
Cc:     jack@...e.com, tytso@....edu, brauner@...nel.org,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org, yukuai3@...wei.com
Subject: Re: [PATCH 1/3] quota: Check next/prev free block number after
 reading from quota file

On Sat 20-08-22 19:05:12, Zhihao Cheng wrote:
> Following process:
>  Init: v2_read_file_info: <3> dqi_free_blk 0 dqi_free_entry 5 dqi_blks 6
> 
>  Step 1. chown bin f_a -> dquot_acquire -> v2_write_dquot:
>   qtree_write_dquot
>    do_insert_tree
>     find_free_dqentry
>      get_free_dqblk
>       write_blk(info->dqi_blocks) // info->dqi_blocks = 6, failure. The
> 	   content in physical block (corresponding to blk 6) is random.
> 
>  Step 2. chown root f_a -> dquot_transfer -> dqput_all -> dqput ->
>          ext4_release_dquot -> v2_release_dquot -> qtree_delete_dquot:
>   dquot_release
>    remove_tree
>     free_dqentry
>      put_free_dqblk(6)
>       info->dqi_free_blk = blk    // info->dqi_free_blk = 6
> 
>  Step 3. drop cache (buffer head for block 6 is released)
> 
>  Step 4. chown bin f_b -> dquot_acquire -> commit_dqblk -> v2_write_dquot:
>   qtree_write_dquot
>    do_insert_tree
>     find_free_dqentry
>      get_free_dqblk
>       dh = (struct qt_disk_dqdbheader *)buf
>       blk = info->dqi_free_blk     // 6
>       ret = read_blk(info, blk, buf)  // The content of buf is random
>       info->dqi_free_blk = le32_to_cpu(dh->dqdh_next_free)  // random blk
> 
>  Step 5. chown bin f_c -> notify_change -> ext4_setattr -> dquot_transfer:
>   dquot = dqget -> acquire_dquot -> ext4_acquire_dquot -> dquot_acquire ->
>           commit_dqblk -> v2_write_dquot -> dq_insert_tree:
>    do_insert_tree
>     find_free_dqentry
>      get_free_dqblk
>       blk = info->dqi_free_blk    // If blk < 0 and blk is not an error
> 				     code, it will be returned as dquot
> 
>   transfer_to[USRQUOTA] = dquot  // A random negative value
>   __dquot_transfer(transfer_to)
>    dquot_add_inodes(transfer_to[cnt])
>     spin_lock(&dquot->dq_dqb_lock)  // page fault
> 
> , which will lead to kernel page fault:
>  Quota error (device sda): qtree_write_dquot: Error -8000 occurred
>  while creating quota
>  BUG: unable to handle page fault for address: ffffffffffffe120
>  #PF: supervisor write access in kernel mode
>  #PF: error_code(0x0002) - not-present page
>  Oops: 0002 [#1] PREEMPT SMP
>  CPU: 0 PID: 5974 Comm: chown Not tainted 6.0.0-rc1-00004
>  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996)
>  RIP: 0010:_raw_spin_lock+0x3a/0x90
>  Call Trace:
>   dquot_add_inodes+0x28/0x270
>   __dquot_transfer+0x377/0x840
>   dquot_transfer+0xde/0x540
>   ext4_setattr+0x405/0x14d0
>   notify_change+0x68e/0x9f0
>   chown_common+0x300/0x430
>   __x64_sys_fchownat+0x29/0x40
> 
> In order to avoid accessing invalid quota memory address, this patch adds
> block number checking of next/prev free block read from quota file.
> 
> Fetch a reproducer in [Link].
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=216372
> Fixes: 1da177e4c3f4152 ("Linux-2.6.12-rc2")

It's better to just have:

CC: stable@...r.kernel.org

here. Fixes tag pointing to kernel release is not very useful.

> --- a/fs/quota/quota_tree.c
> +++ b/fs/quota/quota_tree.c
> @@ -71,6 +71,35 @@ static ssize_t write_blk(struct qtree_mem_dqinfo *info, uint blk, char *buf)
>  	return ret;
>  }
>  
> +static inline int do_check_range(struct super_block *sb, uint val, uint max_val)
> +{
> +	if (val >= max_val) {
> +		quota_error(sb, "Getting block too big (%u >= %u)",
> +			    val, max_val);
> +		return -EUCLEAN;
> +	}
> +
> +	return 0;
> +}

I'd already provide min_val and the string for the message here as well (as
you do in patch 2). It is less churn in the next patch and free blocks
checking actually needs that as well. See below.

> +
> +static int check_free_block(struct qtree_mem_dqinfo *info,
> +			    struct qt_disk_dqdbheader *dh)
> +{
> +	int err = 0;
> +	uint nextblk, prevblk;
> +
> +	nextblk = le32_to_cpu(dh->dqdh_next_free);
> +	err = do_check_range(info->dqi_sb, nextblk, info->dqi_blocks);
> +	if (err)
> +		return err;
> +	prevblk = le32_to_cpu(dh->dqdh_prev_free);
> +	err = do_check_range(info->dqi_sb, prevblk, info->dqi_blocks);
> +	if (err)
> +		return err;

The free block should actually be > QT_TREEOFF so I'd add the check to
do_check_range().

Also rather than having check_free_block(), I'd provide a helper function
like check_dquot_block_header() which will check only free blocks pointers
now and in later patches you can add other checks there.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR