linux-ext4 - Re: [PATCH] ext2: strengthen value length check in ext2_xattr

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Date:   Fri, 24 May 2019 10:33:00 +0200
From:   Jan Kara <jack@...e.cz>
To:     "cgxu519@...o.com.cn" <cgxu519@...o.com.cn>
Cc:     Jan Kara <jack@...e.cz>, jack@...e.com, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext2: strengthen value length check in ext2_xattr_set()

On Fri 24-05-19 14:11:34, cgxu519@...o.com.cn wrote:
> On Wed, 2019-05-22 at 11:50 +0200, Jan Kara wrote:
> > On Wed 22-05-19 16:28:46, Chengguang Xu wrote:
> > > Actually maximum length of a valid entry value is not
> > > ->s_blocksize because header, last entry and entry
> > > name will also occupy some spaces. This patch
> > > strengthens the value length check and return -ERANGE
> > > when the length is larger than allowed maximum length.
> > > 
> > > Signed-off-by: Chengguang Xu <cgxu519@...o.com.cn>
> > 
> > Thanks for the patch! But what's the point of this change? We would return
> > ERANGE instead of ENOSPC? I don't think that's serious enough to warrant
> > changing existing behavior...
> 
> Hi Jan,
> 
> Instead of adding the check here, I propose to change value
> size limit check in ext2_xattr_entry_valid().
> 
> size = le32_to_cpu(entry->e_value_size);
> if (size > end_offs ||
>     le16_to_cpu(entry->e_value_offs) + size > end_offs)
> 
> Change to
> 
> size = EXT2_XATTR_SIZE(le32_to_cpu(entry->e_value_size));
> if (size >= end_offs - sizeof(struct ext2_xattr_header) - sizeof(__u32) ||
>     le16_to_cpu(entry->e_value_offs) + size > end_offs)

I don't think this makes a big difference. Look: end_offs is always aligned to
EXT2_XATTR_PAD (it is always block size) so if entry->e_value_offs is
properly aligned (which we may want to check), then
le16_to_cpu(entry->e_value_offs) + EXT2_XATTR_SIZE(size) > end_offs if and
only if le16_to_cpu(entry->e_value_offs) + size > end_offs.

Also the check le16_to_cpu(entry->e_value_offs) + size > end_offs is the
essential and strongest part - it checks whether the value does not extend
beyond block. The check size > end_offs is needed only for the case where
le16_to_cpu(entry->e_value_offs) + size would overflow and result in a
negative number.

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