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From: Gao Xiang <gaoxiang25@huawei.com>
To: Chao Yu <yuchao0@huawei.com>
Cc: Gao Xiang <xiang@kernel.org>,
	linux-erofs@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4] erofs: support superblock checksum
Date: Wed, 23 Oct 2019 16:45:36 +0800
Message-ID: <20191023084536.GA16289@architecture4> (raw)
In-Reply-To: <f158affb-c5c5-9cbe-d87d-17210bc635fe@huawei.com>

Hi Chao,

On Wed, Oct 23, 2019 at 04:15:29PM +0800, Chao Yu wrote:
> Hi, Xiang, Pratik,
> 
> On 2019/10/23 12:05, Gao Xiang wrote:

<snip>

> >  }
> >  
> > +static int erofs_superblock_csum_verify(struct super_block *sb, void *sbdata)
> > +{
> > +	struct erofs_super_block *dsb;
> > +	u32 expected_crc, nblocks, crc;
> > +	void *kaddr;
> > +	struct page *page;
> > +	int i;
> > +
> > +	dsb = kmemdup(sbdata + EROFS_SUPER_OFFSET,
> > +		      EROFS_BLKSIZ - EROFS_SUPER_OFFSET, GFP_KERNEL);
> > +	if (!dsb)
> > +		return -ENOMEM;
> > +
> > +	expected_crc = le32_to_cpu(dsb->checksum);
> > +	nblocks = le32_to_cpu(dsb->chksum_blocks);
> 
> Now, we try to use nblocks's value before checking its validation, I guess fuzz
> test can easily make the value extreme larger, result in checking latter blocks
> unnecessarily.
> 
> IMO, we'd better
> 1. check validation of superblock to make sure all fields in sb are valid
> 2. use .nblocks to count and check payload blocks following sb

That is quite a good point. :-)

My first thought is to check the following payloads of sb (e.g, some per-fs
metadata should be checked at mount time together. or for small images, check
the whole image at the mount time) as well since if we introduce a new feature
to some kernel version, forward compatibility needs to be considered. So it's
better to make proper scalability, for this case, we have some choices:
 1) limit `chksum_blocks' upbound at runtime (e.g. refuse >= 65536 blocks,
    totally 256M.)
 2) just get rid of the whole `chksum_blocks' mess and checksum the first 4k
    at all, don't consider any latter scalability.

Some perferred idea about this? I plan to release erofs-utils v1.0 tomorrow
and hold up this feature for the next erofs-utils release, but I think we can
get it ready for v5.5 since it is not quite complex feature...

Thanks,
Gao Xiang


  reply index

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-22 18:06 [PATCH-v3] erofs: code for verifying superblock checksum of an erofs image Pratik Shinde
2019-10-23  4:05 ` [PATCH v4] erofs: support superblock checksum Gao Xiang
2019-10-23  8:15   ` Chao Yu
2019-10-23  8:45     ` Gao Xiang [this message]
2019-10-28 12:36       ` Chao Yu
2019-10-28 13:44         ` Gao Xiang
2019-10-28 14:32           ` [PATCH v5] " Gao Xiang
2019-10-30  2:33             ` Chao Yu
2019-10-30  2:56               ` Gao Xiang
2019-10-30  5:08                 ` [PATCH v6] " Gao Xiang
2019-11-04  2:49                   ` [PATCH v7] " Gao Xiang

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