linux-kernel - Re: [PATCH v2] erofs: deprecate superblock checksum feature

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <9299dd4c-c2da-4ed1-8979-87fa44c68f77@t-8ch.de>
Date:   Sun, 30 Jul 2023 16:28:21 +0200
From:   Thomas Weißschuh <thomas@...ch.de>
To:     Gao Xiang <hsiangkao@...ux.alibaba.com>
Cc:     Jingbo Xu <jefflexu@...ux.alibaba.com>, chao@...nel.org,
        huyue2@...lpad.com, linux-erofs@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, Karel Zak <kzak@...hat.com>
Subject: Re: [PATCH v2] erofs: deprecate superblock checksum feature

Hi Gao!

On 2023-07-30 22:01:11+0800, Gao Xiang wrote:
> On 2023/7/30 21:31, Thomas Weißschuh wrote:
> > On 2023-07-17 19:27:03+0800, Jingbo Xu wrote:
> > > Later we're going to try the self-contained image verification.
> > > The current superblock checksum feature has quite limited
> > > functionality, instead, merkle trees can provide better protection
> > > for image integrity.
> > 
> > The crc32c checksum is also used by libblkid to gain more confidence
> > in its filesystem detection.
> > I guess a merkle tree would be much harder to implement.
> > 
> > This is for example used by the mount(8) cli program to allow mounting
> > of devices without explicitly needing to specify a filesystem.
> > 
> > Note: libblkid tests for EROFS_FEATURE_SB_CSUM so at least it won't
> > break when the checksum is removed.

> I'm not sure if we could switch EROFS_FEATURE_SB_CSUM to a simpler
> checksum instead (e.g. just sum each byte up if both
> EROFS_FEATURE_SB_CSUM and COMPAT_XATTR_FILTER bits are set, or
> ignore checksums completely at least in the kernel) if the better
> filesystem detection by using sb chksum is needed (not sure if other
> filesystems have sb chksum or just do magic comparsion)?

Overloading EROFS_FEATURE_SB_CSUM in combination with
COMPAT_XATTR_FILTER would break all existing deployments of libblkid, so
it's not an option.

All other serious and halfway modern filesystems do have superblock
checksums which are also checked by libblkid.

> The main problem here is after xattr name filter feature is added
> (xxhash is generally faster than crc32c), there could be two
> hard-depended hashing algorithms, this increases more dependency
> especially for embededed devices.

>From libblkid side nothing really speaks against a simpler checksum.
XOR is easy to implement and xxhash is already part of libblkid for
other filesystems.

The drawbacks are:
* It would need a completely new feature bit in erofs.
* Old versions of libblkid could not validate checksums on newer
  filesystems.