Re: Kernel 4.14: Using dm-verity with squashfs rootfs - mounting issue

Pintu Agarwal
2021-08-31T13:20:24Z

Hi,

On Tue, 31 Aug 2021 at 00:42, Thomas Petazzoni
<thomas.petazzoni@bootlin.com> wrote:
>
> Hello,
>
> On Mon, 30 Aug 2021 23:48:40 +0530
> Pintu Agarwal <pintu.ping@gmail.com> wrote:
>
> > ohh that means we already have a working reference.
> > If possible can you share the details, even 4.19 or higher will be
> > also a good reference.
> >
> > > > Or, another option is to use the new concept from 5.1 kernel that is:
> > > > dm-mod.create = ?
> > > How are you doing it today without dm-mod.create ?
> > I think in 4.14 we don't have dm-mod.create right ?
>
> No, but you can backport it easily. Back at
> http://lists.infradead.org/pipermail/openwrt-devel/2019-November/025967.html
> I provided backports of this feature to OpenWrt, for the 4.14 and 4.19
> kernels.
>
Yes, I can backport it to our 4.14 Kernel.
Can you share the list of patches to be backported to make it work on 4.14 ?
If it's backported also I need to report to our internal kernel, but
it might be slightly easier.
Please share the details.

> > Here is our kernel command line:
> >
> > [ 0.000000] Kernel command line: ro rootwait
> > console=ttyMSM0,115200,n8 .... verity="95384 11923
> > 16da5e4bbc706e5d90511d2a3dae373b5d878f9aebd522cd614a4faaace6baa3 12026
> > " rootfstype=squashfs ubi.mtd=40,0,30 ubi.block=0,0 root=/dev/dm-0
> > .... init=/sbin/init root=/dev/dm-0 dm="rootfs none ro,0 95384 verity
> > 1 /dev/ubiblock0_0 /dev/mtdblock53 4096 4096 11923 8 sha256
> > 16da5e4bbc706e5d90511d2a3dae373b5d878f9aebd522cd614a4faaace6baa3
> > aee087a5be3b982978c923f566a94613496b417f2af592639bc80d141e34dfe7 10
> > restart_on_corruption ignore_zero_blocks use_fec_from_device
> > /dev/mtdblock53 fec_roots 2 fec_blocks 12026 fec_start 12026" ...
>
> I don't see how this can work without the dm-mod.create feature. Are
> you sure the verity= and dm= kernel arguments exist?

Sorry, I am not a security guy and this was done by someone from the
security team.
But, I know that this is already working with ext4.
The moment we change to squashfs, it does not work.

The only difference with squashfs are:
=> verity metadata are kept on separate volume
=> The rootfstype and related stuff are different
=> verity command line related stuff are almost the same.

Also, you mentioned:
>>> Here, it definitely worked to append the hash tree to the squashfs
>>> image and store them in the same partition.
Can you share some details about it ?
How it can be done since squashfs is readonly.
Do, we need to change some parameters during squashfs image generation ?
{
$(STAGING_DIR_HOST)/bin/mksquashfs4 $(call mkfs_target_dir,$(1)) $@ \
- -nopad -noappend -root-owned \
+ -noappend -root-owned \
}

Also, for the above cmdline, is there any problem with the block size ?
As @Mikulas said before that the block size could be the issue

Also, for squashfs we are passing like this for root=. Is it fine ?
rootfstype=squashfs ubi.mtd=40,0,30 ubi.block=0,0 root=/dev/dm-0

I see that dm-0 is already passed elsewhere so do we really need it ?
I suspect it is not required as a block device.

Thanks,