Re: [PATCH RFC 1/2] dt-bindings: mtd: ubi-volume: add 'volume-is-critical' property

[Daniel Golle]

On Sun, Sep 29, 2024 at 12:03:11PM +0800, Zhihao Cheng wrote:
> 在 2024/9/28 22:38, Daniel Golle 写道:
> > On Sat, Sep 28, 2024 at 03:45:49PM +0200, Krzysztof Kozlowski wrote:
> > > On 28/09/2024 15:09, Daniel Golle wrote:
> > > > On Sat, Sep 28, 2024 at 03:02:47PM +0200, Krzysztof Kozlowski wrote:
> > > > > On 28/09/2024 14:47, Daniel Golle wrote:
> > > > > > Add the 'volume-is-critical' boolean property which marks a UBI volume
> > > > > > as critical for the device to boot. If set it prevents the user from
> > > > > > all kinds of write access to the volume as well as from renaming it or
> > > > > > detaching the UBI device it is located on.
> > > > > > 
> > > > > > Signed-off-by: Daniel Golle <daniel@xxxxxxxxxxxxxx>
> > > > > > ---
> > > > > >   .../devicetree/bindings/mtd/partitions/ubi-volume.yaml   | 9 +++++++++
> > > > > >   1 file changed, 9 insertions(+)
> > > > > > 
> > > > > > diff --git a/Documentation/devicetree/bindings/mtd/partitions/ubi-volume.yaml b/Documentation/devicetree/bindings/mtd/partitions/ubi-volume.yaml
> > > > > > index 19736b26056b..2bd751bb7f9e 100644
> > > > > > --- a/Documentation/devicetree/bindings/mtd/partitions/ubi-volume.yaml
> > > > > > +++ b/Documentation/devicetree/bindings/mtd/partitions/ubi-volume.yaml
> > > > > > @@ -29,6 +29,15 @@ properties:
> > > > > >       description:
> > > > > >         This container may reference an NVMEM layout parser.
> > > > > > +  volume-is-critical:
> > > > > > +    description: This parameter, if present, indicates that the UBI volume
> > > > > > +      contains early-boot firmware images or data which should not be clobbered.
> > > > > > +      If set, it prevents the user from renaming the volume, writing to it or
> > > > > > +      making any changes affecting it, as well as detaching the UBI device it is
> > > > > > +      located on, so direct access to the underlying MTD device is prevented as
> > > > > > +      well.
> > > > > > +    type: boolean
> > > > > 
> > > > > UBI volumes are mapping to partitions 1-to-1, right? So rather I would
> > > > > propose to use partition.yaml - we already have read-only there with
> > > > > very similar description.
> > > > 
> > > > No, that's not the case.
> > > > 
> > > > An MTD partition can be used as UBI device. A UBI device (and hence MTD
> > > > partition) can host *several* UBI volumes.
> > > > 
> > > > Marking the MTD partition as 'read-only' won't work, as UBI needs
> > > > read-write access to perform bad block relocation, scrubbing, ...
> > > 
> > > OK, so not partition but read-only volume.
> > 
> > +1
> > 
> > > 
> > > > 
> > > > Also, typically not all UBI volumes on a UBI device are
> > > > read-only/critical but only a subset of them.
> > > > 
> > > > But you are right that the description is inspired by the description
> > > > of the 'read-only' property in partition.yaml ;)
> > > > 
> > > > I initially thought to also name the property 'read-only', just like
> > > > for MTD partitions. However, as the desired effect goes beyond
> > > > preventing write access to the volume itself, I thought it'd be
> > > > better to use a new name.
> > > 
> > > Yeah, maybe... critical indeed covers multiple cases but is also
> > > subjective. For some bootloader is critical, for other bootloader still
> > > might be fully A/B updateable thus could be modifiable. For others, they
> > > want to use fw_setenv from user-space so not critical at all.
> > 
> > The case I want to cover here is the bootloader itself being stored
> > inside a UBI volume. MediaTek's fork of ARM TrustedFirmware-A bl2 comes
> > with support for UBI and loads BL3 (which is TF-A BL31 and U-Boot, and
> > maybe OP-TEE as well) from a static UBI volume. Removing, renaming or
> > altering that volume results in the device not being able to boot any
> > more and requiring a complicated intervention (at attaching debugging
> > UART and using low-level recovery tool) in order to recover.
> 
> Who removes/renames the 'critical' volume? I suggest to fix it in the upper
> layer(not in kernel). After looking through the patch 2, it seems a hack
> solution.

The enemy is the user, the upper layer is between the keyboard and the
screen. Just like for 'read-only' MTD partitions I'm looking
for a similar solution for UBI which prevents the user from accidentally
deleting or destroying the bootloader, lets say, when logged in via SSH.