Re: [RFC PATCH v12 12/20] dm verity: set DM_TARGET_SINGLETON feature flag

[Fan Wu]

On 2/2/2024 10:51 AM, Mike Snitzer wrote:
> On Tue, Jan 30 2024 at  5:37P -0500,
> Fan Wu <wufan@xxxxxxxxxxxxxxxxxxx> wrote:
>
> > The device-mapper has a flag to mark targets as singleton, which is a
> > required flag for immutable targets. Without this flag, multiple
> > dm-verity targets can be added to a mapped device, which has no
> > practical use cases and will let dm_table_get_immutable_target return
> > NULL. This patch adds the missing flag, restricting only one
> > dm-verity target per mapped device.
> >
> > Signed-off-by: Fan Wu <wufan@xxxxxxxxxxxxxxxxxxx>
> >
> > ---
> > v1-v10:
> >    + Not present
> >
> > v11:
> >    + Introduced
> >
> > v12:
> >    + No changes
> > ---
> >   drivers/md/dm-verity-target.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/md/dm-verity-target.c b/drivers/md/dm-verity-target.c
> > index 14e58ae70521..66a850c02be4 100644
> > --- a/drivers/md/dm-verity-target.c
> > +++ b/drivers/md/dm-verity-target.c
> > @@ -1507,7 +1507,7 @@ int dm_verity_get_root_digest(struct dm_target *ti, u8 **root_digest, unsigned i
> >   
> >   static struct target_type verity_target = {
> >   	.name		= "verity",
> > -	.features	= DM_TARGET_IMMUTABLE,
> > +	.features	= DM_TARGET_SINGLETON | DM_TARGET_IMMUTABLE,
> >   	.version	= {1, 9, 0},
> >   	.module		= THIS_MODULE,
> >   	.ctr		= verity_ctr,
> > --
> > 2.43.0
>
> It is true this change will cause dm_table_get_immutable_target() to
> not return NULL, but: I'm curious how that is meaningful in the
> context of dm-verity? (given the only caller of
> dm_table_get_immutable_target() is request-based DM code in DM core.)
>
> Thanks,
> Mike

Sorry for the confusion. The reference of 
dm_table_get_immutable_target() is only to justify an immutable target 
should also be a 
singleton(https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/drivers/md/dm-table.c#n982). 
It is not directly related to dm-verity.

In the context of dm-verity. I found although veritysetup does ensure 
the dm-verity target as a singleton, users can still use dmsetup to 
configure multiple dm-verity targets within a single map table. This 
leads to a situation where only the first target can be accessed. 
Therefore to prevent this and similar misuse, I propose introducing 
DM_TARGET_SINGLETON to allow the kernel to enforce dm-verity targets as 
singletons.

Thanks,
Fan