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

Fan Wu <wufan@xxxxxxxxxxxxxxxxxxx> · Fri, 2 Feb 2024 19:56:53 -0800

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



