Dne 20.10.2017 v 09:37 Elena Reshetova napsal(a):

quoted

atomic_t variables are currently used to implement reference
counters with the following properties:
  - counter is initialized to 1 using atomic_set()
  - a resource is freed upon counter reaching zero
  - once counter reaches zero, its further
    increments aren't allowed
  - counter schema uses basic atomic operations
    (set, inc, inc_not_zero, dec_and_test, etc.)

Such atomic variables should be converted to a newly provided
refcount_t type and API that prevents accidental counter overflows
and underflows. This is important since overflows and underflows
can lead to use-after-free situation and be exploitable.

The variable table_device.count is used as pure reference counter.
Convert it to refcount_t and fix up the operations.

Suggested-by: Kees Cook <redacted>
Reviewed-by: David Windsor <redacted>
Reviewed-by: Hans Liljestrand <redacted>
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
---
  drivers/md/dm.c | 12 +++++++-----
  1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 4be8532..be12f3f 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c

@@ -24,6 +24,7 @@
  #include <linux/delay.h>
  #include <linux/wait.h>
  #include <linux/pr.h>
+#include <linux/refcount.h>

  #define DM_MSG_PREFIX "core"

@@ -98,7 +99,7 @@ struct dm_md_mempools {

  struct table_device {
  	struct list_head list;
-	atomic_t count;
+	refcount_t count;
  	struct dm_dev dm_dev;
  };

@@ -685,10 +686,11 @@ int dm_get_table_device(struct mapped_device *md,

dev_t dev, fmode_t mode,

quoted

  		format_dev_t(td->dm_dev.name, dev);

-		atomic_set(&td->count, 0);
+		refcount_set(&td->count, 1);
  		list_add(&td->list, &md->table_devices);
+	} else {
+		refcount_inc(&td->count);
  	}
-	atomic_inc(&td->count);
  	mutex_unlock(&md->table_devices_lock);

NACK

This patch (2a0b4682e09d76466f7b8f5e347ae2ff02f033af) currently breaks
accounting of opened devices.

I.e.   multisegment device  (target with 3 segments is not properly accounted)

Could you please explain what exactly happens (i.e. missing/wrong increment?)
or provide a error dump?
By looking at the code, I don't see where the change in the reference counting
could have caused this. 

Best Regards,
Elena.

Patch needs reworking and users of 'dm' and 4.15-rc0 kernel should rather
switch back to 4.14 ATM as it's unclear which other parts can be affected.

Zdenek

quoted

  	*result = &td->dm_dev;

@@ -701,7 +703,7 @@ void dm_put_table_device(struct mapped_device *md,

struct dm_dev *d)

quoted

  	struct table_device *td = container_of(d, struct table_device,

dm_dev);

quoted

  	mutex_lock(&md->table_devices_lock);
-	if (atomic_dec_and_test(&td->count)) {
+	if (refcount_dec_and_test(&td->count)) {
  		close_table_device(td, md);
  		list_del(&td->list);
  		kfree(td);

@@ -718,7 +720,7 @@ static void free_table_devices(struct list_head *devices)
  		struct table_device *td = list_entry(tmp, struct

table_device, list);

quoted

  		DMWARN("dm_destroy: %s still exists with %d

references",

quoted

-		       td->dm_dev.name, atomic_read(&td->count));
+		       td->dm_dev.name, refcount_read(&td->count));
  		kfree(td);
  	}
  }