Dne 20.10.2017 v 09:37 Elena Reshetova napsal(a):
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 <keescook@xxxxxxxxxxxx>
Reviewed-by: David Windsor <dwindsor@xxxxxxxxx>
Reviewed-by: Hans Liljestrand <ishkamiel@xxxxxxxxx>
Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
---
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,
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)
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
*result = &td->dm_dev;
@@ -701,7 +703,7 @@ void dm_put_table_device(struct mapped_device *md, struct dm_dev *d)
struct table_device *td = container_of(d, struct table_device, dm_dev);
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);
DMWARN("dm_destroy: %s still exists with %d references",
- td->dm_dev.name, atomic_read(&td->count));
+ td->dm_dev.name, refcount_read(&td->count));
kfree(td);
}
}
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