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SubjectRe: [PATCH 4/4] dm: convert table_device.count from atomic_t to refcount_t
From
Date
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@chromium.org>
> Reviewed-by: David Windsor <dwindsor@gmail.com>
> Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
> 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,
>
> 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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