The patch titled
Subject: kcov: convert kcov.refcount to refcount_t
has been added to the -mm tree. Its filename is
kcov-convert-kcovrefcount-to-refcount_t.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/kcov-convert-kcovrefcount-to-refcount_t.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/kcov-convert-kcovrefcount-to-refcount_t.patch
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------------------------------------------------------
From: Elena Reshetova <elena.reshetova@xxxxxxxxx>
Subject: kcov: convert kcov.refcount to refcount_t
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 kcov.refcount is used as pure reference counter. Convert it
to refcount_t and fix up the operations.
**Important note for maintainers:
Some functions from refcount_t API defined in lib/refcount.c have
different memory ordering guarantees than their atomic counterparts.
The full comparison can be seen in https://lkml.org/lkml/2017/11/15/57 and
it is hopefully soon in state to be merged to the documentation tree.
Normally the differences should not matter since refcount_t provides
enough guarantees to satisfy the refcounting use cases, but in some rare
cases it might matter. Please double check that you don't have some
undocumented memory guarantees for this variable usage.
For the kcov.refcount it might make a difference
in following places:
- kcov_put(): decrement in refcount_dec_and_test() only
provides RELEASE ordering and control dependency on success
vs. fully ordered atomic counterpart
Link: http://lkml.kernel.org/r/1547634429-772-1-git-send-email-elena.reshetova@xxxxxxxxx
Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
Suggested-by: Kees Cook <keescook@xxxxxxxxxxxx>
Reviewed-by: David Windsor <dwindsor@xxxxxxxxx>
Reviewed-by: Hans Liljestrand <ishkamiel@xxxxxxxxx>
Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Reviewed-by: Andrea Parri <andrea.parri@xxxxxxxxxxxxxxxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
--- a/kernel/kcov.c~kcov-convert-kcovrefcount-to-refcount_t
+++ a/kernel/kcov.c
@@ -20,6 +20,7 @@
#include <linux/debugfs.h>
#include <linux/uaccess.h>
#include <linux/kcov.h>
+#include <linux/refcount.h>
#include <asm/setup.h>
/* Number of 64-bit words written per one comparison: */
@@ -44,7 +45,7 @@ struct kcov {
* - opened file descriptor
* - task with enabled coverage (we can't unwire it from another task)
*/
- atomic_t refcount;
+ refcount_t refcount;
/* The lock protects mode, size, area and t. */
spinlock_t lock;
enum kcov_mode mode;
@@ -228,12 +229,12 @@ EXPORT_SYMBOL(__sanitizer_cov_trace_swit
static void kcov_get(struct kcov *kcov)
{
- atomic_inc(&kcov->refcount);
+ refcount_inc(&kcov->refcount);
}
static void kcov_put(struct kcov *kcov)
{
- if (atomic_dec_and_test(&kcov->refcount)) {
+ if (refcount_dec_and_test(&kcov->refcount)) {
vfree(kcov->area);
kfree(kcov);
}
@@ -312,7 +313,7 @@ static int kcov_open(struct inode *inode
if (!kcov)
return -ENOMEM;
kcov->mode = KCOV_MODE_DISABLED;
- atomic_set(&kcov->refcount, 1);
+ refcount_set(&kcov->refcount, 1);
spin_lock_init(&kcov->lock);
filep->private_data = kcov;
return nonseekable_open(inode, filep);
_
Patches currently in -mm which might be from elena.reshetova@xxxxxxxxx are
kcov-convert-kcovrefcount-to-refcount_t.patch
