+ kcov-convert-kcovrefcount-to-refcount_t.patch added to -mm tree

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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




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