Commit 39e07cb6 authored by Elena Reshetova's avatar Elena Reshetova Committed by Linus Torvalds

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@intel.comSigned-off-by: default avatarElena Reshetova <elena.reshetova@intel.com>
Suggested-by: default avatarKees Cook <keescook@chromium.org>
Reviewed-by: default avatarDavid Windsor <dwindsor@gmail.com>
Reviewed-by: default avatarHans Liljestrand <ishkamiel@gmail.com>
Reviewed-by: default avatarDmitry Vyukov <dvyukov@google.com>
Reviewed-by: default avatarAndrea Parri <andrea.parri@amarulasolutions.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent ec9672d5
......@@ -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_switch);
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, struct file *filep)
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);
......
Markdown is supported
0%
or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment