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. Suggested-by: Kees Cook <[email protected]> Reviewed-by: David Windsor <[email protected]> Reviewed-by: Hans Liljestrand <[email protected]> Signed-off-by: Elena Reshetova <[email protected]> --- kernel/kcov.c | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-) diff --git a/kernel/kcov.c b/kernel/kcov.c index 461c55e..03c174c 100644 --- a/kernel/kcov.c +++ b/kernel/kcov.c @@ -19,6 +19,7 @@ #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: */ @@ -43,7 +44,7 @@ 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; @@ -227,12 +228,12 @@ 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); } @@ -310,7 +311,7 @@ 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); -- 2.7.4
