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 ring_buffer.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 ring_buffer.refcount it might make a difference in following places: - ring_buffer_get(): increment in refcount_inc_not_zero() only guarantees control dependency on success vs. fully ordered atomic counterpart - ring_buffer_put(): decrement in refcount_dec_and_test() only provides RELEASE ordering and control dependency on success vs. fully ordered atomic counterpart Suggested-by: Kees Cook <keescook@xxxxxxxxxxxx> Reviewed-by: David Windsor <dwindsor@xxxxxxxxx> Reviewed-by: Hans Liljestrand <ishkamiel@xxxxxxxxx> Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx> --- kernel/events/core.c | 4 ++-- kernel/events/internal.h | 3 ++- kernel/events/ring_buffer.c | 2 +- 3 files changed, 5 insertions(+), 4 deletions(-) diff --git a/kernel/events/core.c b/kernel/events/core.c index 29c381f..3497c6a 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -5020,7 +5020,7 @@ struct ring_buffer *ring_buffer_get(struct perf_event *event) rcu_read_lock(); rb = rcu_dereference(event->rb); if (rb) { - if (!atomic_inc_not_zero(&rb->refcount)) + if (!refcount_inc_not_zero(&rb->refcount)) rb = NULL; } rcu_read_unlock(); @@ -5030,7 +5030,7 @@ struct ring_buffer *ring_buffer_get(struct perf_event *event) void ring_buffer_put(struct ring_buffer *rb) { - if (!atomic_dec_and_test(&rb->refcount)) + if (!refcount_dec_and_test(&rb->refcount)) return; WARN_ON_ONCE(!list_empty(&rb->event_list)); diff --git a/kernel/events/internal.h b/kernel/events/internal.h index 09b1537..86c5c7f 100644 --- a/kernel/events/internal.h +++ b/kernel/events/internal.h @@ -4,13 +4,14 @@ #include <linux/hardirq.h> #include <linux/uaccess.h> +#include <linux/refcount.h> /* Buffer handling */ #define RING_BUFFER_WRITABLE 0x01 struct ring_buffer { - atomic_t refcount; + refcount_t refcount; struct rcu_head rcu_head; #ifdef CONFIG_PERF_USE_VMALLOC struct work_struct work; diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c index 141aa2c..de12d36 100644 --- a/kernel/events/ring_buffer.c +++ b/kernel/events/ring_buffer.c @@ -284,7 +284,7 @@ ring_buffer_init(struct ring_buffer *rb, long watermark, int flags) else rb->overwrite = 1; - atomic_set(&rb->refcount, 1); + refcount_set(&rb->refcount, 1); INIT_LIST_HEAD(&rb->event_list); spin_lock_init(&rb->event_lock); -- 2.7.4 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
