[PATCH 06/15] kernel: convert perf_event_context.refcount from atomic_t to refcount_t

[Elena Reshetova]

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <elena.reshet...@intel.com>
Signed-off-by: Hans Liljestrand <ishkam...@gmail.com>
Signed-off-by: Kees Cook <keesc...@chromium.org>
Signed-off-by: David Windsor <dwind...@gmail.com>
---
 include/linux/perf_event.h |  3 ++-
 kernel/events/core.c       | 12 ++++++------
 2 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index a3b873f..f7a9802 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -54,6 +54,7 @@ struct perf_guest_info_callbacks {
 #include <linux/perf_regs.h>
 #include <linux/workqueue.h>
 #include <linux/cgroup.h>
+#include <linux/refcount.h>
 #include <asm/local.h>
 
 struct perf_callchain_entry {
@@ -750,7 +751,7 @@ struct perf_event_context {
        int                             nr_stat;
        int                             nr_freq;
        int                             rotate_disable;
-       atomic_t                        refcount;
+       refcount_t                      refcount;
        struct task_struct              *task;
 
        /*
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 1538df9..11d051f 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -1109,7 +1109,7 @@ static void perf_event_ctx_deactivate(struct 
perf_event_context *ctx)
 
 static void get_ctx(struct perf_event_context *ctx)
 {
-       WARN_ON(!atomic_inc_not_zero(&ctx->refcount));
+       refcount_inc(&ctx->refcount);
 }
 
 static void free_ctx(struct rcu_head *head)
@@ -1123,7 +1123,7 @@ static void free_ctx(struct rcu_head *head)
 
 static void put_ctx(struct perf_event_context *ctx)
 {
-       if (atomic_dec_and_test(&ctx->refcount)) {
+       if (refcount_dec_and_test(&ctx->refcount)) {
                if (ctx->parent_ctx)
                        put_ctx(ctx->parent_ctx);
                if (ctx->task && ctx->task != TASK_TOMBSTONE)
@@ -1201,7 +1201,7 @@ perf_event_ctx_lock_nested(struct perf_event *event, int 
nesting)
 again:
        rcu_read_lock();
        ctx = ACCESS_ONCE(event->ctx);
-       if (!atomic_inc_not_zero(&ctx->refcount)) {
+       if (!refcount_inc_not_zero(&ctx->refcount)) {
                rcu_read_unlock();
                goto again;
        }
@@ -1329,7 +1329,7 @@ perf_lock_task_context(struct task_struct *task, int 
ctxn, unsigned long *flags)
                }
 
                if (ctx->task == TASK_TOMBSTONE ||
-                   !atomic_inc_not_zero(&ctx->refcount)) {
+                   !refcount_inc_not_zero(&ctx->refcount)) {
                        raw_spin_unlock(&ctx->lock);
                        ctx = NULL;
                } else {
@@ -3760,7 +3760,7 @@ static void __perf_event_init_context(struct 
perf_event_context *ctx)
        INIT_LIST_HEAD(&ctx->pinned_groups);
        INIT_LIST_HEAD(&ctx->flexible_groups);
        INIT_LIST_HEAD(&ctx->event_list);
-       atomic_set(&ctx->refcount, 1);
+       refcount_set(&ctx->refcount, 1);
 }
 
 static struct perf_event_context *
@@ -9791,7 +9791,7 @@ __perf_event_ctx_lock_double(struct perf_event 
*group_leader,
 again:
        rcu_read_lock();
        gctx = READ_ONCE(group_leader->ctx);
-       if (!atomic_inc_not_zero(&gctx->refcount)) {
+       if (!refcount_inc_not_zero(&gctx->refcount)) {
                rcu_read_unlock();
                goto again;
        }
-- 
2.7.4