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 perf_event_context.refcount is used as pure reference counter. Convert it to refcount_t and fix up the operations. Suggested-by: Kees Cook <keesc...@chromium.org> Reviewed-by: David Windsor <dwind...@gmail.com> Reviewed-by: Hans Liljestrand <ishkam...@gmail.com> Signed-off-by: Elena Reshetova <elena.reshet...@intel.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 79b18a2..ab7f452 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 { @@ -735,7 +736,7 @@ 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 74671e1..7272b47 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)); + WARN_ON(!refcount_inc_not_zero(&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; } @@ -1334,7 +1334,7 @@ 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 { @@ -3813,7 +3813,7 @@ 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 * @@ -9895,7 +9895,7 @@ __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