Messages in this thread Patch in this message | | | Date | Thu, 5 Jan 2017 15:14:29 -0800 | From | Kees Cook <> | Subject | [PATCH] perf: protect group_leader from races that cause ctx |
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From: John Dias <joaodias@google.com>
When moving a group_leader perf event from a software-context to a hardware-context, there's a race in checking and updating that context. The existing locking solution doesn't work; note that it tries to grab a lock inside the group_leader's context object, which you can only get at by going through a pointer that should be protected from these races. If two threads trigger this operation simultaneously, the refcount of 'perf_event_context' will fall to zero and the object may be freed.
To avoid that problem, and to produce a simple solution, we can just use a lock per group_leader to protect all checks on the group_leader's context. The new lock is grabbed and released when no context locks are held.
CVE-2016-6787
Reported-by: Di Shen (@returnsme) of KeenLab (@keen_lab), Tencent Fixes: b04243ef7006 ("perf: Complete software pmu grouping") Cc: stable@vger.kernel.org Signed-off-by: John Dias <joaodias@google.com> Signed-off-by: Kees Cook <keescook@chromium.org> --- include/linux/perf_event.h | 6 ++++++ kernel/events/core.c | 15 +++++++++++++++ 2 files changed, 21 insertions(+)
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h index 4741ecdb9817..a3c102ec5159 100644 --- a/include/linux/perf_event.h +++ b/include/linux/perf_event.h @@ -581,6 +581,12 @@ struct perf_event { int group_caps; struct perf_event *group_leader; + /* + * Protect the pmu, attributes, and context of a group leader. + * Note: does not protect the pointer to the group_leader. + */ + struct mutex group_leader_mutex; + struct pmu *pmu; void *pmu_private; diff --git a/kernel/events/core.c b/kernel/events/core.c index ab15509fab8c..853284604a7b 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -9101,6 +9101,7 @@ perf_event_alloc(struct perf_event_attr *attr, int cpu, if (!group_leader) group_leader = event; + mutex_init(&event->group_leader_mutex); mutex_init(&event->child_mutex); INIT_LIST_HEAD(&event->child_list); @@ -9580,6 +9581,16 @@ SYSCALL_DEFINE5(perf_event_open, group_leader = NULL; } + /* + * Take the group_leader's group_leader_mutex before observing + * anything in the group leader that leads to changes in ctx, + * many of which may be changing on another thread. + * In particular, we want to take this lock before deciding + * whether we need to move_group. + */ + if (group_leader) + mutex_lock(&group_leader->group_leader_mutex); + if (pid != -1 && !(flags & PERF_FLAG_PID_CGROUP)) { task = find_lively_task_by_vpid(pid); if (IS_ERR(task)) { @@ -9855,6 +9866,8 @@ SYSCALL_DEFINE5(perf_event_open, if (move_group) mutex_unlock(&gctx->mutex); mutex_unlock(&ctx->mutex); + if (group_leader) + mutex_unlock(&group_leader->group_leader_mutex); if (task) { mutex_unlock(&task->signal->cred_guard_mutex); @@ -9902,6 +9915,8 @@ SYSCALL_DEFINE5(perf_event_open, if (task) put_task_struct(task); err_group_fd: + if (group_leader) + mutex_unlock(&group_leader->group_leader_mutex); fdput(group); err_fd: put_unused_fd(event_fd); -- 2.7.4
-- Kees Cook Nexus Security
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