From 7ee4109d7ec74f5ed2ae10330f9d0b69b0eeb1c2 Mon Sep 17 00:00:00 2001 From: John Dias Date: Mon, 10 Oct 2016 14:44:30 -0700 Subject: [PATCH] perf: protect group_leader from races that cause ctx double-free 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. 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. Bug: 30955111 Bug: 31095224 Change-Id: If37124c100ca6f4aa962559fba3bd5dbbec8e052 Git-repo: https://android.googlesource.com/kernel/msm Git-commit: 5b87e00be9ca28ea32cab49b92c0386e4a91f730 Signed-off-by: Dennis Cagle --- 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 12d3415a3ef5..9bd7e4b2663c 100644 --- a/include/linux/perf_event.h +++ b/include/linux/perf_event.h @@ -476,6 +476,12 @@ struct perf_event { int nr_siblings; int group_flags; 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; enum perf_event_active_state state; diff --git a/kernel/events/core.c b/kernel/events/core.c index 446dbad75e60..00beacd233fd 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -8066,6 +8066,7 @@ 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); @@ -8505,6 +8506,16 @@ 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)) { @@ -8766,6 +8777,8 @@ 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); @@ -8815,6 +8828,8 @@ err_task: 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);