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Commit 9e4a31bc authored by John Dias's avatar John Dias Committed by Gerrit - the friendly Code Review server
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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
[dcagle@codeaurora.org: Slight merge conflict resolution]
Signed-off-by: default avatarDennis Cagle <d-cagle@codeaurora.org>
parent a7578cfb

include/linux/perf_event.h

+6 −0
Original line number Diff line number Diff line
@@ -584,6 +584,12 @@ 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;

kernel/events/core.c

+15 −0
Original line number Diff line number Diff line
@@ -9942,6 +9942,7 @@ 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);
@@ -10472,6 +10473,16 @@ 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)) {
@@ -10773,6 +10784,8 @@ SYSCALL_DEFINE5(perf_event_open, 
	if (move_group)

		perf_event_ctx_unlock(group_leader, gctx);

	mutex_unlock(&ctx->mutex);

	if (group_leader)

		mutex_unlock(&group_leader->group_leader_mutex);



	if (task) {

		mutex_unlock(&task->signal->cred_guard_mutex);
@@ -10816,6 +10829,8 @@ 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);