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perf: protect group_leader from races that cause ctx double-free

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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: Dennis Cagle <d-cagle@codeaurora.org>
This commit is contained in:
John Dias 2016-10-10 14:44:30 -07:00 committed by Dennis Cagle
parent cb57e6b443
commit 1c046f8869
2 changed files with 21 additions and 0 deletions
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6
include/linux/perf_event.h
View File

@ -579,6 +579,12 @@ 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;

15
kernel/events/core.c
View File

@ -9785,6 +9785,7 @@ 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);
@ -10338,6 +10339,16 @@ 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)) {
@ -10636,6 +10647,8 @@ 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);
@ -10689,6 +10702,8 @@ 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);