[tip:perf/core] perf: Fix race in perf_event_exec()

From: tip-bot for Peter Zijlstra
Date: Wed Jan 06 2016 - 13:47:59 EST


Commit-ID: c127449944659543e5e2423002f08f0af98dba5c
Gitweb: http://git.kernel.org/tip/c127449944659543e5e2423002f08f0af98dba5c
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate: Thu, 10 Dec 2015 20:57:40 +0100
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Wed, 6 Jan 2016 10:52:38 +0100

perf: Fix race in perf_event_exec()

I managed to tickle this warning:

[ 2338.884942] ------------[ cut here ]------------
[ 2338.890112] WARNING: CPU: 13 PID: 35162 at ../kernel/events/core.c:2702 task_ctx_sched_out+0x6b/0x80()
[ 2338.900504] Modules linked in:
[ 2338.903933] CPU: 13 PID: 35162 Comm: bash Not tainted 4.4.0-rc4-dirty #244
[ 2338.911610] Hardware name: Intel Corporation S2600GZ/S2600GZ, BIOS SE5C600.86B.02.02.0002.122320131210 12/23/2013
[ 2338.923071] ffffffff81f1468e ffff8807c6457cb8 ffffffff815c680c 0000000000000000
[ 2338.931382] ffff8807c6457cf0 ffffffff810c8a56 ffffe8ffff8c1bd0 ffff8808132ed400
[ 2338.939678] 0000000000000286 ffff880813170380 ffff8808132ed400 ffff8807c6457d00
[ 2338.947987] Call Trace:
[ 2338.950726] [<ffffffff815c680c>] dump_stack+0x4e/0x82
[ 2338.956474] [<ffffffff810c8a56>] warn_slowpath_common+0x86/0xc0
[ 2338.963195] [<ffffffff810c8b4a>] warn_slowpath_null+0x1a/0x20
[ 2338.969720] [<ffffffff811a49cb>] task_ctx_sched_out+0x6b/0x80
[ 2338.976244] [<ffffffff811a62d2>] perf_event_exec+0xe2/0x180
[ 2338.982575] [<ffffffff8121fb6f>] setup_new_exec+0x6f/0x1b0
[ 2338.988810] [<ffffffff8126de83>] load_elf_binary+0x393/0x1660
[ 2338.995339] [<ffffffff811dc772>] ? get_user_pages+0x52/0x60
[ 2339.001669] [<ffffffff8121e297>] search_binary_handler+0x97/0x200
[ 2339.008581] [<ffffffff8121f8b3>] do_execveat_common.isra.33+0x543/0x6e0
[ 2339.016072] [<ffffffff8121fcea>] SyS_execve+0x3a/0x50
[ 2339.021819] [<ffffffff819fc165>] stub_execve+0x5/0x5
[ 2339.027469] [<ffffffff819fbeb2>] ? entry_SYSCALL_64_fastpath+0x12/0x71
[ 2339.034860] ---[ end trace ee1337c59a0ddeac ]---

Which is a WARN_ON_ONCE() indicating that cpuctx->task_ctx is not
what we expected it to be.

This is because context switches can swap the task_struct::perf_event_ctxp[]
pointer around. Therefore you have to either disable preemption when looking
at current, or hold ctx->lock.

Fix perf_event_enable_on_exec(), it loads current->perf_event_ctxp[]
before disabling interrupts, therefore a preemption in the right place
can swap contexts around and we're using the wrong one.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Alexander Potapenko <glider@xxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
Cc: Kostya Serebryany <kcc@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Sasha Levin <sasha.levin@xxxxxxxxxx>
Cc: Stephane Eranian <eranian@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Vince Weaver <vincent.weaver@xxxxxxxxx>
Cc: syzkaller <syzkaller@xxxxxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/20151210195740.GG6357@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
kernel/events/core.c | 15 +++++----------
1 file changed, 5 insertions(+), 10 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 39cf4a4..fd7de04 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3154,15 +3154,16 @@ static int event_enable_on_exec(struct perf_event *event,
* Enable all of a task's events that have been marked enable-on-exec.
* This expects task == current.
*/
-static void perf_event_enable_on_exec(struct perf_event_context *ctx)
+static void perf_event_enable_on_exec(int ctxn)
{
- struct perf_event_context *clone_ctx = NULL;
+ struct perf_event_context *ctx, *clone_ctx = NULL;
struct perf_event *event;
unsigned long flags;
int enabled = 0;
int ret;

local_irq_save(flags);
+ ctx = current->perf_event_ctxp[ctxn];
if (!ctx || !ctx->nr_events)
goto out;

@@ -3205,17 +3206,11 @@ out:

void perf_event_exec(void)
{
- struct perf_event_context *ctx;
int ctxn;

rcu_read_lock();
- for_each_task_context_nr(ctxn) {
- ctx = current->perf_event_ctxp[ctxn];
- if (!ctx)
- continue;
-
- perf_event_enable_on_exec(ctx);
- }
+ for_each_task_context_nr(ctxn)
+ perf_event_enable_on_exec(ctxn);
rcu_read_unlock();
}

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