Re: perf: use after free in perf_remove_from_context

[Sasha Levin]

On 05/29/2014 03:57 AM, Peter Zijlstra wrote:
> On Wed, May 28, 2014 at 07:52:07PM -0400, Sasha Levin wrote:
>> On 05/14/2014 12:35 PM, Peter Zijlstra wrote:
>>> On Wed, May 14, 2014 at 12:32:26PM -0400, Sasha Levin wrote:
>>>>> -----BEGIN PGP SIGNED MESSAGE-----
>>>>> Hash: SHA1
>>>>>
>>>>> On 05/14/2014 12:29 PM, Peter Zijlstra wrote:
>>>>>>> On Mon, May 12, 2014 at 11:42:33AM -0400, Sasha Levin wrote:
>>>>>>>>> Hi all,
>>>>>>>>>
>>>>>>>>> While fuzzing with trinity inside a KVM tools guest running the latest -next kernel I've stumbled on the following spew. Maybe related to the very recent change in freeing on task exit?
>>>>>>>>>
>>>>>>>>> [ 2509.827261] general protection fault: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC [ 2509.830379] Dumping ftrace buffer: [ 2509.830379]    (ftrace buffer empty) [ 2509.830379] Modules linked in: [ 2509.830379] CPU: 47 PID: 43306 Comm: trinity-c126 Tainted: G        W     3.15.0-rc5-next-20140512-sasha-00019-ga20bc00-dirty #456
>>>>>>>
>>>>>>> Any particular trinity setup? And would you happen to have the seed of that run?
>>>>>
>>>>> Nothing special about trinity options. 400 threads and blacklisting some of the
>>>>> destructive syscalls (umount, reboot, etc).
>>>>>
>>>>> I don't have the seed, but that problem did reproduce again tonight so I can test
>>>>> out debug code if you have something in mind.
>>> Nah, I drew a pretty big blank, which is why I wanted to see if I could
>>> reproduce. If you could share your trinity cmdline I'd be much obliged.
>>> While I did manage to clone (the repo moved since last time) and build
>>> it, I'm not really that handy with it and want to avoid destroying my
>>> machine if possible ;-)
>>
>> Anything I could do to help out with this? It reproduces pretty easily on my
>> configuration so I'd be happy to test out whatever might help.
> 
> Yeah, it takes me days to test anything, and my last guess panned out to
> nothing, at which point I decided I needed to look at the things I'd
> neglected for a bit :/
> 
> Could you see if the below makes any difference? I'll try and get back
> to tracking this.
> 
> ---
>  kernel/events/core.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 9efb1e7858ac..851dc9dc5643 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -7497,8 +7497,10 @@ static void perf_event_exit_task_context(struct task_struct *child, int ctxn)
>  	 */
>  	mutex_lock(&child_ctx->mutex);
>  
> +	rcu_read_lock();
>  	list_for_each_entry_rcu(child_event, &child_ctx->event_list, event_entry)
>  		__perf_event_exit_task(child_event, child_ctx, child);
> +	rcu_read_unlock();
>  
>  	mutex_unlock(&child_ctx->mutex);
>  
> 

It doesn't work out well because we later lock a mutex in sync_child_event().


Thanks,
Sasha
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