linux-kernel - Re: possible deadlock in perf_event_ctx_lock

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Date:   Fri, 27 Oct 2017 17:33:36 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     syzbot 
        <bot+f67ad450a4bd1e42a7bd09f592904b15be39db7a@...kaller.appspotmail.com>
Cc:     acme@...nel.org, alexander.shishkin@...ux.intel.com,
        linux-kernel@...r.kernel.org, mingo@...hat.com,
        syzkaller-bugs@...glegroups.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>, viro@...iv.linux.org.uk
Subject: Re: possible deadlock in perf_event_ctx_lock_nested

On Fri, Oct 27, 2017 at 05:11:37PM +0200, Peter Zijlstra wrote:
> On Fri, Oct 27, 2017 at 01:30:30AM -0700, syzbot wrote:
> 
> > ======================================================
> > WARNING: possible circular locking dependency detected
> > 4.13.0-next-20170911+ #19 Not tainted
> > ------------------------------------------------------
> > syz-executor2/12380 is trying to acquire lock:
> >  (&ctx->mutex){+.+.}, at: [<ffffffff8180923c>]
> > perf_event_ctx_lock_nested+0x1dc/0x3c0 kernel/events/core.c:1210
> > 
> > but task is already holding lock:
> >  (&pipe->mutex/1){+.+.}, at: [<ffffffff81ac0fa6>] pipe_lock_nested
> > fs/pipe.c:66 [inline]
> >  (&pipe->mutex/1){+.+.}, at: [<ffffffff81ac0fa6>] pipe_lock+0x56/0x70
> > fs/pipe.c:74
> > 
> > which lock already depends on the new lock.
> 
> 
> ARRGH!!
> 
> that translates like the below, which is an absolute maze and requires
> at least 5 concurrent callstacks, possibly more.
> 
> We already had a lot of fun with hotplug-perf-ftrace, but the below
> contains more. Let me try and page that previous crap back.
> 
> 
> 
> 	perf_ioctl()
> #0	  perf_event_ctx_lock()		[ctx->mutex]
> 	  perf_event_set_filter  
> #1	    ftrace_profile_set_filter   [event_mutex]
> 		
> 
> 
> 
> 	sys_perf_event_open
> 	  ...
> 	  perf_trace_init
> #1	    mutex_lock [event_mutex]
> 	    trace_event_reg
> 	      tracepoint_probe_register
> #2		mutex_lock() [tracepoints_mutex]
> 		tracepoint_add_func()
> #3		  static_key_slow_inc()	[cpuhotplug_lock]
> 	
> 
> 
> 
> 
> 	cpuhp_setup_state_nocalls
> #3	  cpus_read_lock	[cpuhotplug_lock]
> 	    __cpuhp_setup_state_cpuslocked
> #4	      mutex_lock	[cpuhp_state_mutex]
> 	      cpuhp_issue_call
> #5		cpuhp_invoke_ap_callback()	[cpuhp_state]
> 
> 
> #5	cpuhp_invoke_callback [cpuhp_state]
> 	  ...
> 	    devtmpfs_create_node
> #6	      wait_for_completion()	[&req.done]
> 
> 						devtmpfsd
> 						  handle_create
> #7						    filename_create [sb_writers]
> #6						  complete [&req.done]
> 							
> 
> 
> 
> 
> 
> 	do_splice
> #7	  file_start_write() [sb_writers]
> 	  do_splice_from
> 	    iter_file_splice_write
> #8	      pipe_lock [pipe->mutex]
> 
> 
> 
> 
> 
> 	do_splice
> #8	  pipe_lock	[pipe->mutex]
> 	  do_splice_to
> 	    ...
> #0	      perf_read() [ctx->mutex]
> 		

So arguably that last op, splice_read from a perf fd is fairly
pointless and we could dis-allow that. How about something like the
below?

---
 kernel/events/core.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 04989fb769f0..fd03f3082ee3 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5468,6 +5468,13 @@ static int perf_fasync(int fd, struct file *filp, int on)
 	return 0;
 }
 
+static ssize_t perf_splice_read(struct file *file, loff_t *ppos,
+				struct pipe_inode_info *pope, size_t len,
+				unsigned int flags)
+{
+	return -EOPNOTSUPP;
+}
+
 static const struct file_operations perf_fops = {
 	.llseek			= no_llseek,
 	.release		= perf_release,
@@ -5477,6 +5484,7 @@ static int perf_fasync(int fd, struct file *filp, int on)
 	.compat_ioctl		= perf_compat_ioctl,
 	.mmap			= perf_mmap,
 	.fasync			= perf_fasync,
+	.splice_read		= perf_splice_read,
 };
 
 /*