Subject: Re: [FIX PATCH v1] powerpc/pseries: Fix reference count leak during CPU unplug

From: Bharata B Rao <[email-masked]>
Date: Mon, 13 Mar 2017 15:59:10 +0530

On Thu, Mar 09, 2017 at 01:34:00PM -0800, Tyrel Datwyler wrote:
> On 03/08/2017 08:37 PM, Bharata B Rao wrote:
> > The following warning is seen when a CPU is hot unplugged on a PowerKVM
> > guest:
> 
> Is this the case with cpus present at boot? What about cpus hotplugged
> after boot?

I have observed this for CPUs that are hotplugged.

> 
> My suspicion is that the refcount was wrong to begin with. See my
> comments below. The use of the of_node_put() calls is correct as in each
> case we incremented the ref count earlier in the same function.
> 
> > 
> > refcount_t: underflow; use-after-free.
> > ------------[ cut here ]------------
> > WARNING: CPU: 0 PID: 53 at lib/refcount.c:128 refcount_sub_and_test+0xd8/0xf0
> > Modules linked in:
> > CPU: 0 PID: 53 Comm: kworker/u510:1 Not tainted 4.11.0-rc1 #3
> > Workqueue: pseries hotplug workque pseries_hp_work_fn
> > task: c0000000fb475000 task.stack: c0000000fb81c000
> > NIP: c0000000006f0808 LR: c0000000006f0804 CTR: c0000000007b98c0
> > REGS: c0000000fb81f710 TRAP: 0700   Not tainted  (4.11.0-rc1)
> > MSR: 800000000282b033 <SF,VEC,VSX,EE,FP,ME,IR,DR,RI,LE>
> >   CR: 48002222  XER: 20000000
> > CFAR: c000000000c438e0 SOFTE: 1
> > GPR00: c0000000006f0804 c0000000fb81f990 c000000001573b00 0000000000000026
> > GPR04: 0000000000000000 000000000000016c 667265652e0d0a73 652d61667465722d
> > GPR08: 0000000000000007 0000000000000007 0000000000000001 0000000000000006
> > GPR12: 0000000000002200 c00000000ff40000 c00000000010c578 c0000001f11b9f40
> > GPR16: c0000001fe0312a8 c0000001fe031078 c0000001fe031020 0000000000000001
> > GPR20: 0000000000000000 0000000000000000 c000000001454808 fffffffffffffef7
> > GPR24: 0000000000000000 c0000001f1677648 0000000000000000 0000000000000000
> > GPR28: 0000000010000008 c000000000e4d3d8 0000000000000000 c0000001eaae07d8
> > NIP [c0000000006f0808] refcount_sub_and_test+0xd8/0xf0
> > LR [c0000000006f0804] refcount_sub_and_test+0xd4/0xf0
> > Call Trace:
> > [c0000000fb81f990] [c0000000006f0804] refcount_sub_and_test+0xd4/0xf0 (unreliable)
> > [c0000000fb81f9f0] [c0000000006d04b4] kobject_put+0x44/0x2a0
> > [c0000000fb81fa70] [c0000000009d5284] of_node_put+0x34/0x50
> > [c0000000fb81faa0] [c0000000000aceb8] dlpar_cpu_remove_by_index+0x108/0x130
> > [c0000000fb81fb30] [c0000000000ae128] dlpar_cpu+0x78/0x550
> > [c0000000fb81fbe0] [c0000000000a7b40] handle_dlpar_errorlog+0xc0/0x160
> > [c0000000fb81fc50] [c0000000000a7c74] pseries_hp_work_fn+0x94/0xa0
> > [c0000000fb81fc80] [c000000000102cec] process_one_work+0x23c/0x540
> > [c0000000fb81fd20] [c00000000010309c] worker_thread+0xac/0x620
> > [c0000000fb81fdc0] [c00000000010c6c4] kthread+0x154/0x1a0
> > [c0000000fb81fe30] [c00000000000bbe0] ret_from_kernel_thread+0x5c/0x7c
> > 
> > Fix this by ensuring that of_node_put() is called only from the
> > error path in dlpar_cpu_remove_by_index(). In the normal path,
> > of_node_put() happens as part of dlpar_detach_node().
> > 
> > Signed-off-by: Bharata B Rao <[email-masked]>
> > Cc: Nathan Fontenot <[email-masked]>
> > ---
> > Changes in v1:
> > - Fixed the refcount problem in the userspace driven unplug path
> >   in addition to in-kernel unplug path. (Sachin Sant)
> > 
> > v0: https://patchwork.ozlabs.org/patch/736547/
> > 
> >  arch/powerpc/platforms/pseries/hotplug-cpu.c | 12 ++++++++----
> >  1 file changed, 8 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c
> > index 7bc0e91..c5ed510 100644
> > --- a/arch/powerpc/platforms/pseries/hotplug-cpu.c
> > +++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c
> > @@ -619,7 +619,8 @@ static int dlpar_cpu_remove_by_index(u32 drc_index)
> >  	}
> > 
> >  	rc = dlpar_cpu_remove(dn, drc_index);
> > -	of_node_put(dn);
> > +	if (rc)
> > +		of_node_put(dn);
> 
> I think there is another issue at play here because this is wrong.
> Regardless of whether the dlpar_cpu_remove() succeeds or fails we still
> need of_node_put() for both cases because we incremented the ref count
> earlier in this function with a call to cpu_drc_index_to_dn() call. That
> call doesn't, but shoul, document that it returns a device_node with
> incremented refcount.
> 
> >  	return rc;
> >  }
> > 
> > @@ -856,9 +857,12 @@ static ssize_t dlpar_cpu_release(const char *buf, size_t count)
> >  	}
> > 
> >  	rc = dlpar_cpu_remove(dn, drc_index);
> > -	of_node_put(dn);
> > -
> > -	return rc ? rc : count;
> > +	if (rc) {
> > +		of_node_put(dn);
> > +		return rc;
> > +	} else {
> > +		return count;
> > +	}
> 
> Same comment as above. The call earlier in the function to
> of_find_node_by_path() returned a device_node struct with its ref count
> incremented. So, regardless of whether dlpar_cpu_remove() succeeds or
> fails we need decrement the ref count with of_node_put().
> 
> Looking closer at the call paths for attach and detach one will notice
> that __of_attach_node_sysfs() does not take a device_node reference with
> of_node_get(), but __of_detach_node_sysfs() does a of_node_put(). In the
> old days we use to keep the device tree in /proc. Now it lives in sysfs
> and is symlinked to /proc for userspace ABI reasons. Further, pseries
> was the only platform in those days that did any sort of dynamic OF
> operations. So, in those dark days we were responsible for calling
> of_node_put in dlpar_detach_node() to decrement the of_node_init()
> reference. Looking at the comments in __of_detach_node_sysfs() it seems
> that they expect to decrement that reference there now.
> 
> void __of_detach_node_sysfs(struct device_node *np)
> {
> 
> ...snip...
> 
>         /* finally remove the kobj_init ref */
>         of_node_put(np);
> }
> 

So you suggest that adding of_node_get() to __of_attach_node_sysfs()
is the right fix ?

Regards,
Bharata.