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

[Tyrel Datwyler]

On 03/13/2017 03:29 AM, Bharata B Rao wrote:
> 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.

If removing a cpu present at boot works, but removing one that has been
hotplugged after boot reproduces the problem it is more likely the case
that we failed to take a reference during hotplug or released a
reference we shouldn't have. I'd have to go look at the hot add path.

> 
>>
>> 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 <bharata at linux.vnet.ibm.com>
>>> Cc: Nathan Fontenot <nfont at linux.vnet.ibm.com>
>>> ---
>>> 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 ?

If I understand that this only creates for hot-added cpus then no. Also
for this to be the correct fix I would expect to see this recreate for
all hot-remove operations such as memory and pci devices as well.

-Tyrel

> 
> Regards,
> Bharata.
>