Re: [PATCH v2] mm: shrinker: avoid memleak in alloc_shrinker_info

[Muchun Song <muchun.song@linux.dev>]

> On Oct 16, 2024, at 19:43, Vlastimil Babka <vbabka@suse.cz> wrote:
> On 10/16/24 04:21, Muchun Song wrote:
>> 
>> 
>>> On Oct 16, 2024, at 09:25, chenridong <chenridong@huawei.com> wrote:
>>> On 2024/10/15 14:55, Anshuman Khandual wrote:
>>>> On 10/14/24 16:59, Kirill A. Shutemov wrote:
>>>>> On Mon, Oct 14, 2024 at 03:23:36AM +0000, Chen Ridong wrote:
>>>>>> From: Chen Ridong <chenridong@huawei.com>
>>>>>> A memleak was found as bellow:
>>>>>> unreferenced object 0xffff8881010d2a80 (size 32):
>>>>>>  comm "mkdir", pid 1559, jiffies 4294932666
>>>>>>  hex dump (first 32 bytes):
>>>>>>    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>>>>>>    40 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  @...............
>>>>>>  backtrace (crc 2e7ef6fa):
>>>>>>    [<ffffffff81372754>] __kmalloc_node_noprof+0x394/0x470
>>>>>>    [<ffffffff813024ab>] alloc_shrinker_info+0x7b/0x1a0
>>>>>>    [<ffffffff813b526a>] mem_cgroup_css_online+0x11a/0x3b0
>>>>>>    [<ffffffff81198dd9>] online_css+0x29/0xa0
>>>>>>    [<ffffffff811a243d>] cgroup_apply_control_enable+0x20d/0x360
>>>>>>    [<ffffffff811a5728>] cgroup_mkdir+0x168/0x5f0
>>>>>>    [<ffffffff8148543e>] kernfs_iop_mkdir+0x5e/0x90
>>>>>>    [<ffffffff813dbb24>] vfs_mkdir+0x144/0x220
>>>>>>    [<ffffffff813e1c97>] do_mkdirat+0x87/0x130
>>>>>>    [<ffffffff813e1de9>] __x64_sys_mkdir+0x49/0x70
>>>>>>    [<ffffffff81f8c928>] do_syscall_64+0x68/0x140
>>>>>>    [<ffffffff8200012f>] entry_SYSCALL_64_after_hwframe+0x76/0x7e
>>>>>> In the alloc_shrinker_info function, when shrinker_unit_alloc return
>>>>>> err, the info won't be freed. Just fix it.
>>>>>> Fixes: 307bececcd12 ("mm: shrinker: add a secondary array for shrinker_info::{map, nr_deferred}")
>>>>>> Signed-off-by: Chen Ridong <chenridong@huawei.com>
>>>>>> ---
>>>>>> mm/shrinker.c | 1 +
>>>>>> 1 file changed, 1 insertion(+)
>>>>>> diff --git a/mm/shrinker.c b/mm/shrinker.c
>>>>>> index dc5d2a6fcfc4..92270413190d 100644
>>>>>> --- a/mm/shrinker.c
>>>>>> +++ b/mm/shrinker.c
>>>>>> @@ -97,6 +97,7 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
>>>>>>   err:
>>>>>>  mutex_unlock(&shrinker_mutex);
>>>>>> + kvfree(info);
>>>>>>  free_shrinker_info(memcg);
>>>>>>  return -ENOMEM;
>>>>>> }
>>>>> NAK. If in the future there going to one more error case after
>>>>> rcu_assign_pointer() we will end up with double free.
>>>>> This should be safer:
>>>>> diff --git a/mm/shrinker.c b/mm/shrinker.c
>>>>> index dc5d2a6fcfc4..763fd556bc7d 100644
>>>>> --- a/mm/shrinker.c
>>>>> +++ b/mm/shrinker.c
>>>>> @@ -87,8 +87,10 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
>>>>>  if (!info)
>>>>>  goto err;
>>>>>  info->map_nr_max = shrinker_nr_max;
>>>>> - if (shrinker_unit_alloc(info, NULL, nid))
>>>>> + if (shrinker_unit_alloc(info, NULL, nid)) {
>>>>> + kvfree(info);
>>>>>  goto err;
>>>>> + }
>>>>>  rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, info);
>>>>>  }
>>>>>  mutex_unlock(&shrinker_mutex);
>>>> Agreed, this is what I mentioned earlier as well.
>>>> ------------------------------------------------------------------
>>>> I guess kvfree() should be called just after shrinker_unit_alloc()
>>>> fails but before calling into "goto err"
>>>> ------------------------------------------------------------------
>>> After discussion, it seems that v1 is acceptable.
>>> Hi, Muchun, do you have any other opinions?
>> 
>> I insist on my opinion, not mixing two different approaches
>> to do release resources.
> 
> So instead we mix the cleanup of the whole function with the cleanup of what
> is effectively a per-iteration temporary variable?
> 
> The fact there was already a confusion in this thread about whether it's
> safe and relies on kvfree(NULL) to be a no-op, should be a hint.

Yes. I think someone is confused about my opinion.
I don’t care about whether we should apply this hit.
If we think the hint is tricky, we could add another
label to fix it like I suggested previously. Because
we already use goto-based approaches to
cleanup the resources, why not keeping
consistent? It will be easier for us to add a new
"if" statement and handle the failure case in the future.
For example, if we use his v1 proposal, we should do
the cleanups again for info. But for goto-based
version, we just add another label to do the
cleanups and go to the new label for failure case. goto-based fix is what I insisted on. I copied my previous suggested fix here to clarify my opinion.

--- a/mm/shrinker.c
+++ b/mm/shrinker.c
@@ -88,13 +88,14 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
                      goto err;
              info->map_nr_max = shrinker_nr_max;
              if (shrinker_unit_alloc(info, NULL, nid))
-                       goto err;
+                       goto free;
              rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, info);
      }
      mutex_unlock(&shrinker_mutex);

      return ret;
-
+free:
+       kvfree(info);
err:
      mutex_unlock(&shrinker_mutex);
      free_shrinker_info(memcg);

Muchun,
Thanks.

> 
> So no, I a gree with Kirill and others. Ideally the fix would also move the
> declaration of info into the for loop to make its scope more obvious.
> 
>> Thanks.
>> 
>>> Best regards,
>>> Ridong