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

[Vlastimil Babka <vbabka@suse.cz>]

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.

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
> 
>