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

[chenridong <chenridong@huawei.com>]

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?

Best regards,
Ridong