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

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

> On Oct 16, 2024, at 20:13, Vlastimil Babka <vbabka@suse.cz> wrote:
> 
> On 10/14/24 11:20, Muchun Song wrote:
>> 
>> 
>>>> On Oct 14, 2024, at 17:04, chenridong <chenridong@huawei.com> wrote:
>>> 
>>> 
>>> 
>>> On 2024/10/14 16:43, Muchun Song wrote:
>>>>> On Oct 14, 2024, at 16:13, Anshuman Khandual <anshuman.khandual@arm.com> wrote:
>>>>> 
>>>>> 
>>>>> 
>>>>> On 10/14/24 08:53, 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;
>>>>>> }
>>>>> 
>>>>> There are two scenarios when "goto err:" gets called
>>>>> 
>>>>> - When shrinker_info allocations fails, no kvfree() is required
>>>>> - but after this change kvfree() would be called even
>>>>> when the allocation had failed originally, which does
>>>>>   not sound right
>>>> Yes. In this case, @info is NULL and kvfree could handle NULL.
>>>> It seems strange but the final behaviour correct.
>>>>> 
>>>>> - shrinker_unit_alloc() fails, kvfree() is actually required
>>>>> 
>>>>> I guess kvfree() should be called just after shrinker_unit_alloc()
>>>>> fails but before calling into "goto err".
>>>> We could do it like this, which avoids ambiguity (if someone ignores
>>>> that kvfree could handle NULL). Something like:
>>>> --- 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);
>>>> Thanks.
>>>>> 
>>>>> But curious, should not both kvzalloc_node()/kvfree() be avoided
>>>>> while inside mutex lock to avoid possible lockdep issues ?
>>> How about:
>>> 
>>> diff --git a/mm/shrinker.c b/mm/shrinker.c
>>> index dc5d2a6fcfc4..7baee7f00497 100644
>>> --- a/mm/shrinker.c
>>> +++ b/mm/shrinker.c
>>> @@ -87,9 +87,9 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
>>>                if (!info)
>>>                        goto err;
>>>                info->map_nr_max = shrinker_nr_max;
>>> +               rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, info);
>>>                if (shrinker_unit_alloc(info, NULL, nid))
>>>                        goto err;
>>> -               rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, info);
>>>        }
>>>        mutex_unlock(&shrinker_mutex);
>> 
>> No. We should make sure the @info is fully initialized before others
>> could see it. That's why rcu_assign_pointer is used here.
> 
> If the info is immediately visible, is the failure cleanup
> free_shrinker_info() safe? It uses kvfree(info) and not kvfree_rcu(), and
> shrinker_unit_free() is also doing kfree().

Qi told me that the @info will not visible immediately yesterday.
So non-rcu-based kvfree is safe. Even if this fix could
work properly, it’s a bit strange for me to use
rcu_assign_pointer to assign the @info without full initialization to it.

Muchun,
Thanks.

> 
>>> 
>>> I think this is concise.
>>> 
>>> Best regards,
>>> Ridong
>> 
>> 
>