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

[Anshuman Khandual]

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@xxxxxxxxxx>
>>
>> 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@xxxxxxxxxx>
>> ---
>>  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"
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