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

[Kirill A. Shutemov]

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);
-- 
  Kiryl Shutsemau / Kirill A. Shutemov