linux-kernel - Re: [PATCH v2] mm: shrinker: avoid memleak in alloc_shrinker

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Message-ID: <w7x2gzrpluandokbym2y7sbmfshbfb5wgh4lt4d72bnorb26la@cyavgpacqwhh>
Date: Wed, 16 Oct 2024 13:16:46 +0300
From: "Kirill A. Shutemov" <kirill@...temov.name>
To: Muchun Song <muchun.song@...ux.dev>
Cc: chenridong <chenridong@...wei.com>, 
	Anshuman Khandual <anshuman.khandual@....com>, akpm@...ux-foundation.org, david@...morbit.com, 
	zhengqi.arch@...edance.com, roman.gushchin@...ux.dev, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, wangweiyang2@...wei.com
Subject: Re: [PATCH v2] mm: shrinker: avoid memleak in alloc_shrinker_info

On Wed, Oct 16, 2024 at 10:21:30AM +0800, Muchun Song wrote:
> 
> 
> > On Oct 16, 2024, at 09:25, chenridong <chenridong@...wei.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@...wei.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@...wei.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.

It makes no sense.

This kvfree() is specifically to handle the case when 'info' is allocated,
but not yet assigned to ->shrinker_info. And 'err:' block handles all
other error cases. Putting kvfree() in 'err:' section is double-free
timebomb.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov