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

[Roman Gushchin <roman.gushchin@linux.dev>]

On Wed, Oct 16, 2024 at 07:02:23PM +0200, Vlastimil Babka wrote:
> On 10/16/24 16:08, Muchun Song wrote:
> > 
> > 
> >> On Oct 16, 2024, at 19:43, Vlastimil Babka <vbabka@suse.cz> wrote:
> >> On 10/16/24 04:21, Muchun Song wrote:
> >>> 
> >>> 
> >>>> On Oct 16, 2024, at 09:25, chenridong <chenridong@huawei.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@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?
> >>> 
> >>> I insist on my opinion, not mixing two different approaches
> >>> to do release resources.
> >> 
> >> So instead we mix the cleanup of the whole function with the cleanup of what
> >> is effectively a per-iteration temporary variable?
> >> 
> >> The fact there was already a confusion in this thread about whether it's
> >> safe and relies on kvfree(NULL) to be a no-op, should be a hint.
> > 
> > Yes. I think someone is confused about my opinion.
> > I don’t care about whether we should apply this hit.
> > If we think the hint is tricky, we could add another
> > label to fix it like I suggested previously. Because
> > we already use goto-based approaches to
> > cleanup the resources, why not keeping
> > consistent?
> 
> I think we're rather pragmatic than striving to be consistent for the sake
> of consistency. goto is not the nicest thing in the world, but we (unlike
> other projects) use it where it makes sense to avoid if/else nesting
> explosion. Here for the info it's not the most pragmatic option.
> 
> > It will be easier for us to add a new
> > "if" statement and handle the failure case in the future.
> 
> Let's not overengineer things for hypothetical future.
> 
> > For example, if we use his v1 proposal, we should do
> > the cleanups again for info. But for goto-based
> > version, we just add another label to do the
> > cleanups and go to the new label for failure case. goto-based fix is what I insisted on. I copied my previous suggested fix here to clarify my opinion.
> 
> Again, info is a loop-iteration-local variable, v1 fix making it truly local
> is the way to go. If there are further cleanups added in the loop itself in
> the future, they could hopefully keep being local to the loop as well.
> Cleanup of info outside the loop iteration is breaking its real scope.

+1 to that.

I don't think it's such a big deal and both versions are ok, but I strongly
prefer the original version (without introduction of a new label).

Please, feel free to use
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
with the original version.

Thanks!