Re: [PATCH] mm/memcg: fix NULL pointer dereference in memcg_slab_free_hook()

[Kefeng Wang]

On 2021/7/28 21:23, Michal Hocko wrote:
> On Wed 28-07-21 17:13:48, Wang Hai wrote:
> > When I use kfree_rcu() to free a large memory allocated by
> > kmalloc_node(), the following dump occurs.
> >
> > BUG: kernel NULL pointer dereference, address: 0000000000000020
> > [...]
> > Oops: 0000 [#1] SMP
> > [...]
> > Workqueue: events kfree_rcu_work
> > RIP: 0010:__obj_to_index include/linux/slub_def.h:182 [inline]
> > RIP: 0010:obj_to_index include/linux/slub_def.h:191 [inline]
> > RIP: 0010:memcg_slab_free_hook+0x120/0x260 mm/slab.h:363
> > [...]
> > Call Trace:
> >   kmem_cache_free_bulk+0x58/0x630 mm/slub.c:3293
> >   kfree_bulk include/linux/slab.h:413 [inline]
> >   kfree_rcu_work+0x1ab/0x200 kernel/rcu/tree.c:3300
> >   process_one_work+0x207/0x530 kernel/workqueue.c:2276
> >   worker_thread+0x320/0x610 kernel/workqueue.c:2422
> >   kthread+0x13d/0x160 kernel/kthread.c:313
> >   ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294
> >
> > When kmalloc_node() a large memory, page is allocated, not slab,
> > so when freeing memory via kfree_rcu(), this large memory should not
> > be used by memcg_slab_free_hook(), because memcg_slab_free_hook() is
> > is used for slab.
> >
> > So in this case, there is no need to do anything with this large
> > page in memcg_slab_free_hook(), just skip it.
> >
> > Fixes: 270c6a71460e ("mm: memcontrol/slab: Use helpers to access slab page's memcg_data")
> Are you sure that this commit is really breaking the code. Unless I have
Yes, we confirmed that this commit introduces the bug.
> missed something there shouldn't be any real change wrt. large
> allocations here. page_has_obj_cgroups is just a different name for what
> what page_objcgs is giving us.

maybe we could simply use page_objcgs_check to fix the issue ? we will 
check it again.

diff --git a/mm/slab.h b/mm/slab.h
index f997fd5e42c8..58c01a34e5b8 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -346,7 +346,7 @@ static inline void memcg_slab_free_hook(struct 
kmem_cache *s_orig,
                        continue;

                page = virt_to_head_page(p[i]);
-               objcgs = page_objcgs(page);
+               objcgs = page_objcgs_check(page);
                if (!objcgs)
                        continue;

> I haven't studied the kfree_rcu part but isn't the problem its use of
> kmem_cache_free_bulk or isn't the problem right there in the bulk free?
>
> > Signed-off-by: Wang Hai <wanghai38@xxxxxxxxxx>
> > ---
> >   mm/slab.h | 15 ++++++++++-----
> >   1 file changed, 10 insertions(+), 5 deletions(-)
> >
> > diff --git a/mm/slab.h b/mm/slab.h
> > index 67e06637ff2e..247d3f9c21f7 100644
> > --- a/mm/slab.h
> > +++ b/mm/slab.h
> > @@ -339,15 +339,20 @@ static inline void memcg_slab_free_hook(struct kmem_cache *s_orig,
> >   			continue;
> >   
> >   		page = virt_to_head_page(p[i]);
> > +		if (!s_orig) {
> > +			if (unlikely(!PageSlab(page))) {
> > +				BUG_ON(!PageCompound(page));
the same logical is in build_detached_freelist()
> BUG_ON is not really a good idea here. Why should we crash the kernel
> just because of an unexpected page showing up. Leaking it would be more
> appropriate (the same would apply to kfree btw). I would just warn
> here. Also don't we need any hookd here.  Looking at kfree path it does
> call kfree_hook. Why is that not needed here?
kmem_cache_free_bulk
-- memcg_slab_free_hook  // skip objcgs ops for page
-- build_detached_freelist  // kfree_hook and page free