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

On 2021/7/28 21:23, Michal Hocko wrote:

On Wed 28-07-21 17:13:48, Wang Hai wrote:

quoted

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?

quoted

Signed-off-by: Wang Hai <redacted>
---
  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