Messages in this thread Patch in this message | | | Subject | Re: [PATCH] mm/memcg: fix NULL pointer dereference in memcg_slab_free_hook() | From | Kefeng Wang <> | Date | Wed, 28 Jul 2021 22:21:23 +0800 |
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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@huawei.com> >> --- >> 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
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