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Subject: Re: possible deadlock in bpf_lru_push_free
From:   Yonghong Song <yhs@fb.com>
To:     Hillf Danton <hdanton@sina.com>,
        syzbot <syzbot+122b5421d14e68f29cd1@syzkaller.appspotmail.com>
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        <daniel@iogearbox.net>, <kafai@fb.com>,
        <linux-kernel@vger.kernel.org>, <netdev@vger.kernel.org>,
        <songliubraving@fb.com>, <syzkaller-bugs@googlegroups.com>
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On 2/18/20 9:44 AM, Yonghong Song wrote:
> 
> 
> On 2/16/20 9:23 PM, Hillf Danton wrote:
>>
>> On Sun, 16 Feb 2020 04:17:09 -0800
>>> syzbot has found a reproducer for the following crash on:
>>>
>>> HEAD commit:    2019fc96 Merge 
>>> git://git.kernel.org/pub/scm/linux/kernel/g..
>>> git tree:       net
>>> console output: 
>>> https://urldefense.proofpoint.com/v2/url?u=https-3A__syzkaller.appspot.com_x_log.txt-3Fx-3D1358bb11e00000&d=DwIDAg&c=5VD0RTtNlTh3ycd41b3MUw&r=DA8e1B5r073vIqRrFz7MRA&m=npe_gMkFnfxt6F5dGLs6zsNHWkYM30LkMFOk1_ZR1w8&s=zrgWcBnddWkMWG2zm-9nC8EwvHMsuqw_-EEXwl23XLg&e= 
>>>
>>> kernel config:  
>>> https://urldefense.proofpoint.com/v2/url?u=https-3A__syzkaller.appspot.com_x_.config-3Fx-3D735296e4dd620b10&d=DwIDAg&c=5VD0RTtNlTh3ycd41b3MUw&r=DA8e1B5r073vIqRrFz7MRA&m=npe_gMkFnfxt6F5dGLs6zsNHWkYM30LkMFOk1_ZR1w8&s=kbT6Yw89JDoIWSQtlLJ7sjyNoP2Ulud27GNorna1zQk&e= 
>>>
>>> dashboard link: 
>>> https://urldefense.proofpoint.com/v2/url?u=https-3A__syzkaller.appspot.com_bug-3Fextid-3D122b5421d14e68f29cd1&d=DwIDAg&c=5VD0RTtNlTh3ycd41b3MUw&r=DA8e1B5r073vIqRrFz7MRA&m=npe_gMkFnfxt6F5dGLs6zsNHWkYM30LkMFOk1_ZR1w8&s=U3pdUmrcroaeNsJ9DgFbTlvftQUCUcJ1CW_0NxS8yGA&e= 
>>>
>>> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
>>> syz repro:      
>>> https://urldefense.proofpoint.com/v2/url?u=https-3A__syzkaller.appspot.com_x_repro.syz-3Fx-3D14b67d6ee00000&d=DwIDAg&c=5VD0RTtNlTh3ycd41b3MUw&r=DA8e1B5r073vIqRrFz7MRA&m=npe_gMkFnfxt6F5dGLs6zsNHWkYM30LkMFOk1_ZR1w8&s=TuSfjosRFQW3ArpQwikTtx-dgLLBSMgJfVKtUltqQBM&e= 
>>>
>>>
>>> IMPORTANT: if you fix the bug, please add the following tag to the 
>>> commit:
>>> Reported-by: syzbot+122b5421d14e68f29cd1@syzkaller.appspotmail.com
>>>
>>> ======================================================
>>> WARNING: possible circular locking dependency detected
>>> 5.6.0-rc1-syzkaller #0 Not tainted
>>> ------------------------------------------------------
>>> syz-executor.4/13544 is trying to acquire lock:
>>> ffffe8ffffcba0b8 (&loc_l->lock){....}, at: bpf_common_lru_push_free 
>>> kernel/bpf/bpf_lru_list.c:516 [inline]
>>> ffffe8ffffcba0b8 (&loc_l->lock){....}, at: 
>>> bpf_lru_push_free+0x250/0x5b0 kernel/bpf/bpf_lru_list.c:555
>>>
>>> but task is already holding lock:
>>> ffff888094985960 (&htab->buckets[i].lock){....}, at: 
>>> __htab_map_lookup_and_delete_batch+0x617/0x1540 
>>> kernel/bpf/hashtab.c:1322
>>>
>>> which lock already depends on the new lock.
>>>
>>>
>>> the existing dependency chain (in reverse order) is:
>>>
>>> -> #2 (&htab->buckets[i].lock){....}:
>>>         __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 
>>> [inline]
>>>         _raw_spin_lock_irqsave+0x95/0xcd kernel/locking/spinlock.c:159
>>>         htab_lru_map_delete_node+0xce/0x2f0 kernel/bpf/hashtab.c:593
>>>         __bpf_lru_list_shrink_inactive kernel/bpf/bpf_lru_list.c:220 
>>> [inline]
>>>         __bpf_lru_list_shrink+0xf9/0x470 kernel/bpf/bpf_lru_list.c:266
>>>         bpf_lru_list_pop_free_to_local kernel/bpf/bpf_lru_list.c:340 
>>> [inline]
>>>         bpf_common_lru_pop_free kernel/bpf/bpf_lru_list.c:447 [inline]
>>>         bpf_lru_pop_free+0x87c/0x1670 kernel/bpf/bpf_lru_list.c:499
>>>         prealloc_lru_pop+0x2c/0xa0 kernel/bpf/hashtab.c:132
>>>         __htab_lru_percpu_map_update_elem+0x67e/0xa90 
>>> kernel/bpf/hashtab.c:1069
>>>         bpf_percpu_hash_update+0x16e/0x210 kernel/bpf/hashtab.c:1585
>>>         bpf_map_update_value.isra.0+0x2d7/0x8e0 kernel/bpf/syscall.c:181
>>>         generic_map_update_batch+0x41f/0x610 kernel/bpf/syscall.c:1319
>>>         bpf_map_do_batch+0x3f5/0x510 kernel/bpf/syscall.c:3348
>>>         __do_sys_bpf+0x9b7/0x41e0 kernel/bpf/syscall.c:3460
>>>         __se_sys_bpf kernel/bpf/syscall.c:3355 [inline]
>>>         __x64_sys_bpf+0x73/0xb0 kernel/bpf/syscall.c:3355
>>>         do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
>>>         entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>>
>>> -> #1 (&l->lock){....}:
>>>         __raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline]
>>>         _raw_spin_lock+0x2f/0x40 kernel/locking/spinlock.c:151
>>>         bpf_lru_list_pop_free_to_local kernel/bpf/bpf_lru_list.c:325 
>>> [inline]
>>>         bpf_common_lru_pop_free kernel/bpf/bpf_lru_list.c:447 [inline]
>>>         bpf_lru_pop_free+0x67f/0x1670 kernel/bpf/bpf_lru_list.c:499
>>>         prealloc_lru_pop+0x2c/0xa0 kernel/bpf/hashtab.c:132
>>>         __htab_lru_percpu_map_update_elem+0x67e/0xa90 
>>> kernel/bpf/hashtab.c:1069
>>>         bpf_percpu_hash_update+0x16e/0x210 kernel/bpf/hashtab.c:1585
>>>         bpf_map_update_value.isra.0+0x2d7/0x8e0 kernel/bpf/syscall.c:181
>>>         generic_map_update_batch+0x41f/0x610 kernel/bpf/syscall.c:1319
>>>         bpf_map_do_batch+0x3f5/0x510 kernel/bpf/syscall.c:3348
>>>         __do_sys_bpf+0x9b7/0x41e0 kernel/bpf/syscall.c:3460
>>>         __se_sys_bpf kernel/bpf/syscall.c:3355 [inline]
>>>         __x64_sys_bpf+0x73/0xb0 kernel/bpf/syscall.c:3355
>>>         do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
>>>         entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>>
>>> -> #0 (&loc_l->lock){....}:
>>>         check_prev_add kernel/locking/lockdep.c:2475 [inline]
>>>         check_prevs_add kernel/locking/lockdep.c:2580 [inline]
>>>         validate_chain kernel/locking/lockdep.c:2970 [inline]
>>>         __lock_acquire+0x2596/0x4a00 kernel/locking/lockdep.c:3954
>>>         lock_acquire+0x190/0x410 kernel/locking/lockdep.c:4484
>>>         __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 
>>> [inline]
>>>         _raw_spin_lock_irqsave+0x95/0xcd kernel/locking/spinlock.c:159
>>>         bpf_common_lru_push_free kernel/bpf/bpf_lru_list.c:516 [inline]
>>>         bpf_lru_push_free+0x250/0x5b0 kernel/bpf/bpf_lru_list.c:555
>>>         __htab_map_lookup_and_delete_batch+0x8d4/0x1540 
>>> kernel/bpf/hashtab.c:1374
>>>         htab_lru_map_lookup_and_delete_batch+0x34/0x40 
>>> kernel/bpf/hashtab.c:1491
>>>         bpf_map_do_batch+0x3f5/0x510 kernel/bpf/syscall.c:3348
>>>         __do_sys_bpf+0x1f7d/0x41e0 kernel/bpf/syscall.c:3456
>>>         __se_sys_bpf kernel/bpf/syscall.c:3355 [inline]
>>>         __x64_sys_bpf+0x73/0xb0 kernel/bpf/syscall.c:3355
>>>         do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
>>>         entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>>
>>> other info that might help us debug this:
>>>
>>> Chain exists of:
>>>    &loc_l->lock --> &l->lock --> &htab->buckets[i].lock
>>>
>>>   Possible unsafe locking scenario:
>>>
>>>         CPU0                    CPU1
>>>         ----                    ----
>>>    lock(&htab->buckets[i].lock);
>>>                                 lock(&l->lock);
>>>                                 lock(&htab->buckets[i].lock);
>>>    lock(&loc_l->lock);
>>>
>>>   *** DEADLOCK ***
>>>
>>> 2 locks held by syz-executor.4/13544:
>>>   #0: ffffffff89bac240 (rcu_read_lock){....}, at: 
>>> __htab_map_lookup_and_delete_batch+0x54b/0x1540 
>>> kernel/bpf/hashtab.c:1308
>>>   #1: ffff888094985960 (&htab->buckets[i].lock){....}, at: 
>>> __htab_map_lookup_and_delete_batch+0x617/0x1540 
>>> kernel/bpf/hashtab.c:1322
>>>
>>> stack backtrace:
>>> CPU: 0 PID: 13544 Comm: syz-executor.4 Not tainted 
>>> 5.6.0-rc1-syzkaller #0
>>> Hardware name: Google Google Compute Engine/Google Compute Engine, 
>>> BIOS Google 01/01/2011
>>> Call Trace:
>>>   __dump_stack lib/dump_stack.c:77 [inline]
>>>   dump_stack+0x197/0x210 lib/dump_stack.c:118
>>>   print_circular_bug.isra.0.cold+0x163/0x172 
>>> kernel/locking/lockdep.c:1684
>>>   check_noncircular+0x32e/0x3e0 kernel/locking/lockdep.c:1808
>>>   check_prev_add kernel/locking/lockdep.c:2475 [inline]
>>>   check_prevs_add kernel/locking/lockdep.c:2580 [inline]
>>>   validate_chain kernel/locking/lockdep.c:2970 [inline]
>>>   __lock_acquire+0x2596/0x4a00 kernel/locking/lockdep.c:3954
>>>   lock_acquire+0x190/0x410 kernel/locking/lockdep.c:4484
>>>   __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
>>>   _raw_spin_lock_irqsave+0x95/0xcd kernel/locking/spinlock.c:159
>>>   bpf_common_lru_push_free kernel/bpf/bpf_lru_list.c:516 [inline]
>>>   bpf_lru_push_free+0x250/0x5b0 kernel/bpf/bpf_lru_list.c:555
>>>   __htab_map_lookup_and_delete_batch+0x8d4/0x1540 
>>> kernel/bpf/hashtab.c:1374
>>>   htab_lru_map_lookup_and_delete_batch+0x34/0x40 
>>> kernel/bpf/hashtab.c:1491
>>>   bpf_map_do_batch+0x3f5/0x510 kernel/bpf/syscall.c:3348
>>>   __do_sys_bpf+0x1f7d/0x41e0 kernel/bpf/syscall.c:3456
>>>   __se_sys_bpf kernel/bpf/syscall.c:3355 [inline]
>>>   __x64_sys_bpf+0x73/0xb0 kernel/bpf/syscall.c:3355
>>>   do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
>>>   entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>
>> Reclaim hash table elememt outside bucket lock.
> 
> Thanks for the following patch. Yes, we do have an potential issue
> with the above deadlock if LRU hash map is not preallocated.
> 
> I am not a RCU expert, but maybe you could you help clarify
> one thing below?
> 
>>
>> --- a/kernel/bpf/hashtab.c
>> +++ b/kernel/bpf/hashtab.c
>> @@ -1259,6 +1259,7 @@ __htab_map_lookup_and_delete_batch(struc
>>       u64 elem_map_flags, map_flags;
>>       struct hlist_nulls_head *head;
>>       struct hlist_nulls_node *n;
>> +    struct hlist_nulls_node *node_to_free = NULL;
>>       unsigned long flags;
>>       struct htab_elem *l;
>>       struct bucket *b;
>> @@ -1370,9 +1371,10 @@ again_nocopy:
>>           }
>>           if (do_delete) {
>>               hlist_nulls_del_rcu(&l->hash_node);
>> -            if (is_lru_map)
>> -                bpf_lru_push_free(&htab->lru, &l->lru_node);
>> -            else
>> +            if (is_lru_map) {
>> +                l->hash_node.next = node_to_free;
>> +                node_to_free = &l->hash_node;
> 
> Here, we change "next" pointer. How does this may impact the existing 
> parallel map lookup which does not need to take bucket pointer?

Thanks for Martin for explanation! I think changing l->hash_node.next is
unsafe here as another thread may execute on a different cpu and
traverse the same list. It will see hash_node.next = NULL and it is
unexpected.

How about the following patch?

diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index 2d182c4ee9d9..246ef0f2e985 100644
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -56,6 +56,7 @@ struct htab_elem {
                         union {
                                 struct bpf_htab *htab;
                                 struct pcpu_freelist_node fnode;
+                               struct htab_elem *link;
                         };
                 };
         };
@@ -1256,6 +1257,7 @@ __htab_map_lookup_and_delete_batch(struct bpf_map 
*map,
         void __user *ukeys = u64_to_user_ptr(attr->batch.keys);
         void *ubatch = u64_to_user_ptr(attr->batch.in_batch);
         u32 batch, max_count, size, bucket_size;
+       struct htab_elem *node_to_free = NULL;
         u64 elem_map_flags, map_flags;
         struct hlist_nulls_head *head;
         struct hlist_nulls_node *n;
@@ -1370,9 +1372,14 @@ __htab_map_lookup_and_delete_batch(struct bpf_map 
*map,
                 }
                 if (do_delete) {
                         hlist_nulls_del_rcu(&l->hash_node);
-                       if (is_lru_map)
-                               bpf_lru_push_free(&htab->lru, &l->lru_node);
-                       else
+                       if (is_lru_map) {
+                               /* l->hnode overlaps with * 
l->hash_node.pprev
+                                * in memory. l->hash_node.pprev has been
+                                * poisoned and nobody should access it.
+                                */
+                               l->link = node_to_free;
+                               node_to_free = l;
+                       } else
                                 free_htab_elem(htab, l);
                 }
                 dst_key += key_size;
@@ -1380,6 +1387,13 @@ __htab_map_lookup_and_delete_batch(struct bpf_map 
*map,
         }

         raw_spin_unlock_irqrestore(&b->lock, flags);
+
+       while (node_to_free) {
+               l = node_to_free;
+               node_to_free = node_to_free->link;
+               bpf_lru_push_free(&htab->lru, &l->lru_node);
+       }
+
         /* If we are not copying data, we can go to next bucket and avoid
          * unlocking the rcu.
          */