Subject: Re: [PATCH] bpf, sockmap: fix deadlock in rcu_report_exp_cpu_mult

From: Jakub Sitnicki <[email-masked]>
Date: Mon, 25 Mar 2024 13:23:07 +0100

On Sat, Mar 23, 2024 at 12:08 AM -07, Alexei Starovoitov wrote:
> John,
> please review.
> It seems this bug was causing multiple syzbot reports.

Any chance we could disallow mutating sockhash from interrupt context?

If that is not an option, then this looks like a good start of a fix.
But we also need to cover sock_map_unref->sock_sock_map_del_link called
from sock_hash_delete_elem. It also grabs a spin lock.

Also, sockhash is not the only affected map type. I see we're grabbing a
spin lock in ->map_delete_elem without disabling interrupts as well in:

- sock_map_delete_elem
- reuseport_array_delete_elem
- xsk_map_delete_elem

> On Fri, Mar 22, 2024 at 10:42 PM Edward Adam Davis <[email-masked]> wrote:
>>
>> [Syzbot reported]
>> WARNING: HARDIRQ-safe -> HARDIRQ-unsafe lock order detected
>> 6.8.0-syzkaller-05221-gea80e3ed09ab #0 Not tainted
>> -----------------------------------------------------
>> rcu_exp_gp_kthr/18 [HC0[0]:SC0[2]:HE0:SE0] is trying to acquire:
>> ffff88802b5ab020 (&htab->buckets[i].lock){+...}-{2:2}, at: spin_lock_bh include/linux/spinlock.h:356 [inline]
>> ffff88802b5ab020 (&htab->buckets[i].lock){+...}-{2:2}, at: sock_hash_delete_elem+0xb0/0x300 net/core/sock_map.c:939
>>
>> and this task is already holding:
>> ffffffff8e136558 (rcu_node_0){-.-.}-{2:2}, at: sync_rcu_exp_done_unlocked+0xe/0x140 kernel/rcu/tree_exp.h:169
>> which would create a new lock dependency:
>>  (rcu_node_0){-.-.}-{2:2} -> (&htab->buckets[i].lock){+...}-{2:2}
>>
>> but this new dependency connects a HARDIRQ-irq-safe lock:
>>  (rcu_node_0){-.-.}-{2:2}
>>
>> ... which became HARDIRQ-irq-safe at:
>>   lock_acquire+0x1e4/0x530 kernel/locking/lockdep.c:5754
>>   __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
>>   _raw_spin_lock_irqsave+0xd5/0x120 kernel/locking/spinlock.c:162
>>   rcu_report_exp_cpu_mult+0x27/0x2f0 kernel/rcu/tree_exp.h:238
>>   csd_do_func kernel/smp.c:133 [inline]
>>   __flush_smp_call_function_queue+0xb2e/0x15b0 kernel/smp.c:542
>>   __sysvec_call_function_single+0xa8/0x3e0 arch/x86/kernel/smp.c:271
>>   instr_sysvec_call_function_single arch/x86/kernel/smp.c:266 [inline]
>>   sysvec_call_function_single+0x9e/0xc0 arch/x86/kernel/smp.c:266
>>   asm_sysvec_call_function_single+0x1a/0x20 arch/x86/include/asm/idtentry.h:709
>>   __sanitizer_cov_trace_switch+0x90/0x120
>>   update_event_printk kernel/trace/trace_events.c:2750 [inline]
>>   trace_event_eval_update+0x311/0xf90 kernel/trace/trace_events.c:2922
>>   process_one_work kernel/workqueue.c:3254 [inline]
>>   process_scheduled_works+0xa00/0x1770 kernel/workqueue.c:3335
>>   worker_thread+0x86d/0xd70 kernel/workqueue.c:3416
>>   kthread+0x2f0/0x390 kernel/kthread.c:388
>>   ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:147
>>   ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:243
>>
>> to a HARDIRQ-irq-unsafe lock:
>>  (&htab->buckets[i].lock){+...}-{2:2}
>>
>> ... which became HARDIRQ-irq-unsafe at:
>> ...
>>   lock_acquire+0x1e4/0x530 kernel/locking/lockdep.c:5754
>>   __raw_spin_lock_bh include/linux/spinlock_api_smp.h:126 [inline]
>>   _raw_spin_lock_bh+0x35/0x50 kernel/locking/spinlock.c:178
>>   spin_lock_bh include/linux/spinlock.h:356 [inline]
>>   sock_hash_delete_elem+0xb0/0x300 net/core/sock_map.c:939
>>   0xffffffffa0001b0e
>>   bpf_dispatcher_nop_func include/linux/bpf.h:1234 [inline]
>>   __bpf_prog_run include/linux/filter.h:657 [inline]
>>   bpf_prog_run include/linux/filter.h:664 [inline]
>>   __bpf_trace_run kernel/trace/bpf_trace.c:2381 [inline]
>>   bpf_trace_run2+0x204/0x420 kernel/trace/bpf_trace.c:2420
>>   trace_contention_end+0xd7/0x100 include/trace/events/lock.h:122
>>   __mutex_lock_common kernel/locking/mutex.c:617 [inline]
>>   __mutex_lock+0x2e5/0xd70 kernel/locking/mutex.c:752
>>   futex_cleanup_begin kernel/futex/core.c:1091 [inline]
>>   futex_exit_release+0x34/0x1f0 kernel/futex/core.c:1143
>>   exit_mm_release+0x1a/0x30 kernel/fork.c:1652
>>   exit_mm+0xb0/0x310 kernel/exit.c:542
>>   do_exit+0x99e/0x27e0 kernel/exit.c:865
>>   do_group_exit+0x207/0x2c0 kernel/exit.c:1027
>>   __do_sys_exit_group kernel/exit.c:1038 [inline]
>>   __se_sys_exit_group kernel/exit.c:1036 [inline]
>>   __x64_sys_exit_group+0x3f/0x40 kernel/exit.c:1036
>>   do_syscall_64+0xfb/0x240
>>   entry_SYSCALL_64_after_hwframe+0x6d/0x75
>>
>> other info that might help us debug this:
>>
>>  Possible interrupt unsafe locking scenario:
>>
>>        CPU0                    CPU1
>>        ----                    ----
>>   lock(&htab->buckets[i].lock);
>>                                local_irq_disable();
>>                                lock(rcu_node_0);
>>                                lock(&htab->buckets[i].lock);
>>   <Interrupt>
>>     lock(rcu_node_0);
>>
>>  *** DEADLOCK ***
>> [Fix]
>> Ensure that the context interrupt state is the same before and after using the
>> bucket->lock.
>>
>> Reported-and-tested-by: [email-masked]
>> Signed-off-by: Edward Adam Davis <[email-masked]>
>> ---
>>  net/core/sock_map.c | 10 ++++++----
>>  1 file changed, 6 insertions(+), 4 deletions(-)
>>
>> diff --git a/net/core/sock_map.c b/net/core/sock_map.c
>> index 27d733c0f65e..ae8f81b26e16 100644
>> --- a/net/core/sock_map.c
>> +++ b/net/core/sock_map.c
>> @@ -932,11 +932,12 @@ static long sock_hash_delete_elem(struct bpf_map *map, void *key)
>>         struct bpf_shtab_bucket *bucket;
>>         struct bpf_shtab_elem *elem;
>>         int ret = -ENOENT;
>> +       unsigned long flags;
>>
>>         hash = sock_hash_bucket_hash(key, key_size);
>>         bucket = sock_hash_select_bucket(htab, hash);
>>
>> -       spin_lock_bh(&bucket->lock);
>> +       spin_lock_irqsave(&bucket->lock, flags);
>>         elem = sock_hash_lookup_elem_raw(&bucket->head, hash, key, key_size);
>>         if (elem) {
>>                 hlist_del_rcu(&elem->node);
>> @@ -944,7 +945,7 @@ static long sock_hash_delete_elem(struct bpf_map *map, void *key)
>>                 sock_hash_free_elem(htab, elem);
>>                 ret = 0;
>>         }
>> -       spin_unlock_bh(&bucket->lock);
>> +       spin_unlock_irqrestore(&bucket->lock, flags);
>>         return ret;
>>  }
>>
>> @@ -1136,6 +1137,7 @@ static void sock_hash_free(struct bpf_map *map)
>>         struct bpf_shtab_elem *elem;
>>         struct hlist_node *node;
>>         int i;
>> +       unsigned long flags;
>>
>>         /* After the sync no updates or deletes will be in-flight so it
>>          * is safe to walk map and remove entries without risking a race
>> @@ -1151,11 +1153,11 @@ static void sock_hash_free(struct bpf_map *map)
>>                  * exists, psock exists and holds a ref to socket. That
>>                  * lets us to grab a socket ref too.
>>                  */
>> -               spin_lock_bh(&bucket->lock);
>> +               spin_lock_irqsave(&bucket->lock, flags);
>>                 hlist_for_each_entry(elem, &bucket->head, node)
>>                         sock_hold(elem->sk);
>>                 hlist_move_list(&bucket->head, &unlink_list);
>> -               spin_unlock_bh(&bucket->lock);
>> +               spin_unlock_irqrestore(&bucket->lock, flags);
>>
>>                 /* Process removed entries out of atomic context to
>>                  * block for socket lock before deleting the psock's
>> --
>> 2.43.0
>>