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

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

John,
please review.
It seems this bug was causing multiple syzbot reports.

On Fri, Mar 22, 2024 at 10:42 PM Edward Adam Davis <eadavis@qq.com> 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: syzbot+c4f4d25859c2e5859988@syzkaller.appspotmail.com
> Signed-off-by: Edward Adam Davis <eadavis@qq.com>
> ---
>  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
>