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Date:   Wed, 01 Jul 2020 15:49:54 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     stranche@...eaurora.org
Cc:     netdev@...r.kernel.org, xiyou.wangcong@...il.com,
        johannes.berg@...el.com, subashab@...eaurora.org
Subject: Re: [PATCH net v2] genetlink: remove genl_bind

From: Sean Tranchetti <stranche@...eaurora.org>
Date: Tue, 30 Jun 2020 11:50:17 -0600

> A potential deadlock can occur during registering or unregistering a
> new generic netlink family between the main nl_table_lock and the
> cb_lock where each thread wants the lock held by the other, as
> demonstrated below.
> 
> 1) Thread 1 is performing a netlink_bind() operation on a socket. As part
>    of this call, it will call netlink_lock_table(), incrementing the
>    nl_table_users count to 1.
> 2) Thread 2 is registering (or unregistering) a genl_family via the
>    genl_(un)register_family() API. The cb_lock semaphore will be taken for
>    writing.
> 3) Thread 1 will call genl_bind() as part of the bind operation to handle
>    subscribing to GENL multicast groups at the request of the user. It will
>    attempt to take the cb_lock semaphore for reading, but it will fail and
>    be scheduled away, waiting for Thread 2 to finish the write.
> 4) Thread 2 will call netlink_table_grab() during the (un)registration
>    call. However, as Thread 1 has incremented nl_table_users, it will not
>    be able to proceed, and both threads will be stuck waiting for the
>    other.
> 
> genl_bind() is a noop, unless a genl_family implements the mcast_bind()
> function to handle setting up family-specific multicast operations. Since
> no one in-tree uses this functionality as Cong pointed out, simply removing
> the genl_bind() function will remove the possibility for deadlock, as there
> is no attempt by Thread 1 above to take the cb_lock semaphore.
> 
> Fixes: c380d9a7afff ("genetlink: pass multicast bind/unbind to families")
> Suggested-by: Cong Wang <xiyou.wangcong@...il.com>
> Acked-by: Johannes Berg <johannes.berg@...el.com>
> Reported-by: kernel test robot <lkp@...el.com>
> Signed-off-by: Sean Tranchetti <stranche@...eaurora.org>

Applied and queued up for -stable, thanks.

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