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Subject: Re: [PATCH] skbuff: fix a data race in skb_queue_len()
To:     Qian Cai <cai@lca.pw>, davem@davemloft.net
Cc:     kuba@kernel.org, elver@google.com, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
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From:   Eric Dumazet <eric.dumazet@gmail.com>
Message-ID: <648d6673-bbd8-6634-0174-f9b77194ecfd@gmail.com>
Date:   Mon, 3 Feb 2020 11:41:58 -0800
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On 2/3/20 10:56 AM, Qian Cai wrote:
> sk_buff.qlen can be accessed concurrently as noticed by KCSAN,
> 
>  BUG: KCSAN: data-race in __skb_try_recv_from_queue / unix_dgram_sendmsg
> 
>  read to 0xffff8a1b1d8a81c0 of 4 bytes by task 5371 on cpu 96:
>   unix_dgram_sendmsg+0x9a9/0xb70 include/linux/skbuff.h:1821
> 				 net/unix/af_unix.c:1761
>   ____sys_sendmsg+0x33e/0x370
>   ___sys_sendmsg+0xa6/0xf0
>   __sys_sendmsg+0x69/0xf0
>   __x64_sys_sendmsg+0x51/0x70
>   do_syscall_64+0x91/0xb47
>   entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 
>  write to 0xffff8a1b1d8a81c0 of 4 bytes by task 1 on cpu 99:
>   __skb_try_recv_from_queue+0x327/0x410 include/linux/skbuff.h:2029
>   __skb_try_recv_datagram+0xbe/0x220
>   unix_dgram_recvmsg+0xee/0x850
>   ____sys_recvmsg+0x1fb/0x210
>   ___sys_recvmsg+0xa2/0xf0
>   __sys_recvmsg+0x66/0xf0
>   __x64_sys_recvmsg+0x51/0x70
>   do_syscall_64+0x91/0xb47
>   entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 
> Since only the read is operating as lockless, it could introduce a logic
> bug in unix_recvq_full() due to the load tearing. Fix it by adding
> a READ_ONCE() there.
> 
> Signed-off-by: Qian Cai <cai@lca.pw>
> ---
>  include/linux/skbuff.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 3d13a4b717e9..4b5157164f3e 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -1818,7 +1818,7 @@ static inline struct sk_buff *skb_peek_tail(const struct sk_buff_head *list_)
>   */
>  static inline __u32 skb_queue_len(const struct sk_buff_head *list_)
>  {
> -	return list_->qlen;
> +	return READ_ONCE(list_->qlen);
>  }

We do not want to add READ_ONCE() for all uses of skb_queue_len()

This could hide some real bugs, and could generate slightly less
efficient code in the cases we have the lock held.