Re: [PATCH v3] skbuff: fix a data race in skb_queue_len()

[David Miller]

From: Qian Cai <cai@xxxxxx>
Date: Tue,  4 Feb 2020 13:40:29 -0500

> 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 lockless variant of skb_queue_len() and unix_recvq_full() where
> READ_ONCE() is on the read while WRITE_ONCE() is on the write similar to
> the commit d7d16a89350a ("net: add skb_queue_empty_lockless()").
> 
> Signed-off-by: Qian Cai <cai@xxxxxx>

Applied, thank you.