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SubjectRe: [PATCH] skbuff: fix a data race in skb_queue_len()
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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.



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