Re: [PATCH 5.10, 5.4] net: set SOCK_RCU_FREE before inserting socket into hashtable

["gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>]

On Wed, Sep 04, 2024 at 01:06:45PM +0000, Siddh Raman Pant wrote:
> [ Upstream commit 871019b22d1bcc9fab2d1feba1b9a564acbb6e99 ]
> 
> We've started to see the following kernel traces:
> 
>  WARNING: CPU: 83 PID: 0 at net/core/filter.c:6641 sk_lookup+0x1bd/0x1d0
> 
>  Call Trace:
>   <IRQ>
>   __bpf_skc_lookup+0x10d/0x120
>   bpf_sk_lookup+0x48/0xd0
>   bpf_sk_lookup_tcp+0x19/0x20
>   bpf_prog_<redacted>+0x37c/0x16a3
>   cls_bpf_classify+0x205/0x2e0
>   tcf_classify+0x92/0x160
>   __netif_receive_skb_core+0xe52/0xf10
>   __netif_receive_skb_list_core+0x96/0x2b0
>   napi_complete_done+0x7b5/0xb70
>   <redacted>_poll+0x94/0xb0
>   net_rx_action+0x163/0x1d70
>   __do_softirq+0xdc/0x32e
>   asm_call_irq_on_stack+0x12/0x20
>   </IRQ>
>   do_softirq_own_stack+0x36/0x50
>   do_softirq+0x44/0x70
> 
> __inet_hash can race with lockless (rcu) readers on the other cpus:
> 
>   __inet_hash
>     __sk_nulls_add_node_rcu
>     <- (bpf triggers here)
>     sock_set_flag(SOCK_RCU_FREE)
> 
> Let's move the SOCK_RCU_FREE part up a bit, before we are inserting
> the socket into hashtables. Note, that the race is really harmless;
> the bpf callers are handling this situation (where listener socket
> doesn't have SOCK_RCU_FREE set) correctly, so the only
> annoyance is a WARN_ONCE.
> 
> More details from Eric regarding SOCK_RCU_FREE timeline:
> 
> Commit 3b24d854cb35 ("tcp/dccp: do not touch listener sk_refcnt under
> synflood") added SOCK_RCU_FREE. At that time, the precise location of
> sock_set_flag(sk, SOCK_RCU_FREE) did not matter, because the thread calling
> __inet_hash() owns a reference on sk. SOCK_RCU_FREE was only tested
> at dismantle time.
> 
> Commit 6acc9b432e67 ("bpf: Add helper to retrieve socket in BPF")
> started checking SOCK_RCU_FREE _after_ the lookup to infer whether
> the refcount has been taken care of.
> 
> Fixes: 6acc9b432e67 ("bpf: Add helper to retrieve socket in BPF")
> Reviewed-by: Eric Dumazet <edumazet@google.com>
> Signed-off-by: Stanislav Fomichev <sdf@google.com>
> Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
> Signed-off-by: David S. Miller <davem@davemloft.net>
> [Resolved conflict for 5.10 and below.]
> Signed-off-by: Siddh Raman Pant <siddh.raman.pant@oracle.com>
> ---
>  net/ipv4/inet_hashtables.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Now  queued up, thanks.

greg k-h