netdev - Re: [PATCH v2 net] udp: Set SOCK_RCU_FREE earlier in udp_lib_get

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Message-ID: <CANn89i+pijHpmoKNXm+d+Acs=RVp_wMTKJKhR89R2FOEFaYv-w@mail.gmail.com>
Date: Tue, 9 Jul 2024 12:51:41 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: Kuniyuki Iwashima <kuniyu@...zon.com>
Cc: "David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
	David Ahern <dsahern@...nel.org>, Joe Stringer <joe@...d.net.nz>, 
	Kuniyuki Iwashima <kuni1840@...il.com>, netdev@...r.kernel.org, 
	syzkaller <syzkaller@...glegroups.com>
Subject: Re: [PATCH v2 net] udp: Set SOCK_RCU_FREE earlier in udp_lib_get_port().

On Tue, Jul 9, 2024 at 12:14 PM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
>
> syzkaller triggered the warning [0] in udp_v4_early_demux().
>
> In udp_v[46]_early_demux() and sk_lookup(), we do not touch the refcount
> of the looked-up sk and use sock_pfree() as skb->destructor, so we check
> SOCK_RCU_FREE to ensure that the sk is safe to access during the RCU grace
> period.
>
> Currently, SOCK_RCU_FREE is flagged for a bound socket after being put
> into the hash table.  Moreover, the SOCK_RCU_FREE check is done too early
> in udp_v[46]_early_demux() and sk_lookup(), so there could be a small race
> window:
>
>   CPU1                                 CPU2
>   ----                                 ----
>   udp_v4_early_demux()                 udp_lib_get_port()
>   |                                    |- hlist_add_head_rcu()
>   |- sk = __udp4_lib_demux_lookup()    |
>   |- DEBUG_NET_WARN_ON_ONCE(sk_is_refcounted(sk));
>                                        `- sock_set_flag(sk, SOCK_RCU_FREE)
>
> We had the same bug in TCP and fixed it in commit 871019b22d1b ("net:
> set SOCK_RCU_FREE before inserting socket into hashtable").
>
> Let's apply the same fix for UDP.
>
>

> Fixes: 6acc9b432e67 ("bpf: Add helper to retrieve socket in BPF")
> Reported-by: syzkaller <syzkaller@...glegroups.com>
> Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>

Reviewed-by: Eric Dumazet <edumazet@...gle.com>

Thanks !