Subject: Re: [bpf PATCH v2 1/3] bpf, sockmap: RCU splat with redirect and strparser error or TLS

From: Jakub Sitnicki <[email-masked]>
Date: Fri, 26 Jun 2020 13:11:44 +0200

On Fri, Jun 26, 2020 at 01:12 AM CEST, John Fastabend wrote:
> There are two paths to generate the below RCU splat the first and
> most obvious is the result of the BPF verdict program issuing a
> redirect on a TLS socket (This is the splat shown below). Unlike
> the non-TLS case the caller of the *strp_read() hooks does not
> wrap the call in a rcu_read_lock/unlock. Then if the BPF program
> issues a redirect action we hit the RCU splat.
>
> However, in the non-TLS socket case the splat appears to be
> relatively rare, because the skmsg caller into the strp_data_ready()
> is wrapped in a rcu_read_lock/unlock. Shown here,
>
>  static void sk_psock_strp_data_ready(struct sock *sk)
>  {
> 	struct sk_psock *psock;
>
> 	rcu_read_lock();
> 	psock = sk_psock(sk);
> 	if (likely(psock)) {
> 		if (tls_sw_has_ctx_rx(sk)) {
> 			psock->parser.saved_data_ready(sk);
> 		} else {
> 			write_lock_bh(&sk->sk_callback_lock);
> 			strp_data_ready(&psock->parser.strp);
> 			write_unlock_bh(&sk->sk_callback_lock);
> 		}
> 	}
> 	rcu_read_unlock();
>  }
>
> If the above was the only way to run the verdict program we
> would be safe. But, there is a case where the strparser may throw an
> ENOMEM error while parsing the skb. This is a result of a failed
> skb_clone, or alloc_skb_for_msg while building a new merged skb when
> the msg length needed spans multiple skbs. This will in turn put the
> skb on the strp_wrk workqueue in the strparser code. The skb will
> later be dequeued and verdict programs run, but now from a
> different context without the rcu_read_lock()/unlock() critical
> section in sk_psock_strp_data_ready() shown above. In practice
> I have not seen this yet, because as far as I know most users of the
> verdict programs are also only working on single skbs. In this case no
> merge happens which could trigger the above ENOMEM errors. In addition
> the system would need to be under memory pressure. For example, we
> can't hit the above case in selftests because we missed having tests
> to merge skbs. (Added in later patch)
>
> To fix the below splat extend the rcu_read_lock/unnlock block to
> include the call to sk_psock_tls_verdict_apply(). This will fix both
> TLS redirect case and non-TLS redirect+error case. Also remove
> psock from the sk_psock_tls_verdict_apply() function signature its
> not used there.
>
> [ 1095.937597] WARNING: suspicious RCU usage
> [ 1095.940964] 5.7.0-rc7-02911-g463bac5f1ca79 #1 Tainted: G        W
> [ 1095.944363] -----------------------------
> [ 1095.947384] include/linux/skmsg.h:284 suspicious rcu_dereference_check() usage!
> [ 1095.950866]
> [ 1095.950866] other info that might help us debug this:
> [ 1095.950866]
> [ 1095.957146]
> [ 1095.957146] rcu_scheduler_active = 2, debug_locks = 1
> [ 1095.961482] 1 lock held by test_sockmap/15970:
> [ 1095.964501]  #0: ffff9ea6b25de660 (sk_lock-AF_INET){+.+.}-{0:0}, at: tls_sw_recvmsg+0x13a/0x840 [tls]
> [ 1095.968568]
> [ 1095.968568] stack backtrace:
> [ 1095.975001] CPU: 1 PID: 15970 Comm: test_sockmap Tainted: G        W         5.7.0-rc7-02911-g463bac5f1ca79 #1
> [ 1095.977883] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
> [ 1095.980519] Call Trace:
> [ 1095.982191]  dump_stack+0x8f/0xd0
> [ 1095.984040]  sk_psock_skb_redirect+0xa6/0xf0
> [ 1095.986073]  sk_psock_tls_strp_read+0x1d8/0x250
> [ 1095.988095]  tls_sw_recvmsg+0x714/0x840 [tls]
>
> v2: Improve commit message to identify non-TLS redirect plus error case
>     condition as well as more common TLS case. In the process I decided
>     doing the rcu_read_unlock followed by the lock/unlock inside branches
>     was unnecessarily complex. We can just extend the current rcu block
>     and get the same effeective without the shuffling and branching.
>     Thanks Martin!
>
> Fixes: e91de6afa81c1 ("bpf: Fix running sk_skb program types with ktls")
> Reported-by: Jakub Sitnicki <[email-masked]>
> Reported-by: kernel test robot <[email-masked]>
> Signed-off-by: John Fastabend <[email-masked]>
> ---

Thanks for the detailed explanation.

Acked-by: Jakub Sitnicki <[email-masked]>

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