netdev - Re: [PATCH 1/5] tcp/md5: Don't BUG

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Date:   Thu, 4 Nov 2021 19:55:19 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Dmitry Safonov <dima@...sta.com>, linux-kernel@...r.kernel.org
Cc:     Dmitry Safonov <0x7f454c46@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        David Ahern <dsahern@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Francesco Ruggeri <fruggeri@...sta.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Leonard Crestez <cdleonard@...il.com>,
        linux-crypto@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH 1/5] tcp/md5: Don't BUG_ON() failed kmemdup()



On 11/4/21 6:49 PM, Dmitry Safonov wrote:
> static_branch_unlikely(&tcp_md5_needed) is enabled by
> tcp_alloc_md5sig_pool(), so as long as the code doesn't change
> tcp_md5sig_pool has been already populated if this code is being
> executed.
> 
> In case tcptw->tw_md5_key allocaion failed - no reason to crash kernel:
> tcp_{v4,v6}_send_ack() will send unsigned segment, the connection won't be
> established, which is bad enough, but in OOM situation totally
> acceptable and better than kernel crash.
> 
> Introduce tcp_md5sig_pool_ready() helper.
> tcp_alloc_md5sig_pool() usage is intentionally avoided here as it's
> fast-path here and it's check for sanity rather than point of actual
> pool allocation. That will allow to have generic slow-path allocator
> for tcp crypto pool.
> 
> Signed-off-by: Dmitry Safonov <dima@...sta.com>
> ---
>  include/net/tcp.h        | 1 +
>  net/ipv4/tcp.c           | 5 +++++
>  net/ipv4/tcp_minisocks.c | 5 +++--
>  3 files changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 4da22b41bde6..3e5423a10a74 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -1672,6 +1672,7 @@ tcp_md5_do_lookup(const struct sock *sk, int l3index,
>  #endif
>  
>  bool tcp_alloc_md5sig_pool(void);
> +bool tcp_md5sig_pool_ready(void);
>  
>  struct tcp_md5sig_pool *tcp_get_md5sig_pool(void);
>  static inline void tcp_put_md5sig_pool(void)
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index b7796b4cf0a0..c0856a6af9f5 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -4314,6 +4314,11 @@ bool tcp_alloc_md5sig_pool(void)
>  }
>  EXPORT_SYMBOL(tcp_alloc_md5sig_pool);
>  
> +bool tcp_md5sig_pool_ready(void)
> +{
> +	return tcp_md5sig_pool_populated;
> +}
> +EXPORT_SYMBOL(tcp_md5sig_pool_ready);
>  
>  /**
>   *	tcp_get_md5sig_pool - get md5sig_pool for this user
> diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
> index cf913a66df17..c99cdb529902 100644
> --- a/net/ipv4/tcp_minisocks.c
> +++ b/net/ipv4/tcp_minisocks.c
> @@ -293,11 +293,12 @@ void tcp_time_wait(struct sock *sk, int state, int timeo)
>  			tcptw->tw_md5_key = NULL;
>  			if (static_branch_unlikely(&tcp_md5_needed)) {
>  				struct tcp_md5sig_key *key;
> +				bool err = WARN_ON(!tcp_md5sig_pool_ready());
>  
>  				key = tp->af_specific->md5_lookup(sk, sk);
> -				if (key) {
> +				if (key && !err) {
>  					tcptw->tw_md5_key = kmemdup(key, sizeof(*key), GFP_ATOMIC);
> -					BUG_ON(tcptw->tw_md5_key && !tcp_alloc_md5sig_pool());
> +					WARN_ON_ONCE(tcptw->tw_md5_key == NULL);
>  				}
>  			}
>  		} while (0);
> 

Hmmm.... how this BUG_ON() could trigger exactly ?

tcp_md5_needed can only be enabled after __tcp_alloc_md5sig_pool has succeeded.

This patch, sent during merge-window, is a distraction, sorry.

About renaming : It looks nice, but is a disaster for backports
done for stable releases. Please refrain from doing this.