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

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Date:   Fri, 5 Nov 2021 11:16:32 +0200
From:   Leonard Crestez <cdleonard@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        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>,
        linux-crypto@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH 1/5] tcp/md5: Don't BUG_ON() failed kmemdup()

On 11/5/21 4:55 AM, Eric Dumazet wrote:
> 
> 
> 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.

The tcp_alloc_md5_pool here should just be removed, it no longer does 
anything since md5 pool reference counting and freeing were removed back 
in 2013 by commit 71cea17ed39f ("tcp: md5: remove spinlock usage in fast 
path").