netdev - Re: [PATCH net v3 3/4] ip6mr: Lock RCU before ip6mr_get_table() call in ip6mr_compat

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Message-ID: <cc3c8919-c56f-4a78-80ec-afe7fb028061@nokia.com>
Date: Mon, 14 Oct 2024 17:05:31 +0200
From: Stefan Wiehler <stefan.wiehler@...ia.com>
To: Simon Horman <horms@...nel.org>
Cc: "David S . Miller" <davem@...emloft.net>, David Ahern
 <dsahern@...nel.org>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net v3 3/4] ip6mr: Lock RCU before ip6mr_get_table() call
 in ip6mr_compat_ioctl()

>>>> When IPV6_MROUTE_MULTIPLE_TABLES is enabled, calls to ip6mr_get_table()
>>>> must be done under RCU or RTNL lock. Copy from user space must be
>>>> performed beforehand as we are not allowed to sleep under RCU lock.
>>>>
>>>> Signed-off-by: Stefan Wiehler <stefan.wiehler@...ia.com>
>>>> Fixes: d1db275dd3f6 ("ipv6: ip6mr: support multiple tables")
> 
> ...
> 
>>>> @@ -2004,11 +2020,13 @@ int ip6mr_compat_ioctl(struct sock *sk, unsigned int cmd, void __user *arg)
>>>>                               return -EFAULT;
>>>>                       return 0;
>>>>               }
>>>> -             rcu_read_unlock();
>>>> -             return -EADDRNOTAVAIL;
>>>> -     default:
>>>> -             return -ENOIOCTLCMD;
>>>> +             err = -EADDRNOTAVAIL;
>>>> +             goto out;
>>>>       }
>>>> +
>>>
>>> I think that this out label should be used consistently once rcu_read_lock
>>> has been taken. With this patch applied there seems to be one case on error
>>> where rcu_read_unlock() before returning, and one case where it isn't
>>> (which looks like it leaks the lock).
>>
>> In the remaining two return paths we need to release the RCU lock before
>> calling copy_to_user(), so unfortunately we cannot use a common out label.
> 
> Ok, sure. But can you check that the code always releases the lock?

Yes, it should release the lock in all cases.

Kind regards,

Stefan