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

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Message-ID: <20241011101623.GF66815@kernel.org>
Date: Fri, 11 Oct 2024 11:16:23 +0100
From: Simon Horman <horms@...nel.org>
To: Stefan Wiehler <stefan.wiehler@...ia.com>
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()

On Thu, Oct 10, 2024 at 04:43:34PM +0200, Stefan Wiehler wrote:
> >> 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?