Re: [PATCH net-next-2.6] netlink: netlink_recvmsg() fix

[Eric Dumazet <eric.dumazet@gmail.com>]

Le vendredi 13 août 2010 à 16:35 +0200, Johannes Berg a écrit :
> On Fri, 2010-08-13 at 16:00 +0200, Johannes Berg wrote:
> > On Sun, 2010-07-25 at 21:55 -0700, David Miller wrote:
> > > From: Eric Dumazet <eric.dumazet@gmail.com>
> > > Date: Wed, 21 Jul 2010 10:43:55 +0200
> > > 
> > > > [PATCH net-next-2.6 v3] netlink: netlink_recvmsg() fix
> > > > 
> > > > commit 1dacc76d0014 
> > > > (net/compat/wext: send different messages to compat tasks)
> > > > introduced a race condition on netlink, in case MSG_PEEK is used.
> > > > 
> > > > An skb given by skb_recv_datagram() might be shared, we must copy it
> > > > before any modification, or risk fatal corruption.
> > > > 
> > > > Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
> > > 
> > > Applied, thanks Eric.
> > 
> > I keep getting errors like below in 2.6.35+wireless-testing. Not saying
> > that it's this patch's fault, but it is the only thing I remember
> > touching that area.
> 
> In fact, I think something's wrong with this patch, since my comment
> (that right now unfortunately I no longer fully understand) says:
> 
>                  * If this skb has a frag_list, then here that means that
>                  * we will have to use the frag_list skb for compat tasks
>                  * and the regular skb for non-compat tasks.
>                  *
>                  * The skb might (and likely will) be cloned, so we can't
>                  * just reset frag_list and go on with things -- we need to
>                  * keep that. For the compat case that's easy -- simply get
>                  * a reference to the compat skb and free the regular one
>                  * including the frag. For the non-compat case, we need to
>                  * avoid sending the frag to the user -- so assign NULL but
>                  * restore it below before freeing the skb.
> 
> and that's no longer true, afaict.
> 

Comment was not updated by the patch.

But do you agree temporarly setting frag_list to NULL was a bug ?

Unfortunatly I cannot test this path...

I assume reverting 1235f504aaba removes these errors ?


Its strange we have a double-free on a data part and not a skb_head.

maybe pskb_copy() has a problem with frag_list...


Maybe we can revert the patch and find another way to make sure two
process can not manipulate this skb in // (adding a mutex, or using
RTNL ?)