netdev - Re: [PATCH net] tun, bpf: fix suspicious RCU usage in tun_{attach,detach}

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Date:	Wed, 30 Mar 2016 22:08:10 -0700
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Michal Kubecek <mkubecek@...e.cz>
Cc:	Daniel Borkmann <daniel@...earbox.net>, davem@...emloft.net,
	sasha.levin@...cle.com, jslaby@...e.cz, eric.dumazet@...il.com,
	mst@...hat.com, netdev@...r.kernel.org
Subject: Re: [PATCH net] tun, bpf: fix suspicious RCU usage in
 tun_{attach,detach}_filter

On Thu, Mar 31, 2016 at 07:01:15AM +0200, Michal Kubecek wrote:
> On Wed, Mar 30, 2016 at 06:18:42PM -0700, Alexei Starovoitov wrote:
> > On Thu, Mar 31, 2016 at 02:13:18AM +0200, Daniel Borkmann wrote:
> > > Sasha Levin reported a suspicious rcu_dereference_protected() warning
> > > found while fuzzing with trinity that is similar to this one:
> > > 
> > >   [   52.765684] net/core/filter.c:2262 suspicious rcu_dereference_protected() usage!
> > >   [   52.765688] other info that might help us debug this:
> > >   [   52.765695] rcu_scheduler_active = 1, debug_locks = 1
> > >   [   52.765701] 1 lock held by a.out/1525:
> > >   [   52.765704]  #0:  (rtnl_mutex){+.+.+.}, at: [<ffffffff816a64b7>] rtnl_lock+0x17/0x20
> > >   [   52.765721] stack backtrace:
> > >   [   52.765728] CPU: 1 PID: 1525 Comm: a.out Not tainted 4.5.0+ #264
> > >   [...]
> > >   [   52.765768] Call Trace:
> > >   [   52.765775]  [<ffffffff813e488d>] dump_stack+0x85/0xc8
> > >   [   52.765784]  [<ffffffff810f2fa5>] lockdep_rcu_suspicious+0xd5/0x110
> > >   [   52.765792]  [<ffffffff816afdc2>] sk_detach_filter+0x82/0x90
> > >   [   52.765801]  [<ffffffffa0883425>] tun_detach_filter+0x35/0x90 [tun]
> > >   [   52.765810]  [<ffffffffa0884ed4>] __tun_chr_ioctl+0x354/0x1130 [tun]
> > >   [   52.765818]  [<ffffffff8136fed0>] ? selinux_file_ioctl+0x130/0x210
> > >   [   52.765827]  [<ffffffffa0885ce3>] tun_chr_ioctl+0x13/0x20 [tun]
> > >   [   52.765834]  [<ffffffff81260ea6>] do_vfs_ioctl+0x96/0x690
> > >   [   52.765843]  [<ffffffff81364af3>] ? security_file_ioctl+0x43/0x60
> > >   [   52.765850]  [<ffffffff81261519>] SyS_ioctl+0x79/0x90
> > >   [   52.765858]  [<ffffffff81003ba2>] do_syscall_64+0x62/0x140
> > >   [   52.765866]  [<ffffffff817d563f>] entry_SYSCALL64_slow_path+0x25/0x25
> > > 
> > > Same can be triggered with PROVE_RCU (+ PROVE_RCU_REPEATEDLY) enabled
> > > from tun_attach_filter() when user space calls ioctl(tun_fd, TUN{ATTACH,
> > > DETACH}FILTER, ...) for adding/removing a BPF filter on tap devices.
> > > 
> > > Since the fix in f91ff5b9ff52 ("net: sk_{detach|attach}_filter() rcu
> > > fixes") sk_attach_filter()/sk_detach_filter() now dereferences the
> > > filter with rcu_dereference_protected(), checking whether socket lock
> > > is held in control path.
> > > 
> > > Since its introduction in 994051625981 ("tun: socket filter support"),
> > > tap filters are managed under RTNL lock from __tun_chr_ioctl(). Thus the
> > > sock_owned_by_user(sk) doesn't apply in this specific case and therefore
> > > triggers the false positive.
> > > 
> > > Extend the BPF API with __sk_attach_filter()/__sk_detach_filter() pair
> > > that is used by tap filters and pass in lockdep_rtnl_is_held() for the
> > > rcu_dereference_protected() checks instead.
> > > 
> > > Reported-by: Sasha Levin <sasha.levin@...cle.com>
> > > Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
> > > ---
> > >  drivers/net/tun.c      |  8 +++++---
> > >  include/linux/filter.h |  4 ++++
> > >  net/core/filter.c      | 33 +++++++++++++++++++++------------
> > >  3 files changed, 30 insertions(+), 15 deletions(-)
> > 
> > kinda heavy patch to shut up lockdep.
> > Can we do
> > old_fp = rcu_dereference_protected(sk->sk_filter,
> >                                 sock_owned_by_user(sk) || lockdep_rtnl_is_held());
> > and it always be correct?
> > I think right now tun is the only such user, but if it's correct for tun,
> > it's correct for future users too. If not correct then not correct for tun either.
> > Or I'm missing something?
> 
> Already discussed here:
> 
>   http://thread.gmane.org/gmane.linux.kernel/2158069/focus=405853

I saw that. My point above was challenging 'less accurate' part.