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.le...@oracle.com>
> > > Signed-off-by: Daniel Borkmann <dan...@iogearbox.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.
