Subject: Re: [PATCH net 1/2] tun: hold napi_mutex for all napi operations

From: Stanislav Fomichev <[email-masked]>
Date: Mon, 7 Jan 2019 13:02:31 -0800

On 01/07, Eric Dumazet wrote:
> On Mon, Jan 7, 2019 at 12:02 PM Stanislav Fomichev <[email-masked]> wrote:
> >
> > BUG: unable to handle kernel NULL pointer dereference at 00000000000000d1
> > Call Trace:
> >  ? napi_gro_frags+0xa7/0x2c0
> >  tun_get_user+0xb50/0xf20
> >  tun_chr_write_iter+0x53/0x70
> >  new_sync_write+0xff/0x160
> >  vfs_write+0x191/0x1e0
> >  __x64_sys_write+0x5e/0xd0
> >  do_syscall_64+0x47/0xf0
> >  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> >
> > I think there is a subtle race between sending a packet via tap and
> > attaching it:
> >
> > CPU0:                    CPU1:
> > tun_chr_ioctl(TUNSETIFF)
> >   tun_set_iff
> >     tun_attach
> >       rcu_assign_pointer(tfile->tun, tun);
> >                          tun_fops->write_iter()
> >                            tun_chr_write_iter
> >                              tun_napi_alloc_frags
> >                                napi_get_frags
> >                                  napi->skb = napi_alloc_skb
> >       tun_napi_init
> >         netif_napi_add
> >           napi->skb = NULL
> >                               napi->skb is NULL here
> >                               napi_gro_frags
> >                                 napi_frags_skb
> >                                   skb = napi->skb
> >                                   skb_reset_mac_header(skb)
> >                                   panic()
> >
> > To fix, do the following:
> > * Move rcu_assign_pointer(tfile->tun, tun) to be the last thing we do
> >   in tun_attach(); this should guarantee that when we call tun_get()
> >   we always get an initialized object
> > * As another safeguard, always grab napi_mutex whenever doing any
> >   napi operation; this should prevent napi state change between
> >   calls to napi_get_frags and napi_gro_frags
> >
> > Reported-by: syzbot <[email-masked]>
> > Fixes: 90e33d459407 ("tun: enable napi_gro_frags() for TUN/TAP driver")
> >
> > Signed-off-by: Stanislav Fomichev <[email-masked]>
> > ---
> >  drivers/net/tun.c | 18 +++++++++++++++---
> >  1 file changed, 15 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> > index a4fdad475594..7875f06011f2 100644
> > --- a/drivers/net/tun.c
> > +++ b/drivers/net/tun.c
> > @@ -323,22 +323,30 @@ static void tun_napi_init(struct tun_struct *tun, struct tun_file *tfile,
> >         tfile->napi_enabled = napi_en;
> >         tfile->napi_frags_enabled = napi_en && napi_frags;
> >         if (napi_en) {
> > +               mutex_lock(&tfile->napi_mutex);
> >                 netif_napi_add(tun->dev, &tfile->napi, tun_napi_poll,
> >                                NAPI_POLL_WEIGHT);
> >                 napi_enable(&tfile->napi);
> > +               mutex_unlock(&tfile->napi_mutex);
> >         }
> >  }
> >
> >  static void tun_napi_disable(struct tun_file *tfile)
> >  {
> > -       if (tfile->napi_enabled)
> > +       if (tfile->napi_enabled) {
> > +               mutex_lock(&tfile->napi_mutex);
> >                 napi_disable(&tfile->napi);
> > +               mutex_unlock(&tfile->napi_mutex);
> > +       }
> >  }
> >
> >  static void tun_napi_del(struct tun_file *tfile)
> >  {
> > -       if (tfile->napi_enabled)
> > +       if (tfile->napi_enabled) {
> > +               mutex_lock(&tfile->napi_mutex);
> >                 netif_napi_del(&tfile->napi);
> > +               mutex_unlock(&tfile->napi_mutex);
> > +       }
> >  }
> >
> >  static bool tun_napi_frags_enabled(const struct tun_file *tfile)
> > @@ -856,7 +864,6 @@ static int tun_attach(struct tun_struct *tun, struct file *file,
> >                 err = 0;
> >         }
> >
> > -       rcu_assign_pointer(tfile->tun, tun);
> >         rcu_assign_pointer(tun->tfiles[tun->numqueues], tfile);
> >         tun->numqueues++;
> >
> > @@ -876,6 +883,11 @@ static int tun_attach(struct tun_struct *tun, struct file *file,
> >          * refcnt.
> >          */
> >
> > +       /* All tun_fops depend on tun_get() returning non-null pointer.
> > +        * Thus, assigning tun to a tfile should be the last init operation,
> > +        * otherwise we risk using half-initialized object.
> > +        */
> > +       rcu_assign_pointer(tfile->tun, tun);
> >  out:
> >         return err;
> >  }
> 
> Hmmm I believe the issue is different : We need to call
> tun_napi_init() before doing the publish in the tun->tfiles[] array
> 
> My patch was :
Still fails with your patch.

Maybe the best way is to move both of those publishes (tfile->tun and
tun->tfiles[]) to the end of tun_attach? It looks like tfile->tun
is a publish for syscall side (most of tun_socket_ops and tun_fops call
tun_get which looks at tun->dev and bail out if it's NULL) and tun->tfiles
is (mostly, but not really) for napi side.

Here is a repro I'm using if you want to poke it:

syz-execprog -threaded -collide -repeat=0 -procs=6 <rep.syz>

# See https://goo.gl/kgGztJ for information about syzkaller reproducers.
#{"threaded":true,"collide":true,"repeat":true,"procs":6,"sandbox":"none","fault_call":-1,"tun":true,"tmpdir":true,"cgroups":true,"netdev":true,"resetnet":true,"segv":true}
openat$tun(0xffffffffffffff9c, 0x0, 0x0, 0x0)
r0 = openat$tun(0xffffffffffffff9c,
&(0x7f0000001a00)='/dev/net/tun\x00', 0x0, 0x0)
ioctl$TUNSETIFF(r0, 0x400454ca, &(0x7f0000000300)={"6e72300100",
0x1132})
r1 = socket$kcm(0x2, 0x3, 0x2)
ioctl$PERF_EVENT_IOC_SET_FILTER(r1, 0x8914,
&(0x7f0000000780)="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
 d536a95d96f03d48596e008bf0a028b539cec796cec9bf585eb80fe3e0d26")
perf_event_open$cgroup(&(0x7f0000000440)={0x7, 0x70, 0x1, 0x0, 0x6e36,
0x38000000, 0x0, 0x1, 0x21060, 0x0, 0x0, 0x0, 0x20, 0x3, 0xfc50, 0x9,
0x8000, 0x0, 0x0, 0x0, 0x46c8, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
0x0, 0x0, 0x8, 0x3f, 0x401, 0x8001, 0x8, 0x9, 0x0, 0x0, 0x1, 0x1,
@perf_config_ext={0x100000000, 0x6}, 0x10, 0x5, 0x2, 0x7,
0xfffffffffffffff9, 0x0, 0x7}, 0xffffffffffffffff, 0x5,
0xffffffffffffffff, 0x0)
# 0x2 = O_RDWR
r2 = openat$tun(0xffffffffffffff9c,
&(0x7f0000001a00)='/dev/net/tun\x00', 0x2, 0x0)
#  IFF_TAP         0x0002
#  IFF_NAPI        0x0010
#  IFF_NAPI_FRAGS  0x0020
#  IFF_MULTI_QUEUE 0x0100
#  IFF_MULTICAST = 0x1000
ioctl$TUNSETIFF(r2, 0x400454ca, &(0x7f0000000300)={"6e72300100",
0x1132})
write$cgroup_int(r2, &(0x7f0000000000), 0x17b)

> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index a4fdad47559462fbd049a89f880cc3fb33d1151d..dc751d1cbc21a2e2687c5739b44322cd64d0cb46
> 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -857,15 +857,15 @@ static int tun_attach(struct tun_struct *tun,
> struct file *file,
>         }
> 
>         rcu_assign_pointer(tfile->tun, tun);
> +       if (!tfile->detached) {
> +               tun_napi_init(tun, tfile, napi, napi_frags);
> +               sock_hold(&tfile->sk);
> +       }
>         rcu_assign_pointer(tun->tfiles[tun->numqueues], tfile);
>         tun->numqueues++;
> 
> -       if (tfile->detached) {
> +       if (tfile->detached)
>                 tun_enable_queue(tfile);
> -       } else {
> -               sock_hold(&tfile->sk);
> -               tun_napi_init(tun, tfile, napi, napi_frags);
> -       }
> 
>         if (rtnl_dereference(tun->xdp_prog))
>                 sock_set_flag(&tfile->sk, SOCK_XDP);