Commit 0b7959b6 authored Jan 07, 2019 by Stanislav Fomichev Committed by David S. Miller Jan 10, 2019

tun: publish tfile after it's fully initialized



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()

Move rcu_assign_pointer(tfile->tun) and rcu_assign_pointer(tun->tfiles) 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.

v2 changes:
* remove extra napi_mutex locks/unlocks for napi operations

Reported-by: syzbot <syzkaller@googlegroups.com>
Fixes: 90e33d45 ("tun: enable napi_gro_frags() for TUN/TAP driver")

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent d972f3dc

drivers/net/tun.c

+7 −4

Original line number	Diff line number	Diff line
		@@ -856,10 +856,6 @@ 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++;

		if (tfile->detached) {
		tun_enable_queue(tfile);
		} else {
		@@ -876,6 +872,13 @@ static int tun_attach(struct tun_struct tun, struct file file,
		* refcnt.
		*/

		/* Publish tfile->tun and tun->tfiles only after we've fully
		* initialized tfile; otherwise we risk using half-initialized
		* object.
		*/
		rcu_assign_pointer(tfile->tun, tun);
		rcu_assign_pointer(tun->tfiles[tun->numqueues], tfile);
		tun->numqueues++;
		out:
		return err;
		}

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