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Subject[PATCH 4.19 24/99] tun: publish tfile after its fully initialized
Date
4.19-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Stanislav Fomichev <sdf@google.com>

[ Upstream commit 0b7959b6257322f7693b08a459c505d4938646f2 ]

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: 90e33d459407 ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/net/tun.c | 11 +++++++----
1 file changed, 7 insertions(+), 4 deletions(-)

--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -859,10 +859,6 @@ static int tun_attach(struct tun_struct
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
* 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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