4.20-stable review patch. If anyone has any objections, please let me know.
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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 @@ -852,10 +852,6 @@ 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 { @@ -872,6 +868,13 @@ 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; }