udp: ipv4: fix an use after free in __udp4_lib_rcv()

Dave Jones reported a use after free in UDP stack :

[ 5059.434216] =========================
[ 5059.434314] [ BUG: held lock freed! ]
[ 5059.434420] 3.13.0-rc3+ #9 Not tainted
[ 5059.434520] -------------------------
[ 5059.434620] named/863 is freeing memory ffff88005e960000-ffff88005e96061f, with a lock still held there!
[ 5059.434815]  (slock-AF_INET){+.-...}, at: [<ffffffff8149bd21>] udp_queue_rcv_skb+0xd1/0x4b0
[ 5059.435012] 3 locks held by named/863:
[ 5059.435086]  #0:  (rcu_read_lock){.+.+..}, at: [<ffffffff8143054d>] __netif_receive_skb_core+0x11d/0x940
[ 5059.435295]  #1:  (rcu_read_lock){.+.+..}, at: [<ffffffff81467a5e>] ip_local_deliver_finish+0x3e/0x410
[ 5059.435500]  #2:  (slock-AF_INET){+.-...}, at: [<ffffffff8149bd21>] udp_queue_rcv_skb+0xd1/0x4b0
[ 5059.435734]
stack backtrace:
[ 5059.435858] CPU: 0 PID: 863 Comm: named Not tainted 3.13.0-rc3+ #9 [loadavg: 0.21 0.06 0.06 1/115 1365]
[ 5059.436052] Hardware name:                  /D510MO, BIOS MOPNV10J.86A.0175.2010.0308.0620 03/08/2010
[ 5059.436223]  0000000000000002 ffff88007e203ad8 ffffffff8153a372 ffff8800677130e0
[ 5059.436390]  ffff88007e203b10 ffffffff8108cafa ffff88005e960000 ffff88007b00cfc0
[ 5059.436554]  ffffea00017a5800 ffffffff8141c490 0000000000000246 ffff88007e203b48
[ 5059.436718] Call Trace:
[ 5059.436769]  <IRQ>  [<ffffffff8153a372>] dump_stack+0x4d/0x66
[ 5059.436904]  [<ffffffff8108cafa>] debug_check_no_locks_freed+0x15a/0x160
[ 5059.437037]  [<ffffffff8141c490>] ? __sk_free+0x110/0x230
[ 5059.437147]  [<ffffffff8112da2a>] kmem_cache_free+0x6a/0x150
[ 5059.437260]  [<ffffffff8141c490>] __sk_free+0x110/0x230
[ 5059.437364]  [<ffffffff8141c5c9>] sk_free+0x19/0x20
[ 5059.437463]  [<ffffffff8141cb25>] sock_edemux+0x25/0x40
[ 5059.437567]  [<ffffffff8141c181>] sock_queue_rcv_skb+0x81/0x280
[ 5059.437685]  [<ffffffff8149bd21>] ? udp_queue_rcv_skb+0xd1/0x4b0
[ 5059.437805]  [<ffffffff81499c82>] __udp_queue_rcv_skb+0x42/0x240
[ 5059.437925]  [<ffffffff81541d25>] ? _raw_spin_lock+0x65/0x70
[ 5059.438038]  [<ffffffff8149bebb>] udp_queue_rcv_skb+0x26b/0x4b0
[ 5059.438155]  [<ffffffff8149c712>] __udp4_lib_rcv+0x152/0xb00
[ 5059.438269]  [<ffffffff8149d7f5>] udp_rcv+0x15/0x20
[ 5059.438367]  [<ffffffff81467b2f>] ip_local_deliver_finish+0x10f/0x410
[ 5059.438492]  [<ffffffff81467a5e>] ? ip_local_deliver_finish+0x3e/0x410
[ 5059.438621]  [<ffffffff81468653>] ip_local_deliver+0x43/0x80
[ 5059.438733]  [<ffffffff81467f70>] ip_rcv_finish+0x140/0x5a0
[ 5059.438843]  [<ffffffff81468926>] ip_rcv+0x296/0x3f0
[ 5059.438945]  [<ffffffff81430b72>] __netif_receive_skb_core+0x742/0x940
[ 5059.439074]  [<ffffffff8143054d>] ? __netif_receive_skb_core+0x11d/0x940
[ 5059.442231]  [<ffffffff8108c81d>] ? trace_hardirqs_on+0xd/0x10
[ 5059.442231]  [<ffffffff81430d83>] __netif_receive_skb+0x13/0x60
[ 5059.442231]  [<ffffffff81431c1e>] netif_receive_skb+0x1e/0x1f0
[ 5059.442231]  [<ffffffff814334e0>] napi_gro_receive+0x70/0xa0
[ 5059.442231]  [<ffffffffa01de426>] rtl8169_poll+0x166/0x700 [r8169]
[ 5059.442231]  [<ffffffff81432bc9>] net_rx_action+0x129/0x1e0
[ 5059.442231]  [<ffffffff810478cd>] __do_softirq+0xed/0x240
[ 5059.442231]  [<ffffffff81047e25>] irq_exit+0x125/0x140
[ 5059.442231]  [<ffffffff81004241>] do_IRQ+0x51/0xc0
[ 5059.442231]  [<ffffffff81542bef>] common_interrupt+0x6f/0x6f

We need to keep a reference on the socket, by using skb_steal_sock()
at the right place.

Note that another patch is needed to fix a race in
udp_sk_rx_dst_set(), as we hold no lock protecting the dst.

Fixes: 421b3885 ("udp: ipv4: Add udp early demux")
Reported-by: Dave Jones <davej@redhat.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Shawn Bohrer <sbohrer@rgmadvisors.com>
Signed-off-by: David S. Miller <davem@davemloft.net>