tuntap: dont use skb after netif_rx_ni(skb)

On Wed, 2012-12-12 at 23:16 -0500, Dave Jones wrote:
> Since todays net merge, I see this when I start openvpn..
>
> general protection fault: 0000 [#1] PREEMPT SMP
> Modules linked in: ip6t_REJECT nf_conntrack_ipv6 nf_defrag_ipv6 xt_conntrack nf_conntrack ip6table_filter ip6_tables xfs iTCO_wdt iTCO_vendor_support snd_emu10k1 snd_util_mem snd_ac97_codec coretemp ac97_bus microcode snd_hwdep snd_seq pcspkr snd_pcm snd_page_alloc snd_timer lpc_ich i2c_i801 snd_rawmidi mfd_core snd_seq_device snd e1000e soundcore emu10k1_gp gameport i82975x_edac edac_core vhost_net tun macvtap macvlan kvm_intel kvm binfmt_misc nfsd auth_rpcgss nfs_acl lockd sunrpc btrfs libcrc32c zlib_deflate firewire_ohci sata_sil firewire_core crc_itu_t radeon i2c_algo_bit drm_kms_helper ttm drm i2c_core floppy
> CPU 0
> Pid: 1381, comm: openvpn Not tainted 3.7.0+ #14                  /D975XBX
> RIP: 0010:[<ffffffff815b54a4>]  [<ffffffff815b54a4>] skb_flow_dissect+0x314/0x3e0
> RSP: 0018:ffff88007d0d9c48  EFLAGS: 00010206
> RAX: 000000000000055d RBX: 6b6b6b6b6b6b6b4b RCX: 1471030a0180040a
> RDX: 0000000000000005 RSI: 00000000ffffffe0 RDI: ffff8800ba83fa80
> RBP: ffff88007d0d9cb8 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000101 R12: ffff8800ba83fa80
> R13: 0000000000000008 R14: ffff88007d0d9cc8 R15: ffff8800ba83fa80
> FS:  00007f6637104800(0000) GS:ffff8800bf600000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f563f5b01c4 CR3: 000000007d140000 CR4: 00000000000007f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
> Process openvpn (pid: 1381, threadinfo ffff88007d0d8000, task ffff8800a540cd60)
> Stack:
>  ffff8800ba83fa80 0000000000000296 0000000000000000 0000000000000000
>  ffff88007d0d9cc8 ffffffff815bcff4 ffff88007d0d9ce8 ffffffff815b1831
>  ffff88007d0d9ca8 00000000703f6364 ffff8800ba83fa80 0000000000000000
> Call Trace:
>  [<ffffffff815bcff4>] ? netif_rx+0x114/0x4c0
>  [<ffffffff815b1831>] ? skb_copy_datagram_from_iovec+0x61/0x290
>  [<ffffffff815b672a>] __skb_get_rxhash+0x1a/0xd0
>  [<ffffffffa03b9538>] tun_get_user+0x418/0x810 [tun]
>  [<ffffffff8135f468>] ? delay_tsc+0x98/0xf0
>  [<ffffffff8109605c>] ? __rcu_read_unlock+0x5c/0xa0
>  [<ffffffffa03b9a41>] tun_chr_aio_write+0x81/0xb0 [tun]
>  [<ffffffff81145011>] ? __buffer_unlock_commit+0x41/0x50
>  [<ffffffff811db917>] do_sync_write+0xa7/0xe0
>  [<ffffffff811dc01f>] vfs_write+0xaf/0x190
>  [<ffffffff811dc375>] sys_write+0x55/0xa0
>  [<ffffffff81705540>] tracesys+0xdd/0xe2
> Code: 41 8b 44 24 68 41 2b 44 24 6c 01 de 29 f0 83 f8 03 0f 8e a0 00 00 00 48 63 de 49 03 9c 24 e0 00 00 00 48 85 db 0f 84 72 fe ff ff <8b> 03 41 89 46 08 b8 01 00 00 00 e9 43 fd ff ff 0f 1f 40 00 48
> RIP  [<ffffffff815b54a4>] skb_flow_dissect+0x314/0x3e0
>  RSP <ffff88007d0d9c48>
> ---[ end trace 6d42c834c72c002e ]---
>
>
> Faulting instruction is
>
>    0:	8b 03                	mov    (%rbx),%eax
>
> rbx is slab poison (-20) so this looks like a use-after-free here...
>
>                         flow->ports = *ports;
>  314:   8b 03                   mov    (%rbx),%eax
>  316:   41 89 46 08             mov    %eax,0x8(%r14)
>
> in the inlined skb_header_pointer in skb_flow_dissect
>
> 	Dave
>

commit 96442e42 (tuntap: choose the txq based on rxq) added
a use after free.

Cache rxhash in a temp variable before calling netif_rx_ni()
Reported-by: Dave Jones <davej@redhat.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Jason Wang <jasowang@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -297,13 +297,12 @@ static void tun_flow_cleanup(unsigned long data)
 	spin_unlock_bh(&tun->lock);
 }
 
-static void tun_flow_update(struct tun_struct *tun, struct sk_buff *skb,
+static void tun_flow_update(struct tun_struct *tun, u32 rxhash,
 			    u16 queue_index)
 {
 	struct hlist_head *head;
 	struct tun_flow_entry *e;
 	unsigned long delay = tun->ageing_time;
-	u32 rxhash = skb_get_rxhash(skb);
 
 	if (!rxhash)
 		return;
@@ -1010,6 +1009,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 	int copylen;
 	bool zerocopy = false;
 	int err;
+	u32 rxhash;
 
 	if (!(tun->flags & TUN_NO_PI)) {
 		if ((len -= sizeof(pi)) > total_len)
@@ -1162,12 +1162,13 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 		skb_shinfo(skb)->tx_flags |= SKBTX_DEV_ZEROCOPY;
 	}
 
+	rxhash = skb_get_rxhash(skb);
 	netif_rx_ni(skb);
 
 	tun->dev->stats.rx_packets++;
 	tun->dev->stats.rx_bytes += len;
 
-	tun_flow_update(tun, skb, tfile->queue_index);
+	tun_flow_update(tun, rxhash, tfile->queue_index);
 	return total_len;
 }