Subject: GPF in skb_flow_dissect

From: Dave Jones <[email-masked]>
Date: Wed, 12 Dec 2012 23:16:44 -0500

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