Re: bridge/netfilter: regression in 2.6.39.1

From: Eric Dumazet
Date: Mon Jun 06 2011 - 09:16:17 EST

Le lundi 06 juin 2011 Ã 14:49 +0200, Alexander Holler a Ãcrit :
> Am 06.06.2011 14:12, schrieb Eric Dumazet:
> > Le lundi 06 juin 2011 Ã 13:48 +0200, Alexander Holler a Ãcrit :
> >> Am 06.06.2011 13:15, schrieb Neil Horman:
> >>> On Fri, Jun 03, 2011 at 09:21:06PM +0200, Alexander Holler wrote:
> >>>> Hello,
> >>>>
> >>>> I'm getting a oops in the bridge code in br_change_mtu() with
> >>>> 2.6.39.1. The patch below seems to fix that.
> >>>>
> >>>> I'm not sure about the usage of dst_cow_metrics_generic() in
> >>>> fake_dst_ops, but after having a quick look at it seems to be ok to
> >>>> use that here.
> >>>>
> >>>> Regards,
> >>>>
> >>>> Alexander
> >>>>
> >>> How did the flags of the dst entry on which we're callnig dst_entry_write_ptr
> >>> wind up getting the READ_ONLY flag set on it? I don't see how we'er falling
> >>> into that clause in which we call cow_metrics when we call dst_metric_set. It
> >>> seems like that flag is set erroneously. perhaps we should just update
> >>> fake_rtable.dst to have the correct flags?
> >>> Neil
> >>
> >> It is set by that change:
> >>
> >> --------
> >> @@ -124,7 +128,7 @@ void br_netfilter_rtable_init(struct net_bridge *br)
> >> atomic_set(&rt->dst.__refcnt, 1);
> >> rt->dst.dev = br->dev;
> >> rt->dst.path =&rt->dst;
> >> - dst_metric_set(&rt->dst, RTAX_MTU, 1500);
> >> + dst_init_metrics(&rt->dst, br_dst_default_metrics, true);
> >> rt->dst.flags = DST_NOXFRM;
> >> rt->dst.ops =&fake_dst_ops;
> >> }
> >> --------
> >>
> >> The true in dst_init_metrics() is responsible for that flag.
> >>
> >
> > You are aware this change fixed an oops ?
>
> No, I'm not aware of this. I know almost nothing about what all that
> stuff is doing. For me that change above just introduced an oops through
> an immediate NULL pointer dereference in br_change_mtu().
>
> > read_only in this context means : In case this must be written, we make
> > a COW first
> > (allocate a piece of memory, copy the source in it before applying any
> > change)
> >
> > It would be nice you send us the stack trace, so that we can have a clue
> > of whats going on.
>
> Here is the text as found in my first mail:
> ----
> Commit 42923465fb8d025a2b5153f2e7ab1e6e1058bf00 does here what it
> should prevent, it introduces NULL a dereference.
>

Hmm, this commit uncovers a previous bug, introduced in commit
62fa8a846d7d.

> The above commit uses dst_init_metrics() which sets the metrics as
> read only. As result br_change_mtu() dies in dst_metric_set()
> which calls dst_metrics_write_ptr() which calls
> dst->ops->cow_metrics() if the metrics are read only.
>
> I don't have a system with 2.6.39.1 or 3.0-rcN ready which writes an
> oops to somewhere else than the screen. Just set up a bridge and change
> the MTU for the IF, that will trigger the oops.
> ----
>
> Here is the oops:
>
> ----
> [ 136.546023] BUG: unable to handle kernel NULL pointer dereference at
> (null)
> [ 136.546038] IP: [< (null)>] (null)
> [ 136.546046] *pde = 00000000
> [ 136.546052] Oops: 0000 [#1] SMP
> [ 136.546059] last sysfs file: /sys/devices/virtual/net/mybridge/uevent
> [ 136.546066] Modules linked in: sit tunnel4 tun iwlagn sch_sfq bridge
> stp llc nfs lockd auth_rpcgss nfs_acl sunrpc ipt_LOG xt_recent xt_state
> iptable_filter iptable_nat nf_nat nf_conntrack_ipv4 nf_defrag_ipv4
> xt_addrtype xt_dscp xt_iprange xt_DSCP xt_set ip_set nfnetlink ip6t_LOG
> xt_limit ip6table_filter xt_string xt_owner xt_multiport xt_hashlimit
> xt_conntrack xt_NFQUEUE xt_mark xt_connmark nf_conntrack ip6_tables
> snd_seq_oss snd_seq_midi_event snd_seq snd_seq_device snd_pcm_oss
> snd_mixer_oss uinput ipv6 dm_mod nvidiafb fb_ddc i2c_algo_bit vgastate
> nvidia(P) arc4 uvcvideo videodev ecb snd_hda_codec_realtek iwl4965
> snd_hda_intel iwl_legacy snd_hda_codec uhci_hcd firewire_ohci mac80211
> sdhci_pci snd_hwdep snd_pcm sdhci ehci_hcd intel_agp firewire_core
> cfg80211 snd_timer asus_laptop snd e1000e usbcore sparse_keymap rfkill
> tpm_infineon crc_itu_t intel_gtt mmc_core iTCO_wdt tpm_tis ata_piix
> agpgart tpm video soundcore sg tpm_bios joydev snd_page_alloc [last
> unloaded: microcode]
> [ 136.546235]
> [ 136.546243] Pid: 8415, comm: ip Tainted: P
> 2.6.39.1-00006-g40545b7 #103 ASUSTeK Computer Inc. V1Sn
> /V1Sn
> [ 136.546256] EIP: 0060:[<00000000>] EFLAGS: 00010202 CPU: 0
> [ 136.546268] EIP is at 0x0
> [ 136.546273] EAX: f14a389c EBX: 000005d4 ECX: f80d32c0 EDX: f80d1da1
> [ 136.546279] ESI: f14a3000 EDI: f255bf10 EBP: f15c3b54 ESP: f15c3b48
> [ 136.546285] DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068
> [ 136.546293] Process ip (pid: 8415, ti=f15c2000 task=f4741f80
> task.ti=f15c2000)
> [ 136.546297] Stack:
> [ 136.546301] f80c658f f14a3000 ffffffed f15c3b64 c12cb9c8 f80d1b80
> ffffffa1 f15c3bbc
> [ 136.546315] c12da347 c12d9c7d 00000000 f7670b00 00000000 f80d1b80
> ffffffa6 f15c3be4
> [ 136.546329] 00000004 f14a3000 f255bf20 00000008 f15c3bbc c11d6cae
> 00000000 00000000
> [ 136.546343] Call Trace:
> [ 136.546359] [<f80c658f>] ? br_change_mtu+0x5f/0x80 [bridge]
> [ 136.546372] [<c12cb9c8>] dev_set_mtu+0x38/0x80
> [ 136.546381] [<c12da347>] do_setlink+0x1a7/0x860
> [ 136.546390] [<c12d9c7d>] ? rtnl_fill_ifinfo+0x9bd/0xc70
> [ 136.546400] [<c11d6cae>] ? nla_parse+0x6e/0xb0
> [ 136.546409] [<c12db931>] rtnl_newlink+0x361/0x510
> [ 136.546420] [<c1023240>] ? vmalloc_sync_all+0x100/0x100
> [ 136.546429] [<c1362762>] ? error_code+0x5a/0x60
> [ 136.546438] [<c12db5d0>] ? rtnl_configure_link+0x80/0x80
> [ 136.546446] [<c12db27a>] rtnetlink_rcv_msg+0xfa/0x210
> [ 136.546454] [<c12db180>] ? __rtnl_unlock+0x20/0x20
> [ 136.546463] [<c12ee0fe>] netlink_rcv_skb+0x8e/0xb0
> [ 136.546471] [<c12daf1c>] rtnetlink_rcv+0x1c/0x30
> [ 136.546479] [<c12edafa>] netlink_unicast+0x23a/0x280
> [ 136.546487] [<c12ede6b>] netlink_sendmsg+0x26b/0x2f0
> [ 136.546497] [<c12bb828>] sock_sendmsg+0xc8/0x100
> [ 136.546508] [<c10adf61>] ? __alloc_pages_nodemask+0xe1/0x750
> [ 136.546517] [<c11d0602>] ? _copy_from_user+0x42/0x60
> [ 136.546525] [<c12c5e4c>] ? verify_iovec+0x4c/0xc0
> [ 136.546534] [<c12bd805>] sys_sendmsg+0x1c5/0x200
> [ 136.546542] [<c10c2150>] ? __do_fault+0x310/0x410
> [ 136.546549] [<c10c2c46>] ? do_wp_page+0x1d6/0x6b0
> [ 136.546557] [<c10c47d1>] ? handle_pte_fault+0xe1/0x720
> [ 136.546565] [<c12bd1af>] ? sys_getsockname+0x7f/0x90
> [ 136.546574] [<c10c4ec1>] ? handle_mm_fault+0xb1/0x180
> [ 136.546582] [<c1023240>] ? vmalloc_sync_all+0x100/0x100
> [ 136.546589] [<c10233b3>] ? do_page_fault+0x173/0x3d0
> [ 136.546596] [<c12bd87b>] ? sys_recvmsg+0x3b/0x60
> [ 136.546605] [<c12bdd83>] sys_socketcall+0x293/0x2d0
> [ 136.546614] [<c13629d0>] sysenter_do_call+0x12/0x26
> [ 136.546619] Code: Bad EIP value.
> [ 136.546627] EIP: [<00000000>] 0x0 SS:ESP 0068:f15c3b48
> [ 136.546645] CR2: 0000000000000000
> [ 136.546652] ---[ end trace 6909b560e78934fa ]---
> ----
>
> And if you want the oops on your machine (uisng 2.6.39.1 or 3.0-rcN):
>
> ----
> brctl addbr mybridge
> ip link set mybridge mtu 1234
> oops
> ----

Nice, now please submit a patch with 0972ddb237 as a guideline.

BTW, you could also check other struct dst_ops methods for
bridge/netfilter:

- What about ADVMSS or things like that, see commit 214f45c9



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