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Message-ID: <20140903154703.GE3688@alphalink.fr>
Date: Wed, 3 Sep 2014 17:47:03 +0200
From: Guillaume Nault <g.nault@...halink.fr>
To: Eric Dumazet <eric.dumazet@...il.com>
Cc: Dmitry Petukhov <dmgenp@...il.com>, netdev@...r.kernel.org,
James Chapman <jchapman@...alix.com>,
"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH net] l2tp: fix race while getting PMTU on PPP pseudo-wire
On Wed, Sep 03, 2014 at 06:55:26AM -0700, Eric Dumazet wrote:
> On Wed, 2014-09-03 at 14:12 +0200, Guillaume Nault wrote:
> > Use dst_entry held by sk_dst_get() to retrieve tunnel's PMTU.
> >
> > The dst_mtu(__sk_dst_get(tunnel->sock)) call was racy. __sk_dst_get()
> > could return NULL if tunnel->sock->sk_dst_cache was reset just before the
> > call, thus making dst_mtu() dereference a NULL pointer:
> >
> > [ 1937.661598] BUG: unable to handle kernel NULL pointer dereference at 0000000000000020
> > [ 1937.664005] IP: [<ffffffffa049db88>] pppol2tp_connect+0x33d/0x41e [l2tp_ppp]
> > [ 1937.664005] PGD daf0c067 PUD d9f93067 PMD 0
> > [ 1937.664005] Oops: 0000 [#1] SMP
> > [ 1937.664005] Modules linked in: l2tp_ppp l2tp_netlink l2tp_core ip6table_filter ip6_tables iptable_filter ip_tables ebtable_nat ebtables x_tables udp_tunnel pppoe pppox ppp_generic slhc deflate ctr twofish_generic twofish_x86_64_3way xts lrw gf128mul glue_helper twofish_x86_64 twofish_common blowfish_generic blowfish_x86_64 blowfish_common des_generic cbc xcbc rmd160 sha512_generic hmac crypto_null af_key xfrm_algo 8021q garp bridge stp llc tun atmtcp clip atm ext3 mbcache jbd iTCO_wdt coretemp kvm_intel iTCO_vendor_support kvm pcspkr evdev ehci_pci lpc_ich mfd_core i5400_edac edac_core i5k_amb shpchp button processor thermal_sys xfs crc32c_generic libcrc32c dm_mod usbhid sg hid sr_mod sd_mod cdrom crc_t10dif crct10dif_common ata_generic ahci ata_piix tg3 libahci libata uhci_hcd ptp ehci_hcd pps_core usbcore scsi_mod libphy usb_common [last unloaded: l2tp_core]
> > [ 1937.664005] CPU: 0 PID: 10022 Comm: l2tpstress Tainted: G O 3.17.0-rc1 #1
> > [ 1937.664005] Hardware name: HP ProLiant DL160 G5, BIOS O12 08/22/2008
> > [ 1937.664005] task: ffff8800d8fda790 ti: ffff8800c43c4000 task.ti: ffff8800c43c4000
> > [ 1937.664005] RIP: 0010:[<ffffffffa049db88>] [<ffffffffa049db88>] pppol2tp_connect+0x33d/0x41e [l2tp_ppp]
> > [ 1937.664005] RSP: 0018:ffff8800c43c7de8 EFLAGS: 00010282
> > [ 1937.664005] RAX: ffff8800da8a7240 RBX: ffff8800d8c64600 RCX: 000001c325a137b5
> > [ 1937.664005] RDX: 8c6318c6318c6320 RSI: 000000000000010c RDI: 0000000000000000
> > [ 1937.664005] RBP: ffff8800c43c7ea8 R08: 0000000000000000 R09: 0000000000000000
> > [ 1937.664005] R10: ffffffffa048e2c0 R11: ffff8800d8c64600 R12: ffff8800ca7a5000
> > [ 1937.664005] R13: ffff8800c439bf40 R14: 000000000000000c R15: 0000000000000009
> > [ 1937.664005] FS: 00007fd7f610f700(0000) GS:ffff88011a600000(0000) knlGS:0000000000000000
> > [ 1937.664005] CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
> > [ 1937.664005] CR2: 0000000000000020 CR3: 00000000d9d75000 CR4: 00000000000027e0
> > [ 1937.664005] Stack:
> > [ 1937.664005] ffffffffa049da80 ffff8800d8fda790 000000000000005b ffff880000000009
> > [ 1937.664005] ffff8800daf3f200 0000000000000003 ffff8800c43c7e48 ffffffff81109b57
> > [ 1937.664005] ffffffff81109b0e ffffffff8114c566 0000000000000000 0000000000000000
> > [ 1937.664005] Call Trace:
> > [ 1937.664005] [<ffffffffa049da80>] ? pppol2tp_connect+0x235/0x41e [l2tp_ppp]
> > [ 1937.664005] [<ffffffff81109b57>] ? might_fault+0x9e/0xa5
> > [ 1937.664005] [<ffffffff81109b0e>] ? might_fault+0x55/0xa5
> > [ 1937.664005] [<ffffffff8114c566>] ? rcu_read_unlock+0x1c/0x26
> > [ 1937.664005] [<ffffffff81309196>] SYSC_connect+0x87/0xb1
> > [ 1937.664005] [<ffffffff813e56f7>] ? sysret_check+0x1b/0x56
> > [ 1937.664005] [<ffffffff8107590d>] ? trace_hardirqs_on_caller+0x145/0x1a1
> > [ 1937.664005] [<ffffffff81213dee>] ? trace_hardirqs_on_thunk+0x3a/0x3f
> > [ 1937.664005] [<ffffffff8114c262>] ? spin_lock+0x9/0xb
> > [ 1937.664005] [<ffffffff813092b4>] SyS_connect+0x9/0xb
> > [ 1937.664005] [<ffffffff813e56d2>] system_call_fastpath+0x16/0x1b
> > [ 1937.664005] Code: 10 2a 84 81 e8 65 76 bd e0 65 ff 0c 25 10 bb 00 00 4d 85 ed 74 37 48 8b 85 60 ff ff ff 48 8b 80 88 01 00 00 48 8b b8 10 02 00 00 <48> 8b 47 20 ff 50 20 85 c0 74 0f 83 e8 28 89 83 10 01 00 00 89
> > [ 1937.664005] RIP [<ffffffffa049db88>] pppol2tp_connect+0x33d/0x41e [l2tp_ppp]
> > [ 1937.664005] RSP <ffff8800c43c7de8>
> > [ 1937.664005] CR2: 0000000000000020
> > [ 1939.559375] ---[ end trace 82d44500f28f8708 ]---
> >
> > Signed-off-by: Guillaume Nault <g.nault@...halink.fr>
> > ---
> > net/l2tp/l2tp_ppp.c | 3 ++-
> > 1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
> > index 13752d9..b704a93 100644
> > --- a/net/l2tp/l2tp_ppp.c
> > +++ b/net/l2tp/l2tp_ppp.c
> > @@ -755,7 +755,8 @@ static int pppol2tp_connect(struct socket *sock, struct sockaddr *uservaddr,
> > /* If PMTU discovery was enabled, use the MTU that was discovered */
> > dst = sk_dst_get(tunnel->sock);
> > if (dst != NULL) {
> > - u32 pmtu = dst_mtu(__sk_dst_get(tunnel->sock));
> > + u32 pmtu = dst_mtu(dst);
> > +
> > if (pmtu != 0)
> > session->mtu = session->mru = pmtu -
> > PPPOL2TP_HEADER_OVERHEAD;
>
>
> Probably introduced in commit f34c4a35d879 ("l2tp: take PMTU from tunnel
> UDP socket")
>
Indeed, I forgot to mention it. Thanks for clarification.
> CC Dmitry
>
> Before this patch, the bug could not happen.
>
> Acked-by: Eric Dumazet <edumazet@...gle.com>
> Fixes: f34c4a35d879 ("l2tp: take PMTU from tunnel UDP socket")
>
>
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