This is a note to let you know that I've just added the patch titled

ipv6: mcast: better catch silly mtu values

to the 4.4-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...

The filename of the patch is: ipv6-mcast-better-catch-silly-mtu-values.patch and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.

From foo@baz Sun Dec 31 11:20:35 CET 2017

From: Eric Dumazet edumazet@google.com Date: Mon, 11 Dec 2017 07:03:38 -0800 Subject: ipv6: mcast: better catch silly mtu values

From: Eric Dumazet edumazet@google.com

[ Upstream commit b9b312a7a451e9c098921856e7cfbc201120e1a7 ]

syzkaller reported crashes in IPv6 stack [1]

Xin Long found that lo MTU was set to silly values.

IPv6 stack reacts to changes to small MTU, by disabling itself under RTNL.

But there is a window where threads not using RTNL can see a wrong device mtu. This can lead to surprises, in mld code where it is assumed the mtu is suitable.

Fix this by reading device mtu once and checking IPv6 minimal MTU.

[1] skbuff: skb_over_panic: text:0000000010b86b8d len:196 put:20 head:000000003b477e60 data:000000000e85441e tail:0xd4 end:0xc0 dev:lo ------------[ cut here ]------------ kernel BUG at net/core/skbuff.c:104! invalid opcode: 0000 [#1] SMP KASAN Dumping ftrace buffer: (ftrace buffer empty) Modules linked in: CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.15.0-rc2-mm1+ #39 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 RIP: 0010:skb_panic+0x15c/0x1f0 net/core/skbuff.c:100 RSP: 0018:ffff8801db307508 EFLAGS: 00010286 RAX: 0000000000000082 RBX: ffff8801c517e840 RCX: 0000000000000000 RDX: 0000000000000082 RSI: 1ffff1003b660e61 RDI: ffffed003b660e95 RBP: ffff8801db307570 R08: 1ffff1003b660e23 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000000 R12: ffffffff85bd4020 R13: ffffffff84754ed2 R14: 0000000000000014 R15: ffff8801c4e26540 FS: 0000000000000000(0000) GS:ffff8801db300000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000000000463610 CR3: 00000001c6698000 CR4: 00000000001406e0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 Call Trace: <IRQ> skb_over_panic net/core/skbuff.c:109 [inline] skb_put+0x181/0x1c0 net/core/skbuff.c:1694 add_grhead.isra.24+0x42/0x3b0 net/ipv6/mcast.c:1695 add_grec+0xa55/0x1060 net/ipv6/mcast.c:1817 mld_send_cr net/ipv6/mcast.c:1903 [inline] mld_ifc_timer_expire+0x4d2/0x770 net/ipv6/mcast.c:2448 call_timer_fn+0x23b/0x840 kernel/time/timer.c:1320 expire_timers kernel/time/timer.c:1357 [inline] __run_timers+0x7e1/0xb60 kernel/time/timer.c:1660 run_timer_softirq+0x4c/0xb0 kernel/time/timer.c:1686 __do_softirq+0x29d/0xbb2 kernel/softirq.c:285 invoke_softirq kernel/softirq.c:365 [inline] irq_exit+0x1d3/0x210 kernel/softirq.c:405 exiting_irq arch/x86/include/asm/apic.h:540 [inline] smp_apic_timer_interrupt+0x16b/0x700 arch/x86/kernel/apic/apic.c:1052 apic_timer_interrupt+0xa9/0xb0 arch/x86/entry/entry_64.S:920

Signed-off-by: Eric Dumazet edumazet@google.com Reported-by: syzbot syzkaller@googlegroups.com Tested-by: Xin Long lucien.xin@gmail.com Signed-off-by: David S. Miller davem@davemloft.net Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- net/ipv6/mcast.c | 25 +++++++++++++++---------- 1 file changed, 15 insertions(+), 10 deletions(-)

--- a/net/ipv6/mcast.c +++ b/net/ipv6/mcast.c @@ -1668,16 +1668,16 @@ static int grec_size(struct ifmcaddr6 *p }

static struct sk_buff *add_grhead(struct sk_buff *skb, struct ifmcaddr6 *pmc, - int type, struct mld2_grec **ppgr) + int type, struct mld2_grec **ppgr, unsigned int mtu) { - struct net_device *dev = pmc->idev->dev; struct mld2_report *pmr; struct mld2_grec *pgr;

- if (!skb) - skb = mld_newpack(pmc->idev, dev->mtu); - if (!skb) - return NULL; + if (!skb) { + skb = mld_newpack(pmc->idev, mtu); + if (!skb) + return NULL; + } pgr = (struct mld2_grec *)skb_put(skb, sizeof(struct mld2_grec)); pgr->grec_type = type; pgr->grec_auxwords = 0; @@ -1700,10 +1700,15 @@ static struct sk_buff *add_grec(struct s struct mld2_grec *pgr = NULL; struct ip6_sf_list *psf, *psf_next, *psf_prev, **psf_list; int scount, stotal, first, isquery, truncate; + unsigned int mtu;

if (pmc->mca_flags & MAF_NOREPORT) return skb;

+ mtu = READ_ONCE(dev->mtu); + if (mtu < IPV6_MIN_MTU) + return skb; + isquery = type == MLD2_MODE_IS_INCLUDE || type == MLD2_MODE_IS_EXCLUDE; truncate = type == MLD2_MODE_IS_EXCLUDE || @@ -1724,7 +1729,7 @@ static struct sk_buff *add_grec(struct s AVAILABLE(skb) < grec_size(pmc, type, gdeleted, sdeleted)) { if (skb) mld_sendpack(skb); - skb = mld_newpack(idev, dev->mtu); + skb = mld_newpack(idev, mtu); } } first = 1; @@ -1751,12 +1756,12 @@ static struct sk_buff *add_grec(struct s pgr->grec_nsrcs = htons(scount); if (skb) mld_sendpack(skb); - skb = mld_newpack(idev, dev->mtu); + skb = mld_newpack(idev, mtu); first = 1; scount = 0; } if (first) { - skb = add_grhead(skb, pmc, type, &pgr); + skb = add_grhead(skb, pmc, type, &pgr, mtu); first = 0; } if (!skb) @@ -1790,7 +1795,7 @@ empty_source: mld_sendpack(skb); skb = NULL; /* add_grhead will get a new one */ } - skb = add_grhead(skb, pmc, type, &pgr); + skb = add_grhead(skb, pmc, type, &pgr, mtu); } } if (pgr)

Patches currently in stable-queue which might be from edumazet@google.com are

queue-4.4/net-fix-double-free-and-memory-corruption-in-get_net_ns_by_id.patch queue-4.4/sock-free-skb-in-skb_complete_tx_timestamp-on-error.patch queue-4.4/tcp-md5sig-use-skb-s-saddr-when-replying-to-an-incoming-segment.patch queue-4.4/net-ipv4-fix-for-a-race-condition-in-raw_sendmsg.patch queue-4.4/ipv4-igmp-guard-against-silly-mtu-values.patch queue-4.4/ipv6-mcast-better-catch-silly-mtu-values.patch