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Date: Thu, 26 Jan 2017 09:32:35 -0600 (CST)
From: Roy Keene <lkml@rkeene.org>
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To: netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: ip_rcv_finish() NULL pointer kernel panic
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This bug appears to have existed for a long time:

 	https://www.spinics.net/lists/netdev/msg222459.html

 	http://www.kernelhub.org/?p=2&msg=823752

Though possibly with different things not setting the "input" function 
pointer in the "struct dst_entry".

include/net/dst.h:
  496 static inline int dst_input(struct sk_buff *skb) {
  498         return skb_dst(skb)->input(skb);
  499 }

Is there any reason not to check to see if this pointer is NULL before 
blindly calling it ?


On Thu, 26 Jan 2017, Roy Keene wrote:

> [Resending to netdev from LKML]
>
> All,
>
> 	I am experiencing a kernel panic on different (but identical) 
> hardware in ip_rcv_finish on Linux 4.4.39 (but I can see no changes that 
> would improve the situation on 4.4.44, or master).
>
> Looking at the disassembly at the last stack frame it looks like we are 
> calling a NULL function pointer in net/ipv4/ip_input.c:365
>
> Panic:
>
>
> [ 214.518262] BUG: unable to handle kernel NULL pointer dereference at (null)
> [ 214.612199] IP: [< (null)>] (null)
> [ 214.672744] PGD 0 [ 214.696887] Oops: 0010 [#1] SMP [ 214.735697] Modules 
> linked in: br_netfilter(+) tun 8021q bridge stp llc bonding iTCO_wdt 
> iTCO_vendor_support tpm_tis tpm kvm_intel kvm irqbypass sb_edac edac_core 
> ixgbe mdio ipmi_si ipmi_msghandler lpc_ich mfd_core mousedev evdev igb dca 
> procmemro(O) nokeyctl(O) noptrace(O)
> [ 215.029240] CPU: 34 PID: 0 Comm: swapper/34 Tainted: G O 4.4.39 #1
> [ 215.116720] Hardware name: Cisco Systems Inc UCSC-C220-M3L/UCSC-C220-M3L, 
> BIOS C220M3.2.0.13a.0.0713160937 07/13/16
> [ 215.241644] task: ffff882038fb4380 ti: ffff8810392b0000 task.ti: 
> ffff8810392b0000
> [ 215.331207] RIP: 0010:[<0000000000000000>] [< (null)>] (null)
> [ 215.420877] RSP: 0018:ffff88103fec3880 EFLAGS: 00010286
> [ 215.484436] RAX: ffff881011631000 RBX: ffff881011067100 RCX: 
> 0000000000000000
> [ 215.569836] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 
> ffff881011067100
> [ 215.655234] RBP: ffff88103fec38a8 R08: 0000000000000008 R09: 
> ffff8810116300a0
> [ 215.740629] R10: 0000000000000000 R11: 0000000000000000 R12: 
> ffff881018917dce
> [ 215.826030] R13: ffffffff81c9be00 R14: ffffffff81c9be00 R15: 
> ffff881011630078
> [ 215.911432] FS: 0000000000000000(0000) GS:ffff88103fec0000(0000) 
> knlGS:0000000000000000
> [ 216.008274] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 216.077032] CR2: 0000000000000000 CR3: 0000001011b9d000 CR4: 
> 00000000001406e0
> [ 216.162430] Stack:
> [ 216.186461] ffffffff8157d7f9 ffff881011067100 ffff881018917dce 
> ffff881011630000
> [ 216.275407] ffffffff81c9be00 ffff88103fec3918 ffffffff8157e0db 
> 0000000000000000
> [ 216.364352] 0000000000000000 0000000000000000 0000000000000000 
> 0000000000000000
> [ 216.453301] Call Trace:
> [ 216.482536] <IRQ> [ 216.505533] [<ffffffff8157d7f9>] ? 
> ip_rcv_finish+0x99/0x320
> [ 216.575442] [<ffffffff8157e0db>] ip_rcv+0x25b/0x370
> [ 216.634842] [<ffffffff81540e0b>] __netif_receive_skb_core+0x2cb/0xa20
> [ 216.712965] [<ffffffff81541578>] __netif_receive_skb+0x18/0x60
> [ 216.783801] [<ffffffff815415e3>] netif_receive_skb_internal+0x23/0x80
> [ 216.861921] [<ffffffff8154165c>] netif_receive_skb+0x1c/0x70
> [ 216.930686] [<ffffffffa02f6439>] br_handle_frame_finish+0x1b9/0x5b0 
> [bridge]
> [ 217.016091] [<ffffffff81187a00>] ? ___slab_alloc+0x1d0/0x440
> [ 217.084849] [<ffffffffa0584074>] br_nf_pre_routing_finish+0x174/0x3d0 
> [br_netfilter]
> [ 217.178568] [<ffffffffa0584c07>] ? br_nf_pre_routing+0x97/0x470 
> [br_netfilter]
> [ 217.266052] [<ffffffffa02f6280>] ? br_handle_local_finish+0x80/0x80 
> [bridge]
> [ 217.351450] [<ffffffffa0584d17>] br_nf_pre_routing+0x1a7/0x470 
> [br_netfilter]
> [ 217.437891] [<ffffffff81572f6d>] nf_iterate+0x5d/0x70
> [ 217.499367] [<ffffffff81572fe4>] nf_hook_slow+0x64/0xc0
> [ 217.562928] [<ffffffffa02f69e9>] br_handle_frame+0x1b9/0x290 [bridge]
> [ 217.641048] [<ffffffffa02f6280>] ? br_handle_local_finish+0x80/0x80 
> [bridge]
> [ 217.726446] [<ffffffff81540e82>] __netif_receive_skb_core+0x342/0xa20
> [ 217.804566] [<ffffffff815a7916>] ? tcp4_gro_receive+0x126/0x1d0
> [ 217.876445] [<ffffffff815b7446>] ? inet_gro_receive+0x1c6/0x250
> [ 217.948322] [<ffffffff81541578>] __netif_receive_skb+0x18/0x60
> [ 218.019161] [<ffffffff815415e3>] netif_receive_skb_internal+0x23/0x80
> [ 218.097281] [<ffffffff81542213>] napi_gro_receive+0xc3/0x110
> [ 218.166051] [<ffffffffa00a801f>] ixgbe_clean_rx_irq+0x52f/0xa70 [ixgbe]
> [ 218.246255] [<ffffffffa00a9248>] ixgbe_poll+0x438/0x790 [ixgbe]
> [ 218.318131] [<ffffffff81541a6e>] net_rx_action+0x1ee/0x320
> [ 218.384813] [<ffffffff8109c837>] ? handle_irq_event_percpu+0x167/0x1d0
> [ 218.463973] [<ffffffff8105c3c1>] __do_softirq+0x101/0x280
> [ 218.529608] [<ffffffff8105c69e>] irq_exit+0x8e/0x90
> [ 218.589007] [<ffffffff816dd504>] do_IRQ+0x54/0xd0
> [ 218.646323] [<ffffffff816dba02>] common_interrupt+0x82/0x82
> [ 218.714039] <EOI> [ 218.737040] [<ffffffff814fb4f3>] ? 
> cpuidle_enter_state+0x133/0x2a0
> [ 218.814226] [<ffffffff814fb4cf>] ? cpuidle_enter_state+0x10f/0x2a0
> [ 218.889224] [<ffffffff814fb697>] cpuidle_enter+0x17/0x20
> [ 218.953825] [<ffffffff81093cf1>] cpu_startup_entry+0x2a1/0x300
> [ 219.024663] [<ffffffff8103838d>] start_secondary+0xed/0xf0
> [ 219.091337] Code: Bad RIP value.
> [ 219.131186] RIP [< (null)>] (null)
> [ 219.192770] RSP <ffff88103fec3880>
> [ 219.234483] CR2: 0000000000000000
> [ 219.274121] ---[ end trace 9ce5d4620e3bcbdf ]---
> [ 219.274125] BUG: unable to handle kernel NULL pointer dereference at (null)
> [ 219.274126] IP: [< (null)>] (null)
> [ 219.274126] PGD 0 [ 219.274128] Oops: 0010 [#2] SMP [ 219.274137] Modules 
> linked in: br_netfilter(+) tun 8021q bridge stp llc bonding iTCO_wdt 
> iTCO_vendor_support tpm_tis tpm kvm_intel kvm irqbypass sb_edac edac_core 
> ixgbe mdio ipmi_si ipmi_msghandler lpc_ich mfd_core mousedev evdev igb dca 
> procmemro(O) nokeyctl(O) noptrace(O)
> [ 219.274139] CPU: 7 PID: 0 Comm: swapper/7 Tainted: G D O 4.4.39 #1
> [ 219.274140] Hardware name: Cisco Systems Inc UCSC-C220-M3L/UCSC-C220-M3L, 
> BIOS C220M3.2.0.13a.0.0713160937 07/13/16
> [ 219.274141] task: ffff882038f84380 ti: ffff881039044000 task.ti: 
> ffff881039044000
> [ 219.274142] RIP: 0010:[<0000000000000000>] [< (null)>] (null)
> [ 219.274143] RSP: 0018:ffff88103fce3880 EFLAGS: 00010286
> [ 219.274143] RAX: ffff881
>
>
> Notes:
> ----
> include/linux/skbuff.h:
>
>  734 #define SKB_DST_NOREF   1UL
>  735 #define SKB_DST_PTRMASK ~(SKB_DST_NOREF)
> ...
>  743 static inline struct dst_entry *skb_dst(const struct sk_buff *skb) {
>  745         /* If refdst was not refcounted, check we still are in a
>  746          * rcu_read_lock section
>  747          */
>  748         WARN_ON((skb->_skb_refdst & SKB_DST_NOREF) &&
>  749                 !rcu_read_lock_held() &&
>  750                 !rcu_read_lock_bh_held());
>  751         return (struct dst_entry *)(skb->_skb_refdst & 
> SKB_DST_PTRMASK);
>  752 }
> ---
> include/net/dst.h:
>  496 static inline int dst_input(struct sk_buff *skb) {
>  498         return skb_dst(skb)->input(skb);
>  499 }
> ---
> net/ipv4/ip_input.c:
>
>  359         rt = skb_rtable(skb);
>  360         if (rt->rt_type == RTN_MULTICAST) {
>  361                 IP_UPD_PO_STATS_BH(net, IPSTATS_MIB_INMCAST, skb->len);
>  362         } else if (rt->rt_type == RTN_BROADCAST)
>  363                 IP_UPD_PO_STATS_BH(net, IPSTATS_MIB_INBCAST, skb->len);
>  364
>  365         return dst_input(skb);
> ----
> Expand net/ipv4/ip_input.c:365:
>      return dst_input(skb);
>
> Into:
>      return skb_dst(skb)->input(skb);
>
> Into:
>      return ((struct dst_entry *)(skb->_skb_refdst & 
> SKB_DST_PTRMASK))->input(skb);
>
> Into:
>      return ((struct dst_entry *)(skb->_skb_refdst & (~(1UL))))->input(skb);
>
> Into:
>      return ((struct dst_entry *)(skb->_skb_refdst & 
> 0xfffffffffffffffe))->input(skb);
> ----
> Disassembly with C code next to it
>     0xffffffff8157d7d4 <ip_rcv_finish+116>  movzwl 0xa0(%rdx),%edx 
> | rt = skb_rtable(skb);
>     0xffffffff8157d7db <ip_rcv_finish+123>  cmp    $0x5,%dx 
> | if (rt->rt_type == RTN_MULTICAST) {
>     0xffffffff8157d7df <ip_rcv_finish+127>  je     0xffffffff8157d908 
> <ip_rcv_finish+424>         ....
>     0xffffffff8157d7e5 <ip_rcv_finish+133>  cmp    $0x3,%dx 
> | } else if (rt->rt_type == RTN_BROADCAST)
>     0xffffffff8157d7e9 <ip_rcv_finish+137>  je     0xffffffff8157d87e 
> <ip_rcv_finish+286>         ...
>     0xffffffff8157d7ef <ip_rcv_finish+143>  and    $0xfffffffffffffffe,%rax 
> | skb->_skb_refdst & SKB_DST_PTRMASK
>     0xffffffff8157d7f3 <ip_rcv_finish+147>  mov    %rbx,%rdi
> --> 0xffffffff8157d7f6 <ip_rcv_finish+150>  callq  *0x50(%rax) 
> | ((struct dst_entry *)(skb->_skb_refdst & SKB_DST_PTRMASK))->input(skb)
>     0xffffffff8157d7f9 <ip_rcv_finish+153>  pop    %rbx
>     0xffffffff8157d7fa <ip_rcv_finish+154>  pop    %r12
>     0xffffffff8157d7fc <ip_rcv_finish+156>  pop    %r13
>     0xffffffff8157d7fe <ip_rcv_finish+158>  pop    %r14
>     0xffffffff8157d800 <ip_rcv_finish+160>  pop    %rbp
>     0xffffffff8157d801 <ip_rcv_finish+161>  retq
> ----
>
> This leads me to believe that ((struct dst_entry *)(skb->_skb_refdst & 
> SKB_DST_PTRMASK))->input is NULL and we are jumping there (callq).
>
>
> Under what conditions would ((struct dst_entry *)(skb->_skb_refdst & 
> SKB_DST_PTRMASK))->input be NULL at this point ?
>
> Thanks,
> 	Roy Keene
>