Re: net: GPF in eth_header

[Dmitry Vyukov]

On Mon, Nov 28, 2016 at 8:04 PM, 'Andrey Konovalov' via syzkaller
<syzkaller@xxxxxxxxxxxxxxxx> wrote:
> On Mon, Nov 28, 2016 at 7:50 PM, Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
>> On Sat, 2016-11-26 at 20:07 +0100, Andrey Konovalov wrote:
>>> On Sat, Nov 26, 2016 at 7:28 PM, 'Eric Dumazet' via syzkaller
>>> <syzkaller@xxxxxxxxxxxxxxxx> wrote:
>>> > On Sat, Nov 26, 2016 at 9:30 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
>>> >> Hello,
>>> >>
>>> >> The following program triggers GPF in eth_header:
>>> >>
>>> >> https://gist.githubusercontent.com/dvyukov/613cadf05543b55a419f237e419cd495/raw/5471231523d1a07c3de55f11f87472c2816ee06c/gistfile1.txt
>>> >>
>>> >> On commit 16ae16c6e5616c084168740990fc508bda6655d4 (Nov 24).
>>> >>
>>> >> BUG: unable to handle kernel paging request at ffffed002d14d74a
>>> >> IP: [<ffffffff86be3295>] eth_header+0x75/0x260 net/ethernet/eth.c:88
>>> >> PGD 7fff6067 [   50.787819] PUD 7fff5067
>>> >> PMD 0 [   50.787819]
>>> >> Oops: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN
>>> >> Modules linked in:
>>> >> CPU: 2 PID: 6712 Comm: a.out Not tainted 4.9.0-rc6+ #55
>>> >> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
>>> >> task: ffff88003a1841c0 task.stack: ffff880034d08000
>>> >> RIP: 0010:[<ffffffff86be3295>]  [<ffffffff86be3295>]
>>> >> eth_header+0x75/0x260 net/ethernet/eth.c:88
>>> >> RSP: 0018:ffff880034d0eb68  EFLAGS: 00010a03
>>> >> RAX: 1ffff1002d14d74a RBX: ffff880168a6ba4a RCX: ffff88006a9c7858
>>> >> RDX: 000000000000dd86 RSI: dffffc0000000000 RDI: ffff880168a6ba56
>>> >> RBP: ffff880034d0eb98 R08: 0000000000000000 R09: 0000000000000031
>>> >> R10: 0000000000000002 R11: 0000000000000000 R12: 0000000000000000
>>> >> R13: ffff88006c208d80 R14: 00000000000086dd R15: ffff88006a9c7858
>>> >> FS:  0000000001a02940(0000) GS:ffff88006d000000(0000) knlGS:0000000000000000
>>> >> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>> >> CR2: ffffed002d14d74a CR3: 0000000037373000 CR4: 00000000000006e0
>>> >> Stack:
>>> >>  000000316881ab40 ffff88006a9c76c0 ffff88006881ab40 ffff88006a9c77f8
>>> >>  0000000000000000 dffffc0000000000 ffff880034d0ee98 ffffffff86b31af9
>>> >>  ffffffff8719605c ffff880034d0f0f8 ffffffff000086dd ffffffff86be3220
>>> >> Call Trace:
>>> >>  [<     inline     >] dev_hard_header ./include/linux/netdevice.h:2762
>>> >>  [<ffffffff86b31af9>] neigh_resolve_output+0x659/0xb20 net/core/neighbour.c:1302
>>> >>  [<     inline     >] dst_neigh_output ./include/net/dst.h:464
>>> >>  [<ffffffff8719605c>] ip6_finish_output2+0xb3c/0x2500 net/ipv6/ip6_output.c:121
>>> >>  [<ffffffff871a0b0b>] ip6_finish_output+0x2eb/0x760 net/ipv6/ip6_output.c:139
>>> >>  [<     inline     >] NF_HOOK_COND ./include/linux/netfilter.h:246
>>> >>  [<ffffffff871a1157>] ip6_output+0x1d7/0x9a0 net/ipv6/ip6_output.c:153
>>> >>  [<     inline     >] dst_output ./include/net/dst.h:501
>>> >>  [<ffffffff873312ea>] ip6_local_out+0x9a/0x180 net/ipv6/output_core.c:170
>>> >>  [<ffffffff871a3886>] ip6_send_skb+0xa6/0x340 net/ipv6/ip6_output.c:1712
>>> >>  [<ffffffff871a3bd8>] ip6_push_pending_frames+0xb8/0xe0
>>> >> net/ipv6/ip6_output.c:1732
>>> >>  [<     inline     >] rawv6_push_pending_frames net/ipv6/raw.c:607
>>> >>  [<ffffffff8722acfb>] rawv6_sendmsg+0x250b/0x2c20 net/ipv6/raw.c:920
>>> >>  [<ffffffff8701c4f5>] inet_sendmsg+0x385/0x590 net/ipv4/af_inet.c:734
>>> >>  [<     inline     >] sock_sendmsg_nosec net/socket.c:621
>>> >>  [<ffffffff86a6ea9f>] sock_sendmsg+0xcf/0x110 net/socket.c:631
>>> >>  [<ffffffff86a6ee0b>] sock_write_iter+0x32b/0x620 net/socket.c:829
>>> >>  [<ffffffff81a6f153>] do_iter_readv_writev+0x363/0x670 fs/read_write.c:695
>>> >>  [<ffffffff81a71ba1>] do_readv_writev+0x431/0x9b0 fs/read_write.c:872
>>> >>  [<ffffffff81a726dc>] vfs_writev+0x8c/0xc0 fs/read_write.c:911
>>> >>  [<ffffffff81a72825>] do_writev+0x115/0x2d0 fs/read_write.c:944
>>> >>  [<     inline     >] SYSC_writev fs/read_write.c:1017
>>> >>  [<ffffffff81a75fdc>] SyS_writev+0x2c/0x40 fs/read_write.c:1014
>>> >>  [<ffffffff8814cf85>] entry_SYSCALL_64_fastpath+0x23/0xc6
>>> >> arch/x86/entry/entry_64.S:209
>>> >> Code: 41 83 fe 04 0f 84 aa 00 00 00 e8 17 4e b0 fa 48 8d 7b 0c 48 be
>>> >> 00 00 00 00 00 fc ff df 44 89 f2 66 c1 c2 08 48 89 f8 48 c1 e8 03 <0f>
>>> >> b6 0c 30 48 8d 43 0d 49 89 c0 49 c1 e8 03 41 0f b6 34 30 49
>>> >> RIP  [<ffffffff86be3295>] eth_header+0x75/0x260 net/ethernet/eth.c:88
>>> >>  RSP <ffff880034d0eb68>
>>> >> CR2: ffffed002d14d74a
>>> >> ---[ end trace a73fedfdc11bd60c ]---
>>> >
>>> >
>>> > Hi Dmitry
>>> >
>>> > I could not reproduce the issue. Might need some specific configuration...
>>> >
>>> > loopback device has proper ethernet header (all 0)
>>> >
>>> > Fault happens in :
>>> >
>>> > 0f b6 0c 30             movzbl (%rax,%rsi,1),%ecx
>>> >
>>> > RAX=1ffff1002d14d74a  which is RDI>>3, and RSI=dffffc0000000000
>>> >
>>> > Could this be a KASAN problem ?
>>>
>>> Hi Eric,
>>>
>>> The crash happens when the kernel tries to access shadow for nonmapped memory.
>>>
>>> The issue here is an integer overflow which happens in neigh_resolve_output().
>>> skb_network_offset(skb) can return negative number, but __skb_pull()
>>> accepts unsigned int as len.
>>> As a result, the least significat bit in higher 32 bits of skb->data
>>> gets set and we get an out-of-bounds with offset of 4 GB.
>>>
>>> I've attached a short reproducer, but you either need KASAN or to add
>>> a BUG_ON to see the crash.
>>> In this reproducer skb_network_offset() becomes negative after merging
>>> two ipv6 fragments.
>>>
>>> I actually see multiple places where skb_network_offset() is used as
>>> an argument to skb_pull().
>>> So I guess every place can potentially be buggy.
>>>
>>> Thanks!
>>
>> I can not reproduce the bug on my hosts.
>> Quite hard to debug for me.
>>
>> skb_network_offset() can not be negative at this point, unless there is
>> a bug upper in the stack.
>
> Hi Eric,
>
> As far as I can see, skb_network_offset() becomes negative after
> pskb_pull(skb, (u8 *) (fhdr + 1) - skb->data) in nf_ct_frag6_queue().
> At least I'm able to detect that with a BUG_ON().
>
> Also it seems that the issue is only reproducible (at least with the
> poc I provided) for a short time after boot.


Eric,

Is it enough to debug? Or maybe Andrey can trace some values for you.