Re: KMSAN: uninit-value in pppoe_rcv

Eric Dumazet
2018-09-13T17:35:46Z

On 09/13/2018 07:12 AM, Alexander Potapenko wrote:
> On Thu, Sep 13, 2018 at 3:57 PM Eric Dumazet <eric.dumazet@gmail.com> wrote:
>>
>>
>>
>> On 09/12/2018 03:38 AM, Alexander Potapenko wrote:
>>> On Wed, Sep 12, 2018 at 12:24 PM syzbot
>>> <syzbot+f5f6080811c849739212@syzkaller.appspotmail.com> wrote:
>>>>
>>>> Hello,
>>>>
>>>> syzbot found the following crash on:
>>>>
>>>> HEAD commit: d2d741e5d189 kmsan: add initialization for shmem pages
>>>> git tree: https://github.com/google/kmsan.git/master
>>>> console output: https://syzkaller.appspot.com/x/log.txt?x=1465fc37800000
>>>> kernel config: https://syzkaller.appspot.com/x/.config?x=48f9de3384bcd0f
>>>> dashboard link: https://syzkaller.appspot.com/bug?extid=f5f6080811c849739212
>>>> compiler: clang version 7.0.0 (trunk 329391)
>>>> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=14d6e607800000
>>>> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=10a15b5b800000
>>>>
>>>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>>>> Reported-by: syzbot+f5f6080811c849739212@syzkaller.appspotmail.com
>>>>
>>>> IPVS: ftp: loaded support on port[0] = 21
>>>> ==================================================================
>>>> BUG: KMSAN: uninit-value in __get_item drivers/net/ppp/pppoe.c:172 [inline]
>>>> BUG: KMSAN: uninit-value in get_item drivers/net/ppp/pppoe.c:236 [inline]
>>>> BUG: KMSAN: uninit-value in pppoe_rcv+0xcef/0x10e0
>>>> drivers/net/ppp/pppoe.c:450
>>>> CPU: 0 PID: 4543 Comm: syz-executor355 Not tainted 4.16.0+ #87
>>>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
>>>> Google 01/01/2011
>>>> Call Trace:
>>>> __dump_stack lib/dump_stack.c:17 [inline]
>>>> dump_stack+0x185/0x1d0 lib/dump_stack.c:53
>>>> kmsan_report+0x142/0x240 mm/kmsan/kmsan.c:1067
>>>> __msan_warning_32+0x6c/0xb0 mm/kmsan/kmsan_instr.c:683
>>>> __get_item drivers/net/ppp/pppoe.c:172 [inline]
>>>> get_item drivers/net/ppp/pppoe.c:236 [inline]
>>>> pppoe_rcv+0xcef/0x10e0 drivers/net/ppp/pppoe.c:450
>>>> __netif_receive_skb_core+0x47df/0x4a90 net/core/dev.c:4562
>>>> __netif_receive_skb net/core/dev.c:4627 [inline]
>>>> netif_receive_skb_internal+0x49d/0x630 net/core/dev.c:4701
>>>> netif_receive_skb+0x230/0x240 net/core/dev.c:4725
>>>> tun_rx_batched drivers/net/tun.c:1555 [inline]
>>>> tun_get_user+0x740f/0x7c60 drivers/net/tun.c:1962
>>>> tun_chr_write_iter+0x1d4/0x330 drivers/net/tun.c:1990
>>>> call_write_iter include/linux/fs.h:1782 [inline]
>>>> new_sync_write fs/read_write.c:469 [inline]
>>>> __vfs_write+0x7fb/0x9f0 fs/read_write.c:482
>>>> vfs_write+0x463/0x8d0 fs/read_write.c:544
>>>> SYSC_write+0x172/0x360 fs/read_write.c:589
>>>> SyS_write+0x55/0x80 fs/read_write.c:581
>>>> do_syscall_64+0x309/0x430 arch/x86/entry/common.c:287
>>>> entry_SYSCALL_64_after_hwframe+0x3d/0xa2
>>>> RIP: 0033:0x4447c9
>>>> RSP: 002b:00007fff64c8fc28 EFLAGS: 00000297 ORIG_RAX: 0000000000000001
>>>> RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00000000004447c9
>>>> RDX: 000000000000fd87 RSI: 0000000020000600 RDI: 0000000000000004
>>>> RBP: 00000000006cf018 R08: 00007fff64c8fda8 R09: 00007fff00006bda
>>>> R10: 0000000000005fe7 R11: 0000000000000297 R12: 00000000004020d0
>>>> R13: 0000000000402160 R14: 0000000000000000 R15: 0000000000000000
>>>>
>>>> Uninit was created at:
>>>> kmsan_save_stack_with_flags mm/kmsan/kmsan.c:278 [inline]
>>>> kmsan_internal_poison_shadow+0xb8/0x1b0 mm/kmsan/kmsan.c:188
>>>> kmsan_kmalloc+0x94/0x100 mm/kmsan/kmsan.c:314
>>>> kmsan_slab_alloc+0x11/0x20 mm/kmsan/kmsan.c:321
>>>> slab_post_alloc_hook mm/slab.h:445 [inline]
>>>> slab_alloc_node mm/slub.c:2737 [inline]
>>>> __kmalloc_node_track_caller+0xaed/0x11c0 mm/slub.c:4369
>>>> __kmalloc_reserve net/core/skbuff.c:138 [inline]
>>>> __alloc_skb+0x2cf/0x9f0 net/core/skbuff.c:206
>>>> alloc_skb include/linux/skbuff.h:984 [inline]
>>>> alloc_skb_with_frags+0x1d4/0xb20 net/core/skbuff.c:5234
>>>> sock_alloc_send_pskb+0xb56/0x1190 net/core/sock.c:2085
>>>> tun_alloc_skb drivers/net/tun.c:1532 [inline]
>>>> tun_get_user+0x2242/0x7c60 drivers/net/tun.c:1829
>>>> tun_chr_write_iter+0x1d4/0x330 drivers/net/tun.c:1990
>>>> call_write_iter include/linux/fs.h:1782 [inline]
>>>> new_sync_write fs/read_write.c:469 [inline]
>>>> __vfs_write+0x7fb/0x9f0 fs/read_write.c:482
>>>> vfs_write+0x463/0x8d0 fs/read_write.c:544
>>>> SYSC_write+0x172/0x360 fs/read_write.c:589
>>>> SyS_write+0x55/0x80 fs/read_write.c:581
>>>> do_syscall_64+0x309/0x430 arch/x86/entry/common.c:287
>>>> entry_SYSCALL_64_after_hwframe+0x3d/0xa2
>>>> ==================================================================
>>> I did a little digging before sending the bug upstream.
>>> If I add memset(obj, 0xfe, size) to __kmalloc_reserve(), these 0xfe
>>> bytes are visible in __get_item() at the place where KMSAN reports an
>>> error.
>>>
>>> The problem is somehow related to tun_get_user() creating a fragmented
>>> sk_buff - when I change the call to tun_alloc_skb() so that it
>>> allocates a single buffer the bug goes away.
>>>
>>
>> I guess the following patch would fix the issue
>>
>> (I will submit it more formally)
> No, as far as I can see it doesn't.
> Saving sid before __skb_pull() is still a good idea, but in this
> particular case |ph| doesn't change.
>> diff --git a/drivers/net/ppp/pppoe.c b/drivers/net/ppp/pppoe.c
>> index ce61231e96ea5fe27f512fbd0d80d4609997e508..333e967ed968ea3ff2dda25289f7f657263db2b9 100644
>> --- a/drivers/net/ppp/pppoe.c
>> +++ b/drivers/net/ppp/pppoe.c
>> @@ -423,6 +423,7 @@ static int pppoe_rcv(struct sk_buff *skb, struct net_device *dev,
>> struct pppoe_hdr *ph;
>> struct pppox_sock *po;
>> struct pppoe_net *pn;
>> + __be16 sid;
>> int len;
>>
>> skb = skb_share_check(skb, GFP_ATOMIC);
>> @@ -434,6 +435,7 @@ static int pppoe_rcv(struct sk_buff *skb, struct net_device *dev,
>>
>> ph = pppoe_hdr(skb);
>> len = ntohs(ph->length);

Then ph->length needs to be better validated.

>> + sid = ph->sid;

I'll take a look, thanks.