On 11/18/19 9:25 PM, Oliver Hartkopp wrote:
> On 18/11/2019 20.05, syzbot wrote:
>> Hello,
>>
>> syzbot found the following crash on:
>>
>> HEAD commit:    9c6a7162 kmsan: remove unneeded annotations in bio
>> git tree:       https://github.com/google/kmsan.git master
>> console output: https://syzkaller.appspot.com/x/log.txt?x=14563416e00000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=9e324dfe9c7b0360
>> dashboard link: 
>> https://syzkaller.appspot.com/bug?extid=b02ff0707a97e4e79ebb
>> compiler:       clang version 9.0.0 (/home/glider/llvm/clang 
>> 80fee25776c2fb61e74c1ecb1a523375c2500b69)
>>
>> Unfortunately, I don't have any reproducer for this crash yet.
>>
>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>> Reported-by: syzbot+b02ff0707a97e4e79ebb@syzkaller.appspotmail.com
>>
>> =====================================================
>> BUG: KMSAN: uninit-value in can_receive+0x23c/0x5e0 net/can/af_can.c:649
>> CPU: 1 PID: 3490 Comm: syz-executor.2 Not tainted 5.4.0-rc5+ #0
>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS 
>> Google 01/01/2011
>> Call Trace:
>>   <IRQ>
>>   __dump_stack lib/dump_stack.c:77 [inline]
>>   dump_stack+0x191/0x1f0 lib/dump_stack.c:113
>>   kmsan_report+0x128/0x220 mm/kmsan/kmsan_report.c:108
>>   __msan_warning+0x73/0xe0 mm/kmsan/kmsan_instr.c:245
>>   can_receive+0x23c/0x5e0 net/can/af_can.c:649
>>   can_rcv+0x188/0x3a0 net/can/af_can.c:685
> 
> In line 649 of 5.4.0-rc5+ we can find a while() statement:
> 
> while (!(can_skb_prv(skb)->skbcnt))
> 	can_skb_prv(skb)->skbcnt = atomic_inc_return(&skbcounter);
> 
> In linux/include/linux/can/skb.h we see:
> 
> static inline struct can_skb_priv *can_skb_prv(struct sk_buff *skb)
> {
> 	return (struct can_skb_priv *)(skb->head);
> }
> 
> IMO accessing can_skb_prv(skb)->skbcnt at this point is a valid 
> operation which has no uninitialized value.
> 
> Can this probably be a false positive of KMSAN?

The packet is injected via the packet socket into the kernel. Where does
skb->head point to in this case? When the skb is a proper
kernel-generated skb containing a CAN-2.0 or CAN-FD frame skb->head is
maybe properly initialized?

>   do_softirq kernel/softirq.c:338 [inline]
>   __local_bh_enable_ip+0x184/0x1d0 kernel/softirq.c:190
>   local_bh_enable+0x36/0x40 include/linux/bottom_half.h:32
>   rcu_read_unlock_bh include/linux/rcupdate.h:688 [inline]
>   __dev_queue_xmit+0x38e8/0x4200 net/core/dev.c:3900
>   dev_queue_xmit+0x4b/0x60 net/core/dev.c:3906
>   packet_snd net/packet/af_packet.c:2959 [inline]
>   packet_sendmsg+0x82d7/0x92e0 net/packet/af_packet.c:2984
                                 ^^^^^^^^^^^^^^^^^^^^^^
>   sock_sendmsg_nosec net/socket.c:637 [inline]
>   sock_sendmsg net/socket.c:657 [inline]
>   ___sys_sendmsg+0x14ff/0x1590 net/socket.c:2311
>   __sys_sendmsg net/socket.c:2356 [inline]
>   __do_sys_sendmsg net/socket.c:2365 [inline]
>   __se_sys_sendmsg+0x305/0x460 net/socket.c:2363
>   __x64_sys_sendmsg+0x4a/0x70 net/socket.c:2363
>   do_syscall_64+0xb6/0x160 arch/x86/entry/common.c:291
>   entry_SYSCALL_64_after_hwframe+0x63/0xe7

Marc

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