Re: KMSAN: uninit-value in can_receive

[Oliver Hartkopp]

On 18/11/2019 20.05, syzbot wrote:
> Hello,
>
> syzbot found the following crash on:
>
> HEAD commit:ÂÂÂ 9c6a7162 kmsan: remove unneeded annotations in bio
> git tree:ÂÂÂÂÂÂ https://github.com/google/kmsan.git master
> console output: https://syzkaller.appspot.com/x/log.txt?x=14563416e00000
> kernel config:Â https://syzkaller.appspot.com/x/.config?x=9e324dfe9c7b0360
> dashboard link: 
> https://syzkaller.appspot.com/bug?extid=b02ff0707a97e4e79ebb
> compiler:ÂÂÂÂÂÂ 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@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> =====================================================
> 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:
>  Â<IRQ>
>  Â__dump_stack lib/dump_stack.c:77 [inline]
>  Âdump_stack+0x191/0x1f0 lib/dump_stack.c:113
>  Âkmsan_report+0x128/0x220 mm/kmsan/kmsan_report.c:108
>  Â__msan_warning+0x73/0xe0 mm/kmsan/kmsan_instr.c:245
>  Âcan_receive+0x23c/0x5e0 net/can/af_can.c:649
>  Âcan_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?

Regards,
Oliver