Re: [syzbot] [mptcp?] KMSAN: uninit-value in mptcp_incoming_options

[Paolo Abeni]

Hi.

On Wed, 2023-11-22 at 16:48 -0800, Kuniyuki Iwashima wrote:
> From: syzbot <syzbot+b834a6b2decad004cfa1@xxxxxxxxxxxxxxxxxxxxxxxxx>
> Date: Wed, 22 Nov 2023 07:43:23 -0800
> > Hello,
> > 
> > syzbot found the following issue on:
> > 
> > HEAD commit:    c2d5304e6c64 Merge tag 'platform-drivers-x86-v6.7-2' of gi..
> > git tree:       upstream
> > console+strace: https://syzkaller.appspot.com/x/log.txt?x=1536e3d4e80000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=e32016b84cf917ca
> > dashboard link: https://syzkaller.appspot.com/bug?extid=b834a6b2decad004cfa1
> > compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11469724e80000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13934aaf680000
> > 
> > Downloadable assets:
> > disk image: https://storage.googleapis.com/syzbot-assets/23ea827adf3b/disk-c2d5304e.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/0f964ac588f5/vmlinux-c2d5304e.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/b061be889285/bzImage-c2d5304e.xz
> > 
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+b834a6b2decad004cfa1@xxxxxxxxxxxxxxxxxxxxxxxxx
> > 
> > =====================================================
> > BUG: KMSAN: uninit-value in mptcp_incoming_options+0xc93/0x3a80 net/mptcp/options.c:1197
> 
> #syz test git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git c2d5304e6c648ebcf653bace7e51e0e6742e46c8
> 
> diff --git a/net/mptcp/options.c b/net/mptcp/options.c
> index cd15ec73073e..01066aa7e67d 100644
> --- a/net/mptcp/options.c
> +++ b/net/mptcp/options.c
> @@ -370,6 +370,7 @@ void mptcp_get_options(const struct sk_buff *skb,
>  
>  	/* initialize option status */
>  	mp_opt->suboptions = 0;
> +	mp_opt->use_ack = 0;
>  
>  	length = (th->doff * 4) - sizeof(struct tcphdr);
>  	ptr = (const unsigned char *)(th + 1);
> 

Thanks for the patch! AFAICS it should work, but I think the variant
proposed by Edward:

https://lore.kernel.org/netdev/tencent_B0E02F1D6C009450E8D6EC06CC6C7B5E6C0A@xxxxxx/

should be better, adding the additional instruction only in the code
path needing it. 

Thank!

Paolo