Re: Endless loop in netlink_msg_to_ifaddr

[Rich Felker <dalias@libc.org>]

On Wed, May 30, 2018 at 03:45:29PM +0300, Timo Teras wrote:
> On Wed, 30 May 2018 11:57:03 +0200
> Matej Kupljen <matej.kupljen@gmail.com> wrote:
> 
> > I am using OpenWRT device with MUSL C library version 1.1.19 and I am
> > running custom binary on it. I noticed that during testing my program
> > started using 99% CPU.
> > I build OpenWRT myself so I have all the sources. I attached the
> > gdbserver and checked what is going on.
> 
> Thanks for the report!
> 
> > As you can see the first message in netlink reply has a rta_len set
> > to zero so the list is never traversed, only the first message is
> > received every time.
> > 
> > I am not sure if this is the correct response from netlink, however
> > the program is stucked here.
> >
> > Any ideas?
> > Please CC me in reply.
> 
> That is invalid message to my understanding. Perhaps there's some new
> extensions that allow it. Upstream (linux kernel) RTA_OK does do
> additional checks against this situation.
> 
> The same issue probably affects if_nameindex.
> 
> I think the following should fix it:
> 
> diff --git a/src/network/netlink.h b/src/network/netlink.h
> index 20700ac5..00dc7172 100644
> --- a/src/network/netlink.h
> +++ b/src/network/netlink.h
> @@ -80,13 +80,13 @@ struct ifaddrmsg {
>  #define NLMSG_DATALEN(nlh)	((nlh)->nlmsg_len-sizeof(struct nlmsghdr))
>  #define NLMSG_DATAEND(nlh)	((char*)(nlh)+(nlh)->nlmsg_len)
>  #define NLMSG_NEXT(nlh)		(struct nlmsghdr*)((char*)(nlh)+NETLINK_ALIGN((nlh)->nlmsg_len))
> -#define NLMSG_OK(nlh,end)	((char*)(end)-(char*)(nlh) >= sizeof(struct nlmsghdr))
> +#define NLMSG_OK(nlh,end)	((char*)(end)-(char*)(nlh) >= sizeof(struct nlmsghdr) && (nlh)->nlmsg_len >= sizeof(struct nlmsghdr))
>  
>  #define RTA_DATA(rta)		((void*)((char*)(rta)+sizeof(struct rtattr)))
>  #define RTA_DATALEN(rta)	((rta)->rta_len-sizeof(struct rtattr))
>  #define RTA_DATAEND(rta)	((char*)(rta)+(rta)->rta_len)
>  #define RTA_NEXT(rta)		(struct rtattr*)((char*)(rta)+NETLINK_ALIGN((rta)->rta_len))
> -#define RTA_OK(nlh,end)		((char*)(end)-(char*)(rta) >= sizeof(struct rtattr))
> +#define RTA_OK(rta,end)		((char*)(end)-(char*)(rta) >= sizeof(struct rtattr) && (rta)->rta_len >= sizeof(struct rtattr))
>  
>  #define NLMSG_RTA(nlh,len)	((void*)((char*)(nlh)+sizeof(struct nlmsghdr)+NETLINK_ALIGN(len)))
>  #define NLMSG_RTAOK(rta,nlh)	RTA_OK(rta,NLMSG_DATAEND(nlh))
> 
> 
> Could you try if this fixes it?

I'm still waiting to hear whether this fixed it.

> You will probably need to 'make clean' or at least force recompilation
> of src/network/{getifaddrs,if_nameindex,netlink}.c as the netlink.h
> dependency is not picked up by the makefile automatically.
> 
> @dalias, if the above looks good to you, I am happy to submit properly
> formatted git patch for it.

I don't see anything obvious wrong with the proposed patch, but it
would be nice to have a better understanding of why it's needed and
whether this is a workaround for a kernel bug (present in which
kernels?) or something else.

Rich