Subject: Re: net: GPF in netlink_getsockbyportid

From: Daniel Borkmann <[email-masked]>
Date: Sat, 23 Jan 2016 21:05:11 +0100

On 01/23/2016 08:25 PM, Florian Westphal wrote:
> Dmitry Vyukov <[email-masked]> wrote:
>
> [ CC nf-devel, not sure if its nfnetlink fault or NETLINK_MMAP ]
>
>> The following program causes GPF in netlink_getsockbyportid:
>>
>> // autogenerated by syzkaller (http://github.com/google/syzkaller)
>> #include <pthread.h>
>> #include <stdint.h>
>> #include <string.h>
>> #include <sys/syscall.h>
>> #include <unistd.h>
>>
>> int main()
>> {
>>    syscall(SYS_mmap, 0x20000000ul, 0xe65000ul, 0x3ul, 0x32ul,
>>                   0xfffffffffffffffful, 0x0ul);
>>    int fd = syscall(SYS_socket, 0x10ul, 0x803ul, 0xcul, 0, 0, 0);
>>    *(uint32_t*)0x20e64000 = (uint32_t)0x28;
>>    *(uint32_t*)0x20e64004 = (uint32_t)0x10;
>>    *(uint64_t*)0x20e64008 = (uint64_t)0x0;
>>    *(uint64_t*)0x20e64010 = (uint64_t)0x3;
>>    *(uint64_t*)0x20e64018 = (uint64_t)0xfff;
>>    *(uint16_t*)0x20e64020 = (uint16_t)0x5;
>>    syscall(SYS_write, fd, 0x20e64000ul, 0x28ul, 0, 0, 0);
>>    return 0;
>> }
>
> CONFIG_NETLINK_MMAP and nfnetlink batching strike in unison :-/
>
> root cause is in nfnetlink_rcv_batch():
>
> 296 replay:
> 297         status = 0;
> 298
> 299         skb = netlink_skb_clone(oskb, GFP_KERNEL);
>
> The clone op doesn't copy oskb->sk, so we oops in
> __netlink_alloc_skb -> netlink_getsockbyportid() when nfnetlink_rcv_batch
> tries to send netlink ack.

If indeed oskb is the mmap'ed netlink skb, then it's not even allowed
to call into skb_clone() as it would access skb shared info data that
can be controlled by the user space mmap buffer, iirc, we had that in
the past with nlmon where skb_clone() was accidentally used.