Re: [PATCH net] netlink: fix an use-after-free issue for nlk groups

[Xin Long <lucien.xin@gmail.com>]

On Sat, Sep 9, 2017 at 9:56 AM, Cong Wang <xiyou.wangcong@gmail.com> wrote:
> On Tue, Sep 5, 2017 at 8:47 PM, Xin Long <lucien.xin@gmail.com> wrote:
>> ChunYu found a netlink use-after-free issue by syzkaller:
>>
>> [28448.842981] BUG: KASAN: use-after-free in __nla_put+0x37/0x40 at addr ffff8807185e2378
>> [28448.969918] Call Trace:
>> [...]
>> [28449.117207]  __nla_put+0x37/0x40
>> [28449.132027]  nla_put+0xf5/0x130
>> [28449.146261]  sk_diag_fill.isra.4.constprop.5+0x5a0/0x750 [netlink_diag]
>> [28449.176608]  __netlink_diag_dump+0x25a/0x700 [netlink_diag]
>> [28449.202215]  netlink_diag_dump+0x176/0x240 [netlink_diag]
>> [28449.226834]  netlink_dump+0x488/0xbb0
>> [28449.298014]  __netlink_dump_start+0x4e8/0x760
>> [28449.317924]  netlink_diag_handler_dump+0x261/0x340 [netlink_diag]
>> [28449.413414]  sock_diag_rcv_msg+0x207/0x390
>> [28449.432409]  netlink_rcv_skb+0x149/0x380
>> [28449.467647]  sock_diag_rcv+0x2d/0x40
>> [28449.484362]  netlink_unicast+0x562/0x7b0
>> [28449.564790]  netlink_sendmsg+0xaa8/0xe60
>> [28449.661510]  sock_sendmsg+0xcf/0x110
>> [28449.865631]  __sys_sendmsg+0xf3/0x240
>> [28450.000964]  SyS_sendmsg+0x32/0x50
>> [28450.016969]  do_syscall_64+0x25c/0x6c0
>> [28450.154439]  entry_SYSCALL64_slow_path+0x25/0x25
>>
>> It was caused by no protection between nlk groups' free in netlink_release
>> and nlk groups' accessing in sk_diag_dump_groups. The similar issue also
>> exists in netlink_seq_show().
>>
>> This patch is to defer nlk groups' free in deferred_put_nlk_sk.
>
> This looks odd too, at least not complete.
>
> The netlink sock itself is protected by RCU to speed up
> the lookup path, but not necessarily nlk->groups, at least
> I don't see rcu_dereference() in sk_diag_dump_groups().
> And netlink_realloc_groups() needs fix too, right? Otherwise
> krealloc() could reallocate a brand new memory and
> existing readers will crash too?
We knew it's not good, but this one is not so bad as the one in
netlink_release, as krealloc will not alloc new memory when
the new size < old size, so it may not cause crash.

I was planning to fix it as a long term. As you also said, here needs
more work for nlk->groups, including  proper RCU api for readers.
I will check more.

Thanks.

>
> I am afraid you need more work to make nlk->groups
> RCU friendly. RCU is not just about call_rcu(), both
> readers and writers need to use proper RCU API.