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

On Sat, Sep 9, 2017 at 9:56 AM, Cong Wang [off-list ref] wrote:

On Tue, Sep 5, 2017 at 8:47 PM, Xin Long [off-list ref] wrote:

quoted

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.