From mboxrd@z Thu Jan  1 00:00:00 1970
From: Cong Wang <xiyou.wangcong@gmail.com>
Subject: Re: [PATCH net] netlink: fix an use-after-free issue for nlk groups
Date: Fri, 8 Sep 2017 14:56:36 -0700
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References: <1217baf9faf9d034ba8224cb6362b822c51a0eae.1504669632.git.lucien.xin@gmail.com>
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Cc: network dev <netdev@vger.kernel.org>,
        David Miller <davem@davemloft.net>,
        Florian Westphal <fw@strlen.de>, chunwang@redhat.com,
        syzkaller <syzkaller@googlegroups.com>
To: Xin Long <lucien.xin@gmail.com>
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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?

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.