Re: KASAN: use-after-free Read in __lock_sock (high-risk primitives found)

[SyzScope <syzscope@gmail.com>]

Hi,

This is SyzScope, a research project that aims to reveal high-risk 
primitives from a seemingly low-risk bug (UAF/OOB read, WARNING, BUG, etc.).

We are currently testing seemingly low-risk bugs on syzbot's open 
section(https://syzkaller.appspot.com/upstream), and try to reach out to 
kernel developers if SyzScope discovers any high-risk primitives.

Regrading the bug "KASAN: use-after-free Read in __lock_sock", it seems 
that this bug is still missing a valid patch.

SyzScope reports 8 memory write primitives, and 1 control flow hijacking 
primitives from this bug.

The detailed comments can be found at 
https://sites.google.com/view/syzscope/kasan-use-after-free-read-in-lock_sock

Please let us know if SyzScope indeed helps, and any suggestions/feedback.

On 11/22/2018 6:37 AM, Marcelo Ricardo Leitner wrote:
> On Thu, Nov 22, 2018 at 10:44:16PM +0900, Xin Long wrote:
>> On Thu, Nov 22, 2018 at 10:13 PM Marcelo Ricardo Leitner
>> <marcelo.leitner@gmail.com> wrote:
>>> On Mon, Nov 19, 2018 at 05:57:33PM +0900, Xin Long wrote:
>>>> On Sat, Nov 17, 2018 at 4:18 PM syzbot
>>>> <syzbot+9276d76e83e3bcde6c99@syzkaller.appspotmail.com> wrote:
>>>>> Hello,
>>>>>
>>>>> syzbot found the following crash on:
>>>>>
>>>>> HEAD commit:    ccda4af0f4b9 Linux 4.20-rc2
>>>>> git tree:       upstream
>>>>> console output: https://syzkaller.appspot.com/x/log.txt?x=156cd533400000
>>>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=4a0a89f12ca9b0f5
>>>>> dashboard link: https://syzkaller.appspot.com/bug?extid=9276d76e83e3bcde6c99
>>>>> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
>>>>>
>>>>> Unfortunately, I don't have any reproducer for this crash yet.
>>>>>
>>>>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>>>>> Reported-by: syzbot+9276d76e83e3bcde6c99@syzkaller.appspotmail.com
>>>>>
>>>>> netlink: 5 bytes leftover after parsing attributes in process
>>>>> `syz-executor5'.
>>>>> ==================================================================
>>>>> BUG: KASAN: use-after-free in __lock_acquire+0x36d9/0x4c20
>>>>> kernel/locking/lockdep.c:3218
>>>>> Read of size 8 at addr ffff8881d26d60e0 by task syz-executor1/13725
>>>>>
>>>>> CPU: 0 PID: 13725 Comm: syz-executor1 Not tainted 4.20.0-rc2+ #333
>>>>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
>>>>> Google 01/01/2011
>>>>> Call Trace:
>>>>>    __dump_stack lib/dump_stack.c:77 [inline]
>>>>>    dump_stack+0x244/0x39d lib/dump_stack.c:113
>>>>>    print_address_description.cold.7+0x9/0x1ff mm/kasan/report.c:256
>>>>>    kasan_report_error mm/kasan/report.c:354 [inline]
>>>>>    kasan_report.cold.8+0x242/0x309 mm/kasan/report.c:412
>>>>>    __asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:433
>>>>>    __lock_acquire+0x36d9/0x4c20 kernel/locking/lockdep.c:3218
>>>>>    lock_acquire+0x1ed/0x520 kernel/locking/lockdep.c:3844
>>>>>    __raw_spin_lock_bh include/linux/spinlock_api_smp.h:135 [inline]
>>>>>    _raw_spin_lock_bh+0x31/0x40 kernel/locking/spinlock.c:168
>>>>>    spin_lock_bh include/linux/spinlock.h:334 [inline]
>>>>>    __lock_sock+0x203/0x350 net/core/sock.c:2253
>>>>>    lock_sock_nested+0xfe/0x120 net/core/sock.c:2774
>>>>>    lock_sock include/net/sock.h:1492 [inline]
>>>>>    sctp_sock_dump+0x122/0xb20 net/sctp/diag.c:324
>>>> static int sctp_sock_dump(struct sctp_transport *tsp, void *p)
>>>> {
>>>>          struct sctp_endpoint *ep = tsp->asoc->ep;
>>>>          struct sctp_comm_param *commp = p;
>>>>          struct sock *sk = ep->base.sk; <--- [1]
>>>> ...
>>>>          int err = 0;
>>>>
>>>>          lock_sock(sk);  <--- [2]
>>>>
>>>> Between [1] and [2], an asoc peeloff may happen, still thinking
>>>> how to avoid this.
>>> This race cannot happen more than once for an asoc, so something
>>> like this may be doable:
>>>
>>>          struct sctp_comm_param *commp = p;
>>>          struct sctp_endpoint *ep;
>>>          struct sock *sk;
>>> ...
>>>          int err = 0;
>>>
>>> again:
>>>          ep = tsp->asoc->ep;
>>>          sk = ep->base.sk; <---[3]
>>>          lock_sock(sk);  <--- [2]
>> if peel-off happens between [3] and [2], and sk is freed
>> somewhere, it will panic on [2] when trying to get the
>> sk->lock, no?
> Not sure what protects it, but this construct is also used in BH processing at
> sctp_rcv():
> ...
>          bh_lock_sock(sk); [4]
>
>          if (sk != rcvr->sk) {
>                  /* Our cached sk is different from the rcvr->sk.  This is
>                   * because migrate()/accept() may have moved the association
>                   * to a new socket and released all the sockets.  So now we
>                   * are holding a lock on the old socket while the user may
>                   * be doing something with the new socket.  Switch our veiw
>                   * of the current sk.
>                   */
>                  bh_unlock_sock(sk);
>                  sk = rcvr->sk;
>                  bh_lock_sock(sk);
>          }
> ...
>
> If it is not safe, then we have an issue there too.
> And by [4] that copy on sk is pretty old already.
>
>>>          if (sk != tsp->asoc->ep->base.sk) {
>>>                  /* Asoc was peeloff'd */
>>>                  unlock_sock(sk);
>>>                  goto again;
>>>          }
>>>
>>> Similarly to what we did on cea0cc80a677 ("sctp: use the right sk
>>> after waking up from wait_buf sleep").