Re: KASAN: use-after-free Read in sctp_id2assoc

[Dmitry Vyukov]

On Wed, Oct 10, 2018 at 8:40 PM, Marcelo Ricardo Leitner
<marcelo.leitner@xxxxxxxxx> wrote:
> On Wed, Oct 10, 2018 at 08:28:22PM +0200, Dmitry Vyukov wrote:
>> On Wed, Oct 10, 2018 at 8:13 PM, Marcelo Ricardo Leitner
>> <marcelo.leitner@xxxxxxxxx> wrote:
>> > On Wed, Oct 10, 2018 at 05:28:12PM +0200, Dmitry Vyukov wrote:
>> >> On Fri, Oct 5, 2018 at 4:58 PM, Marcelo Ricardo Leitner
>> >> <marcelo.leitner@xxxxxxxxx> wrote:
>> >> > On Thu, Oct 04, 2018 at 01:48:03AM -0700, syzbot wrote:
>> >> >> Hello,
>> >> >>
>> >> >> syzbot found the following crash on:
>> >> >>
>> >> >> HEAD commit:    4e6d47206c32 tls: Add support for inplace records encryption
>> >> >> git tree:       net-next
>> >> >> console output: https://syzkaller.appspot.com/x/log.txt?x=13834b81400000
>> >> >> kernel config:  https://syzkaller.appspot.com/x/.config?x=e569aa5632ebd436
>> >> >> dashboard link: https://syzkaller.appspot.com/bug?extid=c7dd55d7aec49d48e49a
>> >> >> 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+c7dd55d7aec49d48e49a@xxxxxxxxxxxxxxxxxxxxxxxxx
>> >> >>
>> >> >> netlink: 'syz-executor1': attribute type 1 has an invalid length.
>> >> >> ==================================================================
>> >> >> BUG: KASAN: use-after-free in sctp_id2assoc+0x3a7/0x3e0
>> >> >> net/sctp/socket.c:276
>> >> >> Read of size 8 at addr ffff880195b3eb20 by task syz-executor2/15454
>> >> >>
>> >> >> CPU: 1 PID: 15454 Comm: syz-executor2 Not tainted 4.19.0-rc5+ #242
>> >> >> 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+0x1c4/0x2b4 lib/dump_stack.c:113
>> >> >>  print_address_description.cold.8+0x9/0x1ff mm/kasan/report.c:256
>> >> >>  kasan_report_error mm/kasan/report.c:354 [inline]
>> >> >>  kasan_report.cold.9+0x242/0x309 mm/kasan/report.c:412
>> >> >>  __asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:433
>> >> >>  sctp_id2assoc+0x3a7/0x3e0 net/sctp/socket.c:276
>> >> >
>> >> > I'm not seeing yet how this could happen.
>> >> > All sockopts here are serialized by sock_lock.
>> >> > do_peeloff here would create another socket, but the issue was
>> >> > triggered before that.
>> >> > The same function that freed this memory, also removes the entry from
>> >> > idr mapping, so this entry shouldn't be there anymore.
>> >> >
>> >> > I have only two theories so far:
>> >> > - an issue with IDR/RCU.
>> >> > - something else happened that just the call stacks are not revealing.
>> >>
>> >> The "asoc->base.sk != sk" check after idr_find suggests that we don't
>> >> actually know what sock it belongs to. And if we don't know then
>> >
>> > Right. The check is more because the IDR is global and not per socket
>> > (and we don't want sockets accessing asocs from other sockets), and not
>> > that the asoc may move to another socket in between, but it also
>> > protects from such cases, yes.
>> >
>> >> locking this sock can't help keeping another sock association alive.
>> >> Am I missing something obvious here? Should we take assoc ref while we
>> >
>> > Not sure. Maybe I am.  Thanks for looking into this, btw.
>> >
>> >> are still holding sctp_assocs_id_lock?
>> >
>> > Shouldn't be needed.
>> >
>> > Solely by the call stacks:
>> > - we tried to establish a new asoc from a sctp_connect() call,
>> >   blocking one.
>> > - it slept waiting for the connect
>> > - (something closed the asoc in between the sleeps, because it freed
>> >   the asoc right when waking up on sctp_wait_for_connect())
>> > - it freed the asoc after sleeping on it on sctp_wait_for_connect [A]
>> > - another thread tried to peeloff that asoc [B]
>> >
>> > For [B] to access the asoc in question, it had to take the same sock
>> > lock [A] had taken, and then the idr should not return an asoc in
>> > sctp_i2asoc(). Note that we can't peeloff an asoc twice, thus why
>> > the certainty here.
>> >
>> > If [B] actually kicked in before the sleep resumed, that should have
>> > been fine because it took the same sock lock [A] would have to
>> > re-take. In this case an asoc would have been returned by
>> > sctp_id2asoc(), the asoc would have been moved to a new socket, but
>> > all while holding the original socket sock lock.
>>
>> But why A and B use the same lock?
>>
>> sctp_assocs_id is global, so it contains asocs from all sockets, right?
>> assoc id comes straight from userspaces.
>> So isn't it possible that B uses completely different sock but passes
>> assoc id from the A sock? Then B should find assoc in sctp_assocs_id,
>> and at the point of "asoc->base.sk != sk" check the assoc can be
>> already freed.
>
> That explains it. Somehow I was thinking the issue was for reading
> ->dead instead.  Now it's pretty clear. Then this should be the patch
> we want. Can you please give it a spin? (only compile tested)

syzbot can only test patches for bug with reproducers, this one
doesn't have one (not surprising taking into account subtliness of the
race):
https://github.com/google/syzkaller/blob/master/docs/syzbot.md#testing-patches

It's not possible squeeze in custom patches either:
https://github.com/google/syzkaller/blob/master/docs/syzbot.md#no-custom-patches

But the change looks good to me.

Acked-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>


> While holding the spinlock, an entry cannot be removed from the idr
> and thus it cannot be freed. So even if the app uses an id from
> another socket, it will still be there.
>
> ---8<---
>
> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> index f73e9d38d5ba..a7722f43aa69 100644
> --- a/net/sctp/socket.c
> +++ b/net/sctp/socket.c
> @@ -271,11 +271,10 @@ struct sctp_association *sctp_id2assoc(struct sock *sk, sctp_assoc_t id)
>
>         spin_lock_bh(&sctp_assocs_id_lock);
>         asoc = (struct sctp_association *)idr_find(&sctp_assocs_id, (int)id);
> +       if (asoc && (asoc->base.sk != sk || asoc->base.dead))
> +               asoc = NULL;
>         spin_unlock_bh(&sctp_assocs_id_lock);
>
> -       if (!asoc || (asoc->base.sk != sk) || asoc->base.dead)
> -               return NULL;
> -
>         return asoc;
>  }
>