Re: Use-after-free in ppoll

Dmitry Vyukov
2015-11-23T12:02:36Z

On Sun, Nov 22, 2015 at 7:46 PM, Rainer Weikusat
<rweikusat@mobileactivedefense.com> wrote:
> Dmitry Vyukov <dvyukov@google.com> writes:
>> On Sun, Nov 22, 2015 at 3:32 PM, Rainer Weikusat
>> <rweikusat@mobileactivedefense.com> wrote:
>>> Dmitry Vyukov <dvyukov@google.com> writes:
>>>> Hello,
>>>>
>>>> On commit f2d10565b9bdbb722bd43e6e1a759eeddb9645c8 (Nov 20).
>>>>
>>>> The following program triggers use-after-free:
>>>>
>>>> // autogenerated by syzkaller (http://github.com/google/syzkaller)
>>>> #include <syscall.h>
>>>> #include <string.h>
>>>> #include <stdint.h>
>>>> #include <pthread.h>
>>>>
>>>> void *thread(void *p)
>>>> {
>>>> syscall(SYS_write, (long)p, 0x2000278ful, 0x1ul, 0, 0, 0);
>>>> return 0;
>>>> }
>>>
>>> [...]
>>>
>>>
>>>> long r1 = syscall(SYS_socketpair, 0x1ul, 0x3ul, 0x0ul,
>>>
>>> [...]
>>>
>>>> long r5 = syscall(SYS_close, r2, 0, 0, 0, 0, 0);
>>>> pthread_t th;
>>>> pthread_create(&th, 0, thread, (void*)(long)r3);
>>>
>>> [...]
>>>
>>>> long r21 = syscall(SYS_ppoll, 0x20000ffful, 0x3ul, 0x20000ffcul, 0x20000ffdul, 0x8ul, 0);
>>>> return 0;
>>>> }
>>>
>>> That's one of the already known sequences for triggering this issue:
>
> [...]
>
>> I have not read the code. But I just want to point out that all 3
>> reports are different. For example, in the first one, ppoll both frees
>> the object and then accesses it. That is, it is not write that frees
>> the object.
>
> The call trace is always the same:
>
> [ 2672.994366] [<ffffffff812ca0fa>] __asan_load4+0x6a/0x70
> [ 2672.994366] [<ffffffff81126832>] do_raw_spin_lock+0x22/0x220
> [ 2672.994366] [<ffffffff821d6061>] _raw_spin_lock_irqsave+0x51/0x60
> [ 2672.994366] [<ffffffff8110d748>] remove_wait_queue+0x18/0x80
> [ 2672.994366] [<ffffffff812fddab>] poll_freewait+0x7b/0x130
> [ 2672.994366] [<ffffffff8130063c>] do_sys_poll+0x4dc/0x860
> [ 2672.994366] [<ffffffff81300eb9>] SyS_ppoll+0x1a9/0x310
>
> And if you look at the poll implementation, the important part is this
> (fs/ select.c, do_sys_poll)
>
> fdcount = do_poll(nfds, head, &table, end_time);
> poll_freewait(&table);
>
> do_poll calls the poll routine of the file descriptors which cause
> "enqueuing of something" via poll wait callback. For poll, that's the
> __pollwait routine in select.c:
>
> static void __pollwait(struct file *filp, wait_queue_head_t *wait_address,
> poll_table *p)
> {
> struct poll_wqueues *pwq = container_of(p, struct poll_wqueues, pt);
> struct poll_table_entry *entry = poll_get_entry(pwq);
> if (!entry)
> return;
> entry->filp = get_file(filp);
> entry->wait_address = wait_address;
> entry->key = p->_key;
> init_waitqueue_func_entry(&entry->wait, pollwake);
> entry->wait.private = pwq;
> add_wait_queue(wait_address, &entry->wait);
> }
>
> because of the close, this routine will be called with the peer_wait
> wait_queue_head of the non-closed socket of the socket pair as
> wait_address argument. And poll_freewait calls free_poll_entry for all
> entries on the poll table which is
>
> static void free_poll_entry(struct poll_table_entry *entry)
> {
> remove_wait_queue(entry->wait_address, &entry->wait);
> fput(entry->filp);
> }
>
> but by this time, the wait_address points to freed memory because the
> only thing which kept the socket it belonged to alive after the
> corresponding file descriptor was closed was the reference the other
> socket held. But that was dropped by unix_dgram_sendmsg upon detecting a
> dead peer.

Hi Rainer,

I am not questioning your conclusions. You definitely know better.

Btw, how close are you to a fix that everybody is happy with? I hit
this use-after-free very frequently.