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Subject: Re: [PATCH] sched/psi: Fix use-after-free in poll_freewait()
To:     Suren Baghdasaryan <surenb@google.com>
CC:     Johannes Weiner <hannes@cmpxchg.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <brauner@kernel.org>,
        <linux-kernel@vger.kernel.org>, <linux-fsdevel@vger.kernel.org>,
        Eric Biggers <ebiggers@kernel.org>,
        Oleg Nesterov <oleg@redhat.com>
References: <20230613062306.101831-1-lujialin4@huawei.com>
 <CAJuCfpEoCRHkJF-=1Go9E94wchB4BzwQ1E3vHGWxNe+tEmSJoA@mail.gmail.com>
From:   "lujialin (A)" <lujialin4@huawei.com>
Message-ID: <34f06b52-ff70-dd27-8888-e3e64966d6cd@huawei.com>
Date:   Wed, 14 Jun 2023 11:47:39 +0800
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在 2023/6/14 7:42, Suren Baghdasaryan 写道:
> On Mon, Jun 12, 2023 at 11:24 PM Lu Jialin <lujialin4@huawei.com> wrote:
>>
>> We found a UAF bug in remove_wait_queue as follows:
>>
>> ==================================================================
>> BUG: KASAN: use-after-free in _raw_spin_lock_irqsave+0x71/0xe0
>> Write of size 4 at addr ffff8881150d7b28 by task psi_trigger/15306
>> Call Trace:
>>   dump_stack+0x9c/0xd3
>>   print_address_description.constprop.0+0x19/0x170
>>   __kasan_report.cold+0x6c/0x84
>>   kasan_report+0x3a/0x50
>>   check_memory_region+0xfd/0x1f0
>>   _raw_spin_lock_irqsave+0x71/0xe0
>>   remove_wait_queue+0x26/0xc0
>>   poll_freewait+0x6b/0x120
>>   do_sys_poll+0x305/0x400
>>   do_syscall_64+0x33/0x40
>>   entry_SYSCALL_64_after_hwframe+0x61/0xc6
>>
>> Allocated by task 15306:
>>   kasan_save_stack+0x1b/0x40
>>   __kasan_kmalloc.constprop.0+0xb5/0xe0
>>   psi_trigger_create.part.0+0xfc/0x450
>>   cgroup_pressure_write+0xfc/0x3b0
>>   cgroup_file_write+0x1b3/0x390
>>   kernfs_fop_write_iter+0x224/0x2e0
>>   new_sync_write+0x2ac/0x3a0
>>   vfs_write+0x365/0x430
>>   ksys_write+0xd5/0x1b0
>>   do_syscall_64+0x33/0x40
>>   entry_SYSCALL_64_after_hwframe+0x61/0xc6
>>
>> Freed by task 15850:
>>   kasan_save_stack+0x1b/0x40
>>   kasan_set_track+0x1c/0x30
>>   kasan_set_free_info+0x20/0x40
>>   __kasan_slab_free+0x151/0x180
>>   kfree+0xba/0x680
>>   cgroup_file_release+0x5c/0xe0
>>   kernfs_drain_open_files+0x122/0x1e0
>>   kernfs_drain+0xff/0x1e0
>>   __kernfs_remove.part.0+0x1d1/0x3b0
>>   kernfs_remove_by_name_ns+0x89/0xf0
>>   cgroup_addrm_files+0x393/0x3d0
>>   css_clear_dir+0x8f/0x120
>>   kill_css+0x41/0xd0
>>   cgroup_destroy_locked+0x166/0x300
>>   cgroup_rmdir+0x37/0x140
>>   kernfs_iop_rmdir+0xbb/0xf0
>>   vfs_rmdir.part.0+0xa5/0x230
>>   do_rmdir+0x2e0/0x320
>>   __x64_sys_unlinkat+0x99/0xc0
>>   do_syscall_64+0x33/0x40
>>   entry_SYSCALL_64_after_hwframe+0x61/0xc6
>> ==================================================================
>>
>> If using epoll(), wake_up_pollfree will empty waitqueue and set
>> wait_queue_head is NULL before free waitqueue of psi trigger. But is
>> doesn't work when using poll(), which will lead a UAF problem in
>> poll_freewait coms as following:
>>
>> (cgroup_rmdir)                      |
>> psi_trigger_destroy                 |
>>    wake_up_pollfree(&t->event_wait)  |
> 
> It's important to note that psi_trigger_destroy() calls
> synchronize_rcu() before doing kfree(t), therefore the usage I think
> is valid.
> 
> 
>>      kfree(t)                        |
>>                                      |   (poll_freewait)
>>                                      |     free_poll_entry(pwq->inline_entries + i)
>>                                      |       remove_wait_queue(entry->wait_address)
>>                                      |         spin_lock_irqsave(&wq_head->lock)
>>
>> entry->wait_address in poll_freewait() is t->event_wait in cgroup_rmdir().
>> t->event_wait is free in psi_trigger_destroy before call poll_freewait(),
>> therefore wq_head in poll_freewait() has been already freed, which would
>> lead to a UAF.
>>
>> similar problem for epoll() has been fixed commit c2dbe32d5db5
>> ("sched/psi: Fix use-after-free in ep_remove_wait_queue()").
>> epoll wakeup function ep_poll_callback() will empty waitqueue and set
>> wait_queue_head is NULL when pollflags is POLLFREE and judge pwq->whead
>> is NULL or not before remove_wait_queue in ep_remove_wait_queue(),
>> which will fix the UAF bug in ep_remove_wait_queue.
>>
>> But poll wakeup function pollwake() doesn't do that. To fix the
>> problem, we empty waitqueue and set wait_address is NULL in pollwake() when
>> key is POLLFREE. otherwise in remove_wait_queue, which is similar to
>> epoll().
> 
> Thanks for the patch!
> This seems similar to what ep_poll_callback/ep_remove_wait_queue do,
> which I think makes sense. CC'ing Oleg Nesterov who implemented
> ep_poll_callback/ep_remove_wait_queue logic and Eric Biggers who
> worked on wake_up_pollfree() - both much more knowledgeable in this
> area than me.
> 
> One issue I see with this patch is that the title says "sched/psi:
> ..." while it's fixing polling functionality. The patch is fixing the
> mechanism used by psi triggers, not psi triggers themselves (well it
> does but indirectly). Therefore I suggest changing that prefix to
> something like "select: Fix use-after-free in poll_freewait()"
> 
> Thanks,
> Suren.
> 
>>
>> Fixes: 0e94682b73bf ("psi: introduce psi monitor")
>> Signed-off-by: Lu Jialin <lujialin4@huawei.com>
>> ---
>>   fs/select.c | 20 +++++++++++++++++++-
>>   1 file changed, 19 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/select.c b/fs/select.c
>> index 0ee55af1a55c..e64c7b4e9959 100644
>> --- a/fs/select.c
>> +++ b/fs/select.c
>> @@ -132,7 +132,17 @@ EXPORT_SYMBOL(poll_initwait);
>>
>>   static void free_poll_entry(struct poll_table_entry *entry)
>>   {
>> -       remove_wait_queue(entry->wait_address, &entry->wait);
>> +       wait_queue_head_t *whead;
>> +
>> +       rcu_read_lock();
>> +       /* If it is cleared by POLLFREE, it should be rcu-safe.
>> +        * If we read NULL we need a barrier paired with smp_store_release()
>> +        * in pollwake().
>> +        */
>> +       whead = smp_load_acquire(&entry->wait_address);
>> +       if (whead)
>> +               remove_wait_queue(whead, &entry->wait);
>> +       rcu_read_unlock();
>>          fput(entry->filp);
>>   }
>>
>> @@ -215,6 +225,14 @@ static int pollwake(wait_queue_entry_t *wait, unsigned mode, int sync, void *key
>>          entry = container_of(wait, struct poll_table_entry, wait);
>>          if (key && !(key_to_poll(key) & entry->key))
>>                  return 0;
>> +       if (key_to_poll(key) & POLLFREE) {
>> +               list_del_init(&wait->entry);
>> +               /* wait_address !=NULL protects us from the race with
>> +                * poll_freewait().
>> +                */
>> +               smp_store_release(&entry->wait_address, NULL);
>> +               return 0;
>> +       }
>>          return __pollwake(wait, mode, sync, key);
>>   }
>>
>> --
>> 2.17.1
>>
> .
> 
Thanks for your suggestion.I will correct the commit msg and title in 
v2.And cc to Oleg Nesterov and Eric Biggers.