Re: GPF in shm_lock ipc

[Davidlohr Bueso <dave@stgolabs.net>]

On Mon, 12 Oct 2015, Kirill A. Shutemov wrote:

>On Mon, Oct 12, 2015 at 10:49:45AM -0700, Davidlohr Bueso wrote:
>> diff --git a/ipc/shm.c b/ipc/shm.c
>> index 4178727..9615f19 100644
>> --- a/ipc/shm.c
>> +++ b/ipc/shm.c
>> @@ -385,9 +385,25 @@ static struct mempolicy *shm_get_policy(struct vm_area_struct *vma,
>>  static int shm_mmap(struct file *file, struct vm_area_struct *vma)
>>  {
>> -	struct shm_file_data *sfd = shm_file_data(file);
>> +	struct file *vma_file = vma->vm_file;
>> +	struct shm_file_data *sfd = shm_file_data(vma_file);
>> +	struct ipc_ids *ids = &shm_ids(sfd->ns);
>> +	struct kern_ipc_perm *shp;
>>  	int ret;
>> +	rcu_read_lock();
>> +	shp = ipc_obtain_object_check(ids, sfd->id);
>> +	if (IS_ERR(shp)) {
>> +		ret = -EINVAL;
>> +		goto err;
>> +	}
>> +
>> +	if (!ipc_valid_object(shp)) {
>> +		ret = -EIDRM;
>> +		goto err;
>> +	}
>> +	rcu_read_unlock();
>> +
>
>Hm. Isn't it racy? What prevents IPC_RMID from happening after this point?

Nothing, but that is later caught by shm_open() doing similar checks. We
basically end up doing a check between ->mmap() calls, which is fair imho.
Note that this can occur anywhere in ipc as IPC_RMID is a user request/cmd,
and we try to respect it -- thus you can argue this race anywhere, which is
why we have EIDRM/EINVL. Ultimately the user should not be doing such hacks
_anyway_. So I'm not really concerned about it.

Another similar alternative would be perhaps to make shm_lock() return an
error, and thus propagate that error to mmap return. That way we would have
a silent way out of the warning scenario (afterward we cannot race as we
hold the ipc object lock). However, the users would now have to take this
into account...

      [validity check lockless]
      ->mmap()
      [validity check lock]

>Shouldn't we bump shm_nattch here? Or some other refcount?

At least not shm_nattach, as that would acknowledge a new attachment after
a valid IPC_RMID. But the problem is also with how we check for marked for
deletion segments -- ipc_valid_object() checking the deleted flag. As such,
we always rely on explicitly checking against the deleted flag.

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