On 10/18/19 10:20 AM, Jann Horn wrote: > On Fri, Oct 18, 2019 at 5:55 PM Jens Axboe <axboe@xxxxxxxxx> wrote: >> On 10/18/19 9:00 AM, Jens Axboe wrote: >>> On 10/18/19 8:52 AM, Jann Horn wrote: >>>> On Fri, Oct 18, 2019 at 4:43 PM Jens Axboe <axboe@xxxxxxxxx> wrote: >>>>> >>>>> On 10/18/19 8:40 AM, Jann Horn wrote: >>>>>> On Fri, Oct 18, 2019 at 4:37 PM Jens Axboe <axboe@xxxxxxxxx> wrote: >>>>>>> >>>>>>> On 10/18/19 8:34 AM, Jann Horn wrote: >>>>>>>> On Fri, Oct 18, 2019 at 4:01 PM Jens Axboe <axboe@xxxxxxxxx> wrote: >>>>>>>>> On 10/17/19 8:41 PM, Jann Horn wrote: >>>>>>>>>> On Fri, Oct 18, 2019 at 4:01 AM Jens Axboe <axboe@xxxxxxxxx> wrote: >>>>>>>>>>> This is in preparation for adding opcodes that need to modify files >>>>>>>>>>> in a process file table, either adding new ones or closing old ones. >>>>>>>> [...] >>>>>>>>> Updated patch1: >>>>>>>>> >>>>>>>>> http://git.kernel.dk/cgit/linux-block/commit/?h=for-5.5/io_uring-test&id=df6caac708dae8ee9a74c9016e479b02ad78d436 >>>>>>>> >>>>>>>> I don't understand what you're doing with old_files in there. In the >>>>>>>> "s->files && !old_files" branch, "current->files = s->files" happens >>>>>>>> without holding task_lock(), but current->files and s->files are also >>>>>>>> the same already at that point anyway. And what's the intent behind >>>>>>>> assigning stuff to old_files inside the loop? Isn't that going to >>>>>>>> cause the workqueue to keep a modified current->files beyond the >>>>>>>> runtime of the work? >>>>>>> >>>>>>> I simply forgot to remove the old block, it should only have this one: >>>>>>> >>>>>>> if (s->files && s->files != cur_files) { >>>>>>> task_lock(current); >>>>>>> current->files = s->files; >>>>>>> task_unlock(current); >>>>>>> if (cur_files) >>>>>>> put_files_struct(cur_files); >>>>>>> cur_files = s->files; >>>>>>> } >>>>>> >>>>>> Don't you still need a put_files_struct() in the case where "s->files >>>>>> == cur_files"? >>>>> >>>>> I want to hold on to the files for as long as I can, to avoid unnecessary >>>>> shuffling of it. But I take it your worry here is that we'll be calling >>>>> something that manipulates ->files? Nothing should do that, unless >>>>> s->files is set. We didn't hide the workqueue ->files[] before this >>>>> change either. >>>> >>>> No, my worry is that the refcount of the files_struct is left too >>>> high. From what I can tell, the "do" loop in io_sq_wq_submit_work() >>>> iterates over multiple instances of struct sqe_submit. If there are >>>> two sqe_submit instances with the same ->files (each holding a >>>> reference from the get_files_struct() in __io_queue_sqe()), then: >>>> >>>> When processing the first sqe_submit instance, current->files and >>>> cur_files are set to $user_files. >>>> When processing the second sqe_submit instance, nothing happens >>>> (s->files == cur_files). >>>> After the loop, at the end of the function, put_files_struct() is >>>> called once on $user_files. >>>> >>>> So get_files_struct() has been called twice, but put_files_struct() >>>> has only been called once. That leaves the refcount too high, and by >>>> repeating this, an attacker can make the refcount wrap around and then >>>> cause a use-after-free. >>> >>> Ah now I see what you are getting at, yes that's clearly a bug! I wonder >>> how we best safely can batch the drops. We can track the number of times >>> we've used the same files, and do atomic_sub_and_test() in a >>> put_files_struct_many() type addition. But that would leave us open to >>> the issue you describe, where someone could maliciously overflow the >>> files ref count. >>> >>> Probably not worth over-optimizing, as long as we can avoid the >>> current->files task lock/unlock and shuffle. >>> >>> I'll update the patch. >> >> Alright, this incremental on top should do it. And full updated patch >> here: >> >> http://git.kernel.dk/cgit/linux-block/commit/?h=for-5.5/io_uring-test&id=40449c5a3d3b16796fa13e9469c69d62986e961c >> >> Let me know what you think. > > Ignoring the locking elision, basically the logic is now this: > > static void io_sq_wq_submit_work(struct work_struct *work) > { > struct io_kiocb *req = container_of(work, struct io_kiocb, work); > struct files_struct *cur_files = NULL, *old_files; > [...] > old_files = current->files; > [...] > do { > struct sqe_submit *s = &req->submit; > [...] > if (cur_files) > /* drop cur_files reference; borrow lifetime must > * end before here */ > put_files_struct(cur_files); > /* move reference ownership to cur_files */ > cur_files = s->files; > if (cur_files) { > task_lock(current); > /* current->files borrows reference from cur_files; > * existing borrow from previous loop ends here */ > current->files = cur_files; > task_unlock(current); > } > > [call __io_submit_sqe()] > [...] > } while (req); > [...] > /* existing borrow ends here */ > task_lock(current); > current->files = old_files; > task_unlock(current); > if (cur_files) > /* drop cur_files reference; borrow lifetime must > * end before here */ > put_files_struct(cur_files); > } > > If you run two iterations of this loop, with a first element that has > a ->files pointer and a second element that doesn't, then in the > second run through the loop, the reference to the files_struct will be > dropped while current->files still points to it; current->files is > only reset after the loop has ended. If someone accesses > current->files through procfs directly after that, AFAICS you'd get a > use-after-free. Amazing how this is still broken. You are right, and it's especially annoying since that's exactly the case I originally talked about (not flipping current->files if we don't have to). I just did it wrong, so we'll leave a dangling pointer in ->files. The by far most common case is if one sqe has a files it needs to attach, then others that also have files will be the same set. So I want to optimize for the case where we only flip current->files once when we see the files, and once when we're done with the loop. Let me see if I can get this right... -- Jens Axboe
