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.
