Re: [PATCH 12/15] io_uring: add support for pre-mapped user IO buffers

[Jens Axboe]

On 1/16/19 1:53 PM, Dave Chinner wrote:
> On Wed, Jan 16, 2019 at 10:50:00AM -0700, Jens Axboe wrote:
>> If we have fixed user buffers, we can map them into the kernel when we
>> setup the io_context. That avoids the need to do get_user_pages() for
>> each and every IO.
> .....
>> +                    return -ENOMEM;
>> +    } while (atomic_long_cmpxchg(&ctx->user->locked_vm, cur_pages,
>> +                                    new_pages) != cur_pages);
>> +
>> +    return 0;
>> +}
>> +
>> +static int io_sqe_buffer_unregister(struct io_ring_ctx *ctx)
>> +{
>> +    int i, j;
>> +
>> +    if (!ctx->user_bufs)
>> +            return -EINVAL;
>> +
>> +    for (i = 0; i < ctx->sq_entries; i++) {
>> +            struct io_mapped_ubuf *imu = &ctx->user_bufs[i];
>> +
>> +            for (j = 0; j < imu->nr_bvecs; j++) {
>> +                    set_page_dirty_lock(imu->bvec[j].bv_page);
>> +                    put_page(imu->bvec[j].bv_page);
>> +            }
> 
> Hmmm, so we call set_page_dirty() when the gup reference is dropped...
> 
> .....
> 
>> +static int io_sqe_buffer_register(struct io_ring_ctx *ctx, void __user *arg,
>> +                              unsigned nr_args)
>> +{
> 
> .....
> 
>> +            down_write(&current->mm->mmap_sem);
>> +            pret = get_user_pages_longterm(ubuf, nr_pages, FOLL_WRITE,
>> +                                            pages, NULL);
>> +            up_write(&current->mm->mmap_sem);
> 
> Thought so. This has the same problem as RDMA w.r.t. using
> file-backed mappings for the user buffer.  It is not synchronised
> against truncate, hole punches, async page writeback cleaning the
> page, etc, and so can lead to data corruption and/or kernel panics.
> 
> It also can't be used with DAX because the above problems are
> actually a user-after-free of storage space, not just a dangling
> page reference that can be cleaned up after the gup pin is dropped.
> 
> Perhaps, at least until we solve the GUP problems w.r.t. file backed
> pages and/or add and require file layout leases for these reference,
> we should error out if the  user buffer pages are file-backed
> mappings?

Thanks for taking a look at this.

I'd be fine with that restriction, especially since it can get relaxed
down the line. Do we have an appropriate API for this? And why isn't
get_user_pages_longterm() that exact API already? Would seem that most
(all?) callers of this API is currently broken then.

-- 
Jens Axboe