Re: [PATCH 05/18] Add io_uring IO interface

[Jann Horn <jannh@google.com>]

On Tue, Jan 29, 2019 at 2:07 AM Jann Horn <jannh@google.com> wrote:
> On Mon, Jan 28, 2019 at 10:35 PM Jens Axboe <axboe@kernel.dk> wrote:
> > The submission queue (SQ) and completion queue (CQ) rings are shared
> > between the application and the kernel. This eliminates the need to
> > copy data back and forth to submit and complete IO.
> [...]
> > +static bool io_get_sqring(struct io_ring_ctx *ctx, struct sqe_submit *s)
> > +{
> > +       struct io_sq_ring *ring = ctx->sq_ring;
> > +       unsigned head;
> > +
> > +       /*
> > +        * The cached sq head (or cq tail) serves two purposes:
> > +        *
> > +        * 1) allows us to batch the cost of updating the user visible
> > +        *    head updates.
> > +        * 2) allows the kernel side to track the head on its own, even
> > +        *    though the application is the one updating it.
> > +        */
> > +       head = ctx->cached_sq_head;
> > +       smp_rmb();
> > +       if (head == READ_ONCE(ring->r.tail))
> > +               return false;
> > +
> > +       head = ring->array[head & ctx->sq_mask];
> > +       if (head < ctx->sq_entries) {
> > +               s->index = head;
> > +               s->sqe = &ctx->sq_sqes[head];
>
> ring->array can be mapped writable into userspace, right? If so: This
> looks like a double-read issue; the compiler might assume that
> ring->array is not modified concurrently and perform separate memory
> accesses for the "if (head < ctx->sq_entries)" check and the
> "&ctx->sq_sqes[head]" computation. Please use READ_ONCE()/WRITE_ONCE()
> for all accesses to memory that userspace could concurrently modify in
> a malicious way.
>
> There have been some pretty severe security bugs caused by missing
> READ_ONCE() annotations around accesses to shared memory; see, for
> example, https://www.blackhat.com/docs/us-16/materials/us-16-Wilhelm-Xenpwn-Breaking-Paravirtualized-Devices.pdf
> . Slides 35-48 show how the code "switch (op->cmd)", where "op" is a
> pointer to shared memory, allowed an attacker to break out of a Xen
> virtual machine because the compiler generated multiple memory
> accesses.

Oh, actually, it's even worse (comments with "//" added by me):

io_sq_thread() does this:

        do {
                // sqes[i].sqe is pointer to shared memory, result of
                // io_sqe_needs_user() is unreliable
                if (all_fixed && io_sqe_needs_user(sqes[i].sqe))
                        all_fixed = false;

                i++;
                if (i == ARRAY_SIZE(sqes))
                        break;
        } while (io_get_sqring(ctx, &sqes[i]));
        // sqes[...].sqe are pointers to shared memory

        io_commit_sqring(ctx);

        /* Unless all new commands are FIXED regions, grab mm */
        if (!all_fixed && !cur_mm) {
                mm_fault = !mmget_not_zero(ctx->sqo_mm);
                if (!mm_fault) {
                        use_mm(ctx->sqo_mm);
                        cur_mm = ctx->sqo_mm;
                }
        }

        inflight += io_submit_sqes(ctx, sqes, i, mm_fault);

Then the shared memory pointers go into io_submit_sqes():

static int io_submit_sqes(struct io_ring_ctx *ctx, struct sqe_submit *sqes,
                          unsigned int nr, bool mm_fault)
{
        struct io_submit_state state, *statep = NULL;
        int ret, i, submitted = 0;
        // sqes[...].sqe are pointers to shared memory
        [...]
        for (i = 0; i < nr; i++) {
                if (unlikely(mm_fault))
                        ret = -EFAULT;
                else
                        ret = io_submit_sqe(ctx, &sqes[i], statep);
                [...]
        }
        [...]
}

And on into io_submit_sqe():

static int io_submit_sqe(struct io_ring_ctx *ctx, struct sqe_submit *s,
                         struct io_submit_state *state)
{
        [...]
        ret = __io_submit_sqe(ctx, req, s, true, state);
        [...]
}

And there it gets interesting:

static int __io_submit_sqe(struct io_ring_ctx *ctx, struct io_kiocb *req,
                           struct sqe_submit *s, bool force_nonblock,
                           struct io_submit_state *state)
{
        // s->sqe is a pointer to shared memory
        const struct io_uring_sqe *sqe = s->sqe;
        // sqe is a pointer to shared memory
        ssize_t ret;

        if (unlikely(s->index >= ctx->sq_entries))
                return -EINVAL;
        req->user_data = sqe->user_data;

        ret = -EINVAL;
        // switch() on read from shared memory, potential instruction pointer
        // control
        switch (sqe->opcode) {
        [...]
        case IORING_OP_READV:
                if (unlikely(sqe->buf_index))
                        return -EINVAL;
                ret = io_read(req, sqe, force_nonblock, state);
                break;
        [...]
        case IORING_OP_READ_FIXED:
                ret = io_read(req, sqe, force_nonblock, state);
                break;
        [...]
        }
        [...]
}

On into io_read():

static ssize_t io_read(struct io_kiocb *req, const struct io_uring_sqe *sqe,
                       bool force_nonblock, struct io_submit_state *state)
{
[...]
        // sqe is a pointer to shared memory
        ret = io_prep_rw(req, sqe, force_nonblock, state);
        [...]
}

And then io_prep_rw() does multiple reads even in the source code:

static int io_prep_rw(struct io_kiocb *req, const struct io_uring_sqe *sqe,
                      bool force_nonblock, struct io_submit_state *state)
{
        struct io_ring_ctx *ctx = req->ctx;
        struct kiocb *kiocb = &req->rw;
        int ret;

        // sqe is a pointer to shared memory

        // double-read of sqe->flags, see end of function
        if (sqe->flags & IOSQE_FIXED_FILE) {
                // double-read of sqe->fd for the bounds check and the
array access, potential OOB pointer read
                if (unlikely(!ctx->user_files || sqe->fd >= ctx->nr_user_files))
                        return -EBADF;
                kiocb->ki_filp = ctx->user_files[sqe->fd];
                req->flags |= REQ_F_FIXED_FILE;
        } else {
                kiocb->ki_filp = io_file_get(state, sqe->fd);
        }
        if (unlikely(!kiocb->ki_filp))
                return -EBADF;
        kiocb->ki_pos = sqe->off;
        kiocb->ki_flags = iocb_flags(kiocb->ki_filp);
        kiocb->ki_hint = ki_hint_validate(file_write_hint(kiocb->ki_filp));
        // three reads of sqe->ioprio, bypassable capability check
        if (sqe->ioprio) {
                ret = ioprio_check_cap(sqe->ioprio);
                if (ret)
                        goto out_fput;

                kiocb->ki_ioprio = sqe->ioprio;
        } else
                kiocb->ki_ioprio = get_current_ioprio();
        [...]
        return 0;
out_fput:
        // double-read of sqe->flags, changed value can lead to
unbalanced refcount
        if (!(sqe->flags & IOSQE_FIXED_FILE))
                io_file_put(state, kiocb->ki_filp);
        return ret;
}

Please create a local copy of the request before parsing it to keep
the data from changing under you. Additionally, it might make sense to
annotate every pointer to shared memory with a comment, or something
like that, to ensure that anyone looking at the code can immediately
see for which pointers special caution is required on access.

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